COOL STARS 22
24–28 June 2024, San Diego, California, USA
The "Cambridge Workshops on Cool Stars, Stellar Systems and the Sun" are held biennially and have evolved to be the premier conference series for cool star research.
We reserve the right to make adjustments to the schedule.
| Time | Sunday | Monday | Tuesday | Wednesday | Thursday | Friday |
| 6/23 | 6/24 | 6/25 | 6/26 | 6/27 | 6/28 | |
| Topic | New Insights into Star Formation and Evolution | Milky Way-scale Science and Big Data | The Sun and Cool Stars in the Time Domain | Cool Stars as Stellar Systems | Brown Dwarfs and Giant Exoplanets: Future Prospects and Thirty Years of Discovery | |
| 08:00-08:45 | Registration & welcome address | |||||
| 08:45-09:15 | Invited Talk | Invited Talk | Invited Talk | Invited Talk | Invited Talk | |
| 09:15-10:00 | Contributed Talks | Contributed Talks | Contributed Talks | Contributed Talks | Contributed Talks | |
| 10:00-10:15 | Frank Shu in memoriam | Coffee break + Posters | Coffee break + Posters | Coffee break + Posters | Coffee break + Posters | |
| 10:15-10:50 | Coffee break + Posters | |||||
| 10:50-11:25 | Invited Talk | Invited Talk | Invited Talk | Invited Talk | Invited Talk | |
| 11:25-12:30 | Contributed Talks | Contributed Talks | Contributed Talks | Contributed Talks | Contributed Talks | |
| 12:30-14:00 | Lunch break | Lunch break | Lunch break | Lunch break | Concluding remarks & Picnic | |
| 14:00-15:00 | Splinter sessions | Splinter sessions | Excursions | Splinter sessions | ||
| 15:00-15:30 | Networking & Professional Development Early Career Researcher Event | |||||
| 15:30-16:00 | Coffee break + Posters | Coffee break + Posters | Banquet | |||
| 16:00-17:30 | Splinter sessions | Splinter sessions | ||||
| 17:30-18:00 | Posters | Posters | ||||
| 18:00-18:30 | Conference Reception & Pre-registration | |||||
| 18:30-19:00 | ||||||
| 19:00-20:30 | ||||||
| 20:30-21:00 | ||||||
| 22:00-22:00 |
The meeting will cover the following five science themes:
Recent advances in observational and theoretical tools allow us to now study star formation age ages and scales previously unavailable to astronomers. Radio and IR instruments routinely penetrate to depths on 100 Av. Meanwhile, interferometric techniques are resolving angular size to a few milli-arcsecs. These tools put us on the precipice of directly testing theoretical principles of the flow of matter from the molecular cloud through the disk down to the star…and back out again. We focus in this session on stars from just before the onset of Class 0 to the very end of Class II (user-defined).
The gathering and analysis of Milky Way-scale samples of stars is already underway, but the available data will expand dramatically in the next few years with the full Gaia dataset, Rubin/LSST, Euclid, and Roman. What new science will emerge from these datasets? How can these datasets be exploited? What additional data is needed? What tools need to be developed to analyze the data?
We are entering a new era of continuous monitoring of the Sun and other cool stars. Kepler, K2, and TESS have provided new views of stellar activity cycles, seismology, and explosive events on scales previously not accessible. In situ, high spectral resolution, and spatially-resolved data of the Sun and its immediate environment have shown how the temporal regime is necessary to understand the physical processes responsible for its variability, and by extension, the variability of other stars. Many unsolved problems remain in the time-domain, from the origin of activity cycle variability and angular momentum evolution over long timescales, to the physics of transient explosive events such as flares of all sizes, and coronal mass ejections. In this session, we focus on the origin and manifestation of the time-dependent physical processes in cool stars and the Sun, and we invite contributions in both theoretical and observational areas.
This session aims to put cool stars in the context of stellar systems. It includes all aspects of the creation and interactions of cool stars: their fundamental properties, their multiplicity at close and wide separations, their chemical abundances, their magnetic fields, and activity. The session also includes cool stars as hosts of planetary systems and/or disks and their interactions.
Cool Stars 9 in 1995 marked the first definitive detections of both brown dwarfs and exoplanets, with the announcements of Teide 1, GL 229 B, and 51 Pegasi b all occurring in the same year. The past 30 years have led to extraordinary growth in the fields of brown dwarf and exoplanet science. The spectral classes L, T, and Y have been defined and refined. A theoretical framework has been developed, centered on our growing understanding of chemistry and clouds in substellar atmospheres. However, many open fields of research remain. How do atmospheric dynamics shape chemistry, clouds, and weather? How are brown dwarf magnetic fields driven, and to what extent do they differ from cool stars and giant planets? How do brown dwarfs form, and how can we infer their formation mechanisms from atmospheric compositions or other properties? What is the nature of planetary systems around cool stars and brown dwarfs? In this session, we will present a retrospective view of the immense progress made over the past three decades, discuss the wealth of data coming from new facilities such as JWST, and look forward to exciting prospects of future facilities (ELTs, space missions such as the Nancy Roman Space Telescope, and more) in terms of unraveling the mysteries of substellar objects.
In parallel with the tremendous increase in high-resolution observations of young, low-mass protostars using the latest interferometers, significant progress has been made over the last ten years in our understanding of disk formation, outflows and protostars, thanks to multidimensional numerical simulations. Today, numerical experiments run on supercomputers and achieve unprecedented numerical resolution and temporal evolution. Using the results of recent 3D radiation magnetohydrodynamics simulations, I will review our current understanding of the formation and evolution of the protostellar disk, outflow and protostar through the different phases and scales of dense low-mass core collapse. I will show to what extent the results of numerical experiments can address the fundamental problems of angular momentum and magnetic flux conservation. Finally, I will detail the questions still open and give some perspectives on the next generation of numerical experiments.
Protoplanetary disks are the necessary consequence of the gravitational collapse of the dense molecular cloud cores and the subsequent birthplace of planetary systems. Numerous studies have investigated the properties of disks in the more mature Class II stage, either theoretically by numerical simulations from pre-defined initial conditions or observationally by modelling of their dust continuum and line emission from specific molecular tracers, and compared the results from the two standpoints. However, few have evaluated the main limitations at work when determining the embedded Class 0/I disk properties from continuum observations. In this talk, we present our first attempt to assess the accuracy of some critical disk parameters, namely their radii and masses, in Class 0 systems, as derived on actual ALMA observational data, with the corresponding physical parameters accessible to modellers in numerical simulations. To that ends, we followed the approach of performing full post-processing of the numerical simulations and applying on the synthetic observations the same techniques used by observers to obtain the physical properties. We then demonstrate how their sizes and masses vary from the gas kinematics analyses to the dust continuum modelling and provide implications for the possible uncertainties in the modelling of these objects from observations.
Most of our knowledge about how stars form comes from observations of the nearest star-forming regions. These all share similar properties (e.g., mass, density, radiation field, metallicity, etc.), and therefore do not give us representative picture of star formation in our Galaxy or in the Universe. Local environment may play a pivotal role in shaping future stars and their planetary systems. Specifically, massive stars can be particularly destructive to the circumstellar disks hosting young planets due to their strong UV radiation, which can remove matter from the disk. These harsh conditions are characteristic of massive star-forming regions such as the Carina Nebula Complex. Here, we present a spectroscopic study with VLT/MUSE of the population of young, low-mass stars in Carina and their disks by means of accretion. The UV radiation in Carina spans few orders of magnitudes, we assess the impact of environment on accretion properties testing their dependence on the level of the perceived UV radiation. We complement the investigation with measurements of forbidden atomic emission lines tracing disk photoevaporation. We discuss our results in the context of theoretical predictions of external photoevaporation and observational studies of other star-forming sites. Our findings support the scenario of externally evaporated disks and show that with available instruments a detailed spectroscopic study of young stellar populations is possible even in regions at large distances.
While the population of brown dwarfs has been extensively studied in nearby star-forming regions (d < 400 pc), theories of brown dwarf formation suggest that high gas or stellar densities, as well as the presence of massive OB stars, may stimulate the formation of brown dwarfs compared to stars. Therefore, it is imperative to extend the study of the brown dwarf population to massive young clusters, characterized by significantly different star-forming environments than those found in our immediate vicinity. One of such regions is Westerlund 1, located at a distance of 4 kpc and with an estimated mass of 52000 $M_\odot$, it is the closest supermassive star cluster to the Sun and possibly the most massive cluster in the Milky Way. We have recently obtained deep JWST/NIRCAM observations of Westerlund 1 within the EWOCS (Extended Westerlund 1 and 2 Open Clusters Survey) project. One of the primary goals of this project is to derive the mass function of the cluster and investigate whether the formation of brown dwarfs is influenced in such an extreme environment. In this contribution, I will present the NIRCAM products and the first results on the (sub)stellar initial mass function of the cluster, comparing it with recent studies on nearby star-forming regions and other massive young clusters in the Milky Way.
This paper presents a theoretical explanation for the collection of low-mass binary systems (with roughly Jovian masses) that were recently discovered in the Orion Nebula Cluster. In this scenario, binary systems are produced through the collapse of gas parcels with high densities $n\sim10^7$ cm$^{-3}$, cold temperatures $T\sim10$ K, and rapid rotation rates $\Omega\sim30$ km s$^{-1}$ pc$^{-1}$. With these initial conditions, a given parcel resides near the threshold for opacity limited fragmentation and has mass of order $M\sim10M_{\rm Jup}$. The parcel collapses to form a disk-like structure with centrifugal radius $R_C\sim100$ AU, and the disk subsequently fragments into multiple bodies. This paper estimates the conditions required for initial collapse and disk instability, and outlines the parameter space for which binaries are the energetically favored outcome. A suite of numerical simulations is then carried out to illustrate the possible outcomes from disk fragmentation. The initial conditions required to form the observed binary systems lie at the extremes of the expected parameter distributions, but nonetheless can be realized within typical cluster environments.
Deciphering the physical processes occurring in the inner region of protoplanetary disks is key to understand the environmental conditions of (terrestrial) planet formation and evolution. Thanks to the exquisite performance of recent instruments, star-disk interactions can now be probed at sub-astronomical unit scales, providing a unique insight of the place where close-in planets are. Noatbly, with its unique milliarcsecond angular resolution, the near-infrared interferometric instrument GRAVITY of the VLTI has studied the 0.1–5AU central region in a homogeneous sample of a hundred protoplanetary disks around T Tauri, Herbig Ae-Be and high-mass young stars. This allows us to look for trends with the properties of the central star and the disk morphology within a statistical approach, and to investigate the connection with the well-known disk structures at larger scales within a multi-wavelength and multi-technique approach. Owing its high spectral resolution in the K-band, GRAVITY has spatially resolved in a sample of T Tauri stars the Br_gamma emitting regions associated to hot hydrogen gas, which allowed us to differentiate between possible star-disk interaction mechanisms, and to directly monitor the magnetospheric accretion flows. The potential yield of the new GRAVITY+ upgrade, in particular towards the study of early Class I phases of YSOs in various star forming regions, will be discussed, as well as the possible legacy of GRAVITY(+) observations to refine theoretical physico-chemical models of disks.
Some Classical T Tauri Stars (CTTS) are strong accretors ($\dot{M}_{acc}\sim$ a few $10^{-7} M_\odot/yr$) , offering a unique opportunity to study magnetospheric accretion in a sustained regime ($\sim$100 times larger than in other CTTS). We present unique results from multi-technique observations of the strongly accreting CTTS S CrA N, offering a glimpse into the potential of the upcoming VLTI/GRAVITY+. This upgraded instrument will enhance sensitivity, making most CTTS accessible for near-infrared interferometry with a spatial resolution of a few stellar radii. Our observation of S CrA N, combining GRAVITY and CFHT/ESPaDOnS, unveils its dusty disk's sublimation front and magnetic topology, making possible a monitoring of accretion flows at a 1-hour cadence. GRAVITY+ promises to extend this capability to a larger CTTS sample, opening a statistical analyses era and enhancing comprehension of magnetospheric accretion. Combining GRAVITY+ with spectro-polarimeters like ESPaDOnS and SPIRou could broaden investigations to younger, more embedded sources, marking a milestone in near-infrared interferometry.
Classical T Tauri stars coevolve with their surrounding protoplanetary disks, with the two being connected by the stellar magnetic field. Magnetospheric accretion of disk material onto the star is a primary driver of the system’s evolution. It dictates the timescale of planet formation by removing disk material over time; it drives angular momentum transport; and it produces an accretion shock that irradiates the disk with NUV-dominated emission. This NUV continuum emission is observable only from space, and ground-based observables are less robust tracers of accretion. Therefore, we have undertaken the largest NUV accretion study of T Tauri stars using the Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) DDT Program as part of the ODYSSEUS and PENELLOPE collaborations. In this talk, I will present our results from consistently measuring the stellar and accretion properties of $\sim 60$ T Tauri stars using an accretion shock model. We find that ground-based studies systematically underestimate accretion rates by a factor of $\sim 3$ for a given T Tauri star. Further, on a population level, accretion rates are systematically underestimated for a given stellar mass. We propose a new $M_\star - \dot{M}_{\rm acc}$ relation based on our NUV accretion rate measurements, which increases the median $\dot{M}_{\rm acc}$ by a factor of 5-6. We will discuss the implications of our work for models of disk dispersal and planet formation.
Improving stellar structure and evolution models is key to interpreting the high-quality stellar data produced by space missions like Kepler, TESS, and PLATO. We use the multi-dimensional, time implicit, fully compressible, hydrodynamic large eddy simulation code MUSIC (Multi-dimensional Stellar Implicit Code) to study how a star's mass affects the properties of stellar convection and overshooting. Stellar evolution models typically set the amount of overshooting using a fixed percentage of the pressure scale height. By comparing global simulations of convection in stars that have different masses, but identical evolutionary stages, we demonstrate that overshooting beneath a convective envelope is dependent on mass. This result extends the findings of recent observational (Claret and Torres 2018) and numerical (Baraffe et al 2023) studies that overshooting above a convective core is mass dependent. We also establish the mass dependence of other parameters that describe stellar convection, including the filling factor and the plume interaction parameter.
Stellar age is a key fundamental property for understanding the habitability, evolution, and formation of exoplanets. In order to estimate precise ages, we also need to understand the evolution of more fundamental properties such as mass, radius and effective temperature. However, current theoretical models over-predict effective temperatures, and under-predict radii, compared to observations of M dwarfs (radius inflation problem), which are likely to host Earth-like exoplanets. In this talk I will present a model independent method to estimate stellar radii and how we used it to study the problem of radius inflation. We calibrated the Gaia surface brightness-color relation (SBCR) for low-mass stars which we combined with Gaia DR3 parallaxes to estimate radii. We found that radius inflation is correlated to magnetic activity for single stars, and we calibrated the percentage of radius inflation as a function of Hα emission and mass. This correlation could explain the difference between models and observations for M dwarf radii, and get us a step closer to understand M dwarf evolution.
The nearest stars provide a fundamental constraint for our understanding of stellar physics and the Galaxy. The nearby sample serves as an anchor where all objects can be seen and understood with precise data. It provides benchmark stars and brown dwarfs that can be used to define calibration samples, as well as targets for focused planetary searches. The space mission Gaia and its unprecedented high precision parallax measurements gave the opportunity to refine the nearby census. The space mission Euclid and its large and deep near-infrared survey is poised to dramatically increase the census of the lowest mass stars and brown dwarfs in the Solar vicinity. I will review the recent efforts made to map the solar neighbourhood, to study the demographics of stars and substellar objects in our Galaxy.
We present results from a six-year campaign to gather multi-epoch, high-resolution spectra of an all-sky, volume-complete sample of 413 M dwarfs with masses 10-30% that of the Sun that lie within 15 parsecs. We report weighted mean systemic radial velocities and rotational broadening measurements ($v \sin i$) for our targets. Our typical intra-star RV uncertainties are less than $30$ m s$^{-1}$ for the effectively single, slowly rotating targets in our sample, and are less than 1 km s$^{-1}$ for more rapidly rotating stars. The majority of the presumed single stars in our sample ($71\pm3$%) do not exhibit rotational broadening above our detection limit. Our radial velocities, combined with precise astrometric data, allow us to calculate Galactic space motions, with which we calculate thin and thick disk membership probabilities. We determine that the majority of our full sample (81%) are highly probable thin disk members. We report new multi-lined systems and identify additional targets with velocity variations indicative of long-period companions. Finally, we calculate multiplicity rates for our sample and find no significant difference between the stellar multiplicity rates of the thin disk and thick disk populations in our sample. Our survey more than triples the number of these fully-convective stars with both high-resolution spectroscopic and astrometric data.
A star and its planets are born from a single cloud of gas and dust, so the chemical compositions of planet host stars encode information on the processes that govern planet formation. Measuring the compositions of M dwarf planet hosts is particularly important because the small sizes and masses of M dwarfs make them ideal for exoplanet discoveries. M dwarfs are also common, comprising ~70% of the stars in our galaxy. However, constraining M dwarf chemistry is a long-standing challenge; M dwarfs are cool enough for molecules to form in their atmospheres, which create complex spectral features that physical models struggle to reproduce. To solve this problem, I constructed the first-ever fully automated model for measuring M dwarf abundances across many elements with The Cannon, a data-driven framework. I will apply this model to Transiting Exoplanet Survey Satellite (TESS) M dwarf planet hosts with spectroscopic observations from the Sloan Digital Sky Survey (SDSS-V). This will enable us to explore connections between planet properties (e.g., occurrence rates, architectures) and M dwarf host chemistry, and thus shed light on dominant planet formation pathways around the most common stars.
In this talk, I present the determination of basic atmospheric parameters and chemical abundances in 1600+ main-sequence stars within 100 pc of the solar neighborhood. We used APOGEE-2 infrared spectra and a specially developed spectral fitting code called tonalli. We then used those atmospheric parameters in combination with a list of 35 atomic lines and the BACCHUS code to determine the abundance of Mg, Al, Si, Ca, and Fe in our sample. Our results indicate that atmospheric parameters determined with tonalli agree with previous determinations like those made by ASPCAP or Gaia DR3 for G, K, and M type stars ($T_{eff}$ between 6000 and $\sim$3200 K). tonalli allows us to obtain atmospheric parameters primarily from the stellar spectra without introducing a posteriori calibration. We established an infrared color-temperature relation that can be used to determine the temperature of any main-sequence star, a great alternative to characterize large stellar samples. Additionally, the characterization we made for our sample is a valuable resource for training neuronal networks or machine learning codes. We have applied our methods to a sample of more than 500 young stars in four groups (Orion A, B, OB1, and $\lambda$ Ori) of the Orion complex. Our initial findings show no significant differences in the chemical abundances of stars from different Orion groups, suggesting that the Orion complex is chemically homogeneous in the five elements examined in this study.
Detached eclipsing binaries are the most precise method to measure fundamental stellar parameters. Starting from binary star candidates identified by the All-Sky Automated Survey for Supernovae (ASAS-SN), we use PHOEBE to determine the sum of the fractional radii, the ratio of effective temperatures, the inclinations, and the eccentricities for more than 35,000 detached eclipsing binaries. By combining our results with Gaia distances and 3-dimensional dust maps, we examine the properties of the systems as a function of their absolute magnitude and evolutionary state. We visually inspect all the light curves to verify the model fits and use TESS light curves to identify and characterize 766 binaries with evidence for “extra physics”, such as spots, pulsations, and doubly-eclipsing binaries. Finally, we use radial velocities to measure masses and radii of eclipsing red giants, which are vastly underrepresented in current catalogs. We start with Gaia DR3 spectroscopic orbits and measure the masses of 61 binaries, including 12 red giants. We are performing dedicated spectroscopic follow-up for an additional 20 eclipsing red giants.
I will present some recent development in the study of coeval populations of stars in our neighborhood of the Galaxy, most of which were made possible by the advent of the Gaia mission. I will demonstrate the newly released Montreal Open Clusters and Associations (MOCA) database, a new resource for the community that includes a large number of observables and calculated properties for approximately 500,000 stars in 5,000 nearby associations and open clusters, comprising many stellar associations that were only recently discovered. I will also go over some of the online tools that we made available to the public at www.mocadb.ca, allowing users to explore the database in an interactive manner, or to connect directly via simple Python scripting, and will outline some of the current projects that rely on MOCAdb to further the census of nearby young stars, planemos and age-calibrated exoplanets. I will end with future progress on these topics that will be made possible with future missions such as the final Gaia data releases, the Rubin observatory, the Roman space telescope and the Euclid mission.
Young associations record star formation histories spanning tens of millions of years, revealing the initiation, progression, and termination of star formation long after the dispersal of the natal cloud. Through the SPYGLASS program, I am expanding this record by mapping the extensive and often poorly characterized network of clusters and associations in the solar neighborhood. Our most recent survey update reveals 116 young associations within 1 kpc in Gaia DR3, providing a powerful resource for studies of large-scale population statistics as well as star formation patterns on both local and spiral arm scales. I have already spectroscopically observed members of over a dozen young associations in this sample, providing radial velocities and youth indicators that can reconstruct entire star formation histories through age measurements and kinematic traceback. Results in two young associations have already revealed distinct nodes in which co-spatial star formation takes place, which may represent the clearest discrete unit of star formation. I am currently reconstructing star formation patterns in the much larger associations of Cep-Her and Sco-Cen, which are bridging the gap between local star formation and the patterns that drive star formation on galactic scales. Combining large-scale surveys with regional reconstructions of star formation, I am revealing processes guiding star formation ranging from local association scales to the scale of galactic structure.
We present a novel approach for empirically calibrating synthetic spectra of main-sequence stars and apply it to extensive survey databases for investigating Galactic stellar and dust structures. Our method involves comparing synthetic spectra with observations of cluster sequences and individual stars with spectroscopic data, providing a first-order correction to theoretical models. By applying these calibrated models, we derive metallicities of stars from various photometric databases and construct chemo-dynamical distributions of Galactic stars by combining our data with Gaia astrometry. In particular, they unveil a remarkably narrow sequence in metallicity versus orbital rotational velocity space for high proper-motion stars. This observation lends support to a scenario suggesting a starburst during the Milky Way's encounter with the progenitor of Gaia Sausage/Enceladus, which eventually led to the deposition of metal-enriched gas onto the disk. In addition, we construct an all-sky three-dimensional extinction map from the largest collection of low-resolution spectra in Gaia DR3 by determining both the reddening and metallicity of main-sequence stars along each line-of-sight, which extends up to 3 kpc from the Sun. We discuss the implications of our technique in relation to future large photometric surveys, particularly considering its potential contributions to upcoming endeavors such as the LSST.
Recent space-based missions have ushered in a new era of observational astronomy, where high cadence photometric light curves for millions of stars in the solar neighborhood can improve our understanding of the Milky Way’s dynamical history. One open problem in Galactic dynamics is understanding the timescale on which the Galaxy’s spiral arms pass through the Solar Neighborhood. Simulations have shown that spiral arms can leave signatures, or “wrinkles”, in kinematic-space, which are now observationally confirmed with high precision kinematic data from the Gaia mission. However, kinematics alone is not sufficient to disentangle the history of spiral structure. Using photometry from TESS and gyrochronology age-rotation relations, we are calculating stellar age distributions in wrinkles to place a timestamp on transient spiral arm passages. We have built a series of TESS rotation period measurement and validation pipelines that were tested on 1200+ stars with average recovery rates of 88% (Rampalli+2023). We apply these pipelines to measure periods for and determine gyro ages for ~34,000 TESS stars in non-wrinkle and wrinkle space. We pair gyrochronologically-determined ages with Ca II activity measurements from Gaia and Li measurements from Galah to ascertain youth of rapidly rotating stars. We present an age distribution of stars that place constraints on when spiral arm passages occurred. Early evidence suggests these passages could have happened as recently as 120 Myr ago.
Fundamental parameters of low-mass stars (temperature, mass, metallicity) can potentially be determined by their location on an HR diagram with sufficiently accurate photometry and parallaxes. This is, however, complicated by the fact that 20-40 percent of low-mass stars are predicted to be unresolved binaries and appear more luminous compared to single stars with comparable fundamental parameters. We present a method to separate out the binary stars from the single-star main sequence K and M dwarfs with their Gaia DR3 XP spectra. Using Gaia DR3, we create a sample of stars with pristine astrometry and photometry composed of single stars and equal mass binaries within 100 parsecs. We then iteratively train Random Forest Regression (RFR) models to predict absolute magnitude and color given a star’s RP spectral coefficients. After each model, we remove the stars which are predicted to be significantly dimmer than their measured absolute magnitude from training future models. This method converges on an RFR model trained only on the single stars within the 100 parsec sample. We then use this model to predict the absolute magnitudes of the full 200 parsec sample of K and M dwarfs and find that 29 percent of the sample is significantly more luminous than our prediction. This method provides a novel approach at breaking the multiplicity-metallicity degeneracy of the lower main sequence and can be a useful tool for wide-field surveys to predict the multiplicity of a given source.
Abstract TBD
The diverse rotational velocities of solar-type stars at birth, and their evolution, present a compelling puzzle. Traditional models based on polytropic stellar winds have fallen short in explaining the observed transition from a broad range of initial rotational speeds to the well-documented Skumanich spin-down phase, without resorting to arbitrary limits on the wind mass-loss rate. To shed light on this issue, we carried out a set of Alfvén wave powered wind simulations covering the Sun and twenty-seven open cluster stars, aged between 0.04 to 0.6 Gyr, all of which are characterized by observationally derived surface magnetic maps and rotation rates. The results are revealing: we observe torques and mass loss rates compatible with an exponential spin-down among young, fast-rotating stars, transitioning to the Skumanich spin-down in their older, slower counterparts. Remarkably, this bifurcation in spin-down behaviour emerges naturally from our model, without the need for predefined limits on wind mass loss. The findings of this study suggest that the observed distribution of stellar rotation rates may be a direct consequence of magnetic field strength saturation in faster rotators. Furthermore, our work offers new insights into the interplay between stellar magnetic fields and wind dynamics, enhancing our understanding of the processes driving stellar spin-down. This research charts a course for future explorations into the magnetic mechanisms influencing stellar rotation.
The 22-yr-long solar magnetic cycle consists of two consecutive 11-yr sunspot cycles, and exhibits a polarity reversal at sunspot maximum. The large scale magnetic field is complex at cycle maximum, and assumes a more simple, dipolar geometry at minimum. Although solar dynamo theories have progressively become more sophisticated, the details as to how the dynamo operates and sustains magnetic fields are still subject of research. In this context, observing the magnetic field evolution of Sun-like stars advances our understanding on how key dynamo ingredients, like stellar mass and rotation, influence internal dynamo processes. In this talk, I present the long-term spectropolarimetric monitoring of six Sun-like stars performed within the BCool programme. The masses of our stars are at most 6 percent larger than solar and the rotation periods span 3.5-21 d, so it is a practical sample to study magnetic evolution for Sun-like interiors and distinct activity levels. We applied Zeeman-Doppler imaging to map the large-scale stellar magnetic field from circular polarisation spectra collected with ESPaDOnS and Narval over 10-15 yr. We found that our solar analogs exhibit a dipolar field with clear polarity switches over 3-11 yr, while our fast-rotating stars have a complex, temporally stable field topology, with oscillations in field strength. These results emphasise the sensitivity of magnetic cycles to stellar fundamental parameters, specifically the stellar rotation period.
I will present a new catalogue of ~800 radio stars detected at <2 GHz: the Sydney Radio Star Catalogue (SRSC). Approximately half of the stars in the catalogue have multiple radio detections, revealing information about the variability of radio stars. Many of the stars are K and M dwarfs, as well as some ultra-cool dwarfs. The previous best-known radio star catalogue, the Wendker catalogue, contained approximately 200 radio stars detected at <2 GHz. This means that the population, properties and variability of radio stars are not well known. If a star is detected in the radio this provides information about the chromosphere, the magnetic fields and potential impact on orbiting exoplanets. Stellar radio variability tells us more about these properties, as well as rotational velocities, different emission mechanisms, and activity cycles. For example, the Variables and Slow Transients (VAST) survey with the Australian SKA Pathfinder (ASKAP) has been observing Galactic Fields every few weeks for more than a year and will continue to do so for three more years. VAST provides unprecedented monitoring for stars in those fields. I will present properties of the population of radio stars in the SRSC and the light curve variability of individual cool stars. We are excited to share the potential of radio observations to reveal properties of the radio star population and to study individual objects.
This presentation introduces a novel method to measure stellar activity through precise differential temperature measurement. The method is inspired by precision radial velocity algorithms, used in the temperature space; they lead to a sub-Kelvin accuracy, using the correlated changes in fractional depth of all spectroscopic features in a given spectrum. The technique is based on a library of spectra with known temperatures, enabling the derivation of temperature changes and uncertainties for each spectral line and a combined differential temperature accuracy that is unprecedented and opens a new tool for stellar physics. We demonstrate the power of this technique by retrieving rotation periods of inactive stars (e.g., Barnard star), in retrieving the starspot evolution in moderately active stars (e.g., Proxima, Epsilon Eridani) as well as very young objects (e.g., AU Mic). We also showcase the detection of an equivalent to the detection of the Rossiter effect in temperature space, where a transiting planet changes the disk-averaged temperature of the host star, providing a unique constraint on the geometry of exoplanet systems.
The radial velocity (RV) scatter that arises due to time-varying inhomogeneities on stellar surfaces is now the largest source of error for the dynamical detection and characterization of exoplanets. As such, a host of different mitigation methods exist with the goal of disentangling stellar signals from spectral shifts due to orbiting planets. I will present contemporaneous Sun-as-a-star observations taken by HARPS, HARPS-N, EXPRES, and NEID and discuss the potential and limitations of this combined data set for testing mitigation methods. I will give an overview of current mitigation methods employed as part of the Extreme Stellar Signals Project (ESSP), an international research network of scientists developing such methods. I will conclude with a framework showing the existence of a low-dimensional latent space that can capture relevant stellar surface variation in the spectral domain.
Space-based photometric surveys have shown that one to a few percent of cool stars younger than 200 million years exhibit complex, but highly structured and periodic optical light curves. The most likely interpretation for these ``complex periodic variables'', after correcting for the line-of-sight viewing angle, is that up to a quarter of young cool stars host transient circumstellar clumps of material that can be magnetically confined to corotate with the star for hundreds of rotation cycles. The composition of this material is not currently known: dust would suggest an extrinsic origin (e.g., debris from planetesimal collisions), while partially-ionized gas could suggest an intrinsic origin (e.g., a stellar wind). After summarizing the existing body of evidence, I will highlight how a new census using TESS has yielded the brightest and closest complex periodic variables known. We have acquired simultaneous time-series optical spectroscopy and photometry of these new objects, and we are finding that the circumstellar clumps of material contain significant quantities of hydrogen. This rules out ``dust-only'' scenarios, and implies either a purely stellar origin for the phenomenon, or a mixed-composition scenario in which both gas and dust are present.
Coronal mass ejections (CMEs) are a dominant contributor to space weather in the solar system, with the potential to catastrophically erode planetary atmospheres. Traditional stellar activity probes, such as flares, cannot indicate if a CME is present. A characteristic radio burst (called type-II burst) is an unambiguous CME signature but there have been no such bursts detected due to a lack of sensitivity and time on sky. I will show that by using the solar flare-CME relationship, we should be able to find >50 type-II bursts within the Low-Frequency Array Two-Metre Sky Survey. I will present a progress report of our ambitious project to search over 12 petabytes of data to identify extrasolar CMEs. In particular, I will present our discovery of the first extrasolar type II burst, which contains the first unambiguous CME signatures, detected by the Low-Frequency Array. Lastly, I will end with presenting plans for a more ambitious search to discover extrasolar (so-called) type-III bursts that trace energetic particle events.
A recent investigation of the highest amplitude infrared variable stars in the Milky Way discovered an unexpected new population of giant stars that show multi-year dips of several magnitudes in their light curves. Intriguingly, the stars are strongly clustered in the Nuclear Disc of the Milky Way, which co-locates with the Central Molecular Zone (CMZ). These dips seen in the 10 year VVV light curves are asymmetric, showing somewhat disparate profiles and at present there is no evidence that they are periodic events. Spectra suggest that these stars have super-solar metallicities which would be consistent with the Nuclear Disc location. Following the initial discovery of 21 dipping giants, further searches of the VVV data have found many more examples with lower amplitudes, tracing out a spatial structure slightly offset from the Galactic Centre, somewhat resembling the structure of the CMZ. The stars appear likely to be on the Asymptotic Giant Branch but they do not pulsate. The colour-magnitude behaviour indicates extinction by sub-micron grains but Spitzer photometry rules out any large envelope of cool circumstellar matter. We place these discoveries in the context of past work and discuss two possible explanations for the phenomenon: (i) dust puffs in the line of sight, similar to R Cor Bor stars and (ii) long period eclipses by circumstellar matter associated with a companion.
Gyrochronology, the field of age-dating stars using mainly their rotation periods and masses, is ideal for inferring the ages of individual main-sequence stars. However, modern gyrochronology relies heavily on empirical calibrations, and the lack of old benchmark ages for main-sequence stars means no gyrochronology models have been calibrated for stars older than 4 Gyr. In this talk, we present the first empirical gyrochronology relation capable of inferring ages for single, main-sequence stars between 0.67 Gyr to 14 Gyr, using a Gaussian Process model conditioned on kinematic ages (∼ 1 - 14 Gyr) and known clusters ages (0.67 - 3.8 Gyr). I will show this model is self-consistent with wide-binary pairs and has an average uncertainty of just over 1 Gyr with asteroseismic ages. With this model, I will showcase the largest gyrochronology age sample for ∼ 100,000 stars within 1.5 kpc of the Sun with period measurements from Kepler and ZTF, and 353 unique planet host stars.
This presentation delves into the intricate task of unraveling the stellar properties of M dwarfs, shedding light on both the current achievements and the future pathways in this challenging endeavor. The pursuit of exoplanets orbiting M dwarfs has ignited a renewed interest in their stellar characteristics, propelling advancements in observational techniques and theoretical models. We navigate through various methodologies employed to extract fundamental parameters of these low-mass stars, with particular emphasis on the pivotal role played by binary/multiple systems and benchmark samples in understanding their intrinsic nature. Despite the significant progress made, challenges persist, notably in reconciling discrepancies between methodologies. However, it is precisely these challenges that motivate our collective efforts to overcome them in the near future. Interdisciplinary collaborations persist in propelling us towards a holistic understanding of M dwarfs and their roles as stellar hosts. This journey not only enriches our comprehension of individual stars but also deepens our understanding of planetary systems and our Galaxy at large.
Although we are accustomed to considering binary star systems in terms of their immediately observable properties such as separation, their full orbits reveal a much richer picture. This context is particularly difficult to achieve for the lowest-mass stars because mapping their orbits takes more time for a given semi-major axis. We present our multi-method survey -- astrometric, visual, and spectroscopic -- of $\sim$200 M dwarf multiples in the solar neighborhood, from which we compare the orbits of systems with respect to their masses, mass ratios, and orbital periods. With these orbits that span 1 day to 30 years, our goal is to contextualize these systems' dynamics -- to reveal their behavior as a population. We find that orbits longer than a few years tend to be more eccentric if the stars are more massive, indicating a mass-dependent difference in the environments that drove these systems' early dynamical evolution. Along the same lines, M dwarfs' orbits are less eccentric than those of FGK binaries. The eccentricity distribution does not depend strongly on mass ratio or whether or not a given pair has a third companion. These orbits also constrain the tidal circularization period for M dwarf pairs at $\sim$7 days, shorter than that of FGK binaries. In this talk we will expand on these results in the context of the completeness of the parameter space thus far, and in particular the questions that remain open around lowest-mass stars and their lower-mass companions.
As binarity is a direct outcome of formation, studying multiplicity across all ranges of masses and separations is key to fully understand stellar formation mechanisms. Here, we present results from a JWST/NIRCam+NIRISS campaign aimed at investigating the multiplicity of the coldest and least massive objects produced by star-forming processes. We searched for close binary companions to 22 nearby Y-type brown dwarfs, all cooler than 500 K, that represent the bottom end of the observed Initial Mass Function in the Solar neighborhood. One binary system, WISE 0336 AB, was discovered in our search and represents the first Y+Y binary system. With an estimated temperature of smaller than 300 K, the companion bridges the gap between the coldest known brown dwarf, WISE 0855, and the rest of the Y-type population. Thanks to JWST’s exceptional infrared sensitivity to extremely cold objects, and its unmatched spatial resolution at these wavelengths, we placed strong constraints on the multiplicity outputs of star formation inside unexplored regions of the parameter space, allowing to test whether the trends in companion occurrence rate, separation and mass ratio distribution seen for more massive objects extend down to the very lowest masses and temperatures.
Small planets ($R_{p} < 4 R_{\oplus}$) are divided into rocky super-Earths and gaseous sub-Neptunes separated by a radius gap, but the mechanisms that produce these distinct planet populations remain unclear. Binary stars are the only main-sequence systems with an observable record of the protoplanetary disk lifetime and mass reservoir, and the demographics of planets in binaries may provide insights into planet formation and subsequent evolution. To investigate the radius distribution of planets in binary star systems, we observed 190 binary systems hosting 264 confirmed and candidate transiting planets detected by the Kepler mission and recharacterized the planets while accounting for the observational biases introduced by the secondary star. We found that the population of planets in close binaries ($\rho \leq 100$ au) is significantly different from the planet population in wider binaries ($\rho \geq 300$ au) or single stars. In contrast to planets around single stars, planets in close binaries appear to have a unimodal radius distribution with a peak near the expected super-Earth peak of $R_{p} \sim 1.3 R_{\oplus}$ and a suppressed population of sub-Neptunes. We conclude that we are observing the direct impact of a reduced disk lifetime, smaller mass reservoir, and possible altered distribution of solids reducing the sub-Neptune formation efficiency.
Exoplanetary magnetic fields play an important role in modulating atmospheric mass-loss and potentially shielding the atmosphere from the impact of energetic particles in the stellar wind. The planetary fields are an important ingredient in assessing the prospects for habitability; however, there remains few empirical constraints on exoplanetary magnetic properties. The recent detections of radio emission from the nearby exoplanet host, YZ Ceti, suggest that the star is possibly interacting with its rocky innermost planet. These radio emissions are characterized by strong circular polarization, and appear to repeat within consistent orbital phase windows dictated by the orbital position of YZ Ceti b. The strength of the radio bursts are further consistent with theoretical predictions of the star-planet interaction. We will discuss our ongoing efforts to characterize this system and test potential alternative interpretations of the radio emissions. Our efforts in fully characterizing the magnetic properties of the star with spectropolarimetry will let us transform the radio detections into bona fide constraints on an exoplanet magnetic field. Understanding the interactions of stars with planets requires an interdisciplinary systems approach. The advent of radio star-planet interactions illustrates how radio observatories will make significant advances at this vanguard of understanding planet magnetic fields, habitability, and the different kinds of extrasolar magnetospheres.
Magnetism is among the most relevant and exciting physical mechanisms in the Sun and cool stars. Its ramifications are ubiquitous, yet its observational characterization and modeling pose significant challenges. The talk will provide an overview about our current understanding of magnetic field generation and magnetic activity in Sun-like and low-mass stars. Special focus will be placed on delineating the interplay between various magnetic field and activity indicators that can be useful for understanding magnetism and characterizing activity-induced stellar variability.
The relationship between magnetic activity and Rossby number---the rotation period divided by the ``convective turnover time'' is well-established for low mass stars (roughly F-type and later). However, in present stellar evolution models convection is largely a simplified theoretical phenomenon rather than an observational one, and our understanding of both stellar convection and magnetic dynamos is still incomplete. In the Rossby-activity relation, it is generally assumed that the convective turnover time should be calculated near the base of the convective envelope in low mass stars, which in turn is based on the assumption that the dynamo is situated there as well. Using 1D stellar evolution models, we have re-calibrated the convective turnover time vs stellar mass relationship. Taking the Rossby-activity relationship as ground truth, we use measured stellar X-ray luminosities and rotation periods to derive empirical convective turnover times. By comparison with model turnover times as a function of depth in the stellar envelope, we can locate the (implied) predominant region of dynamo activity within the envelope. I will present results that suggest that the dynamo does not lie at base of the convection zone (or tachocline) for all low mass stars. I will also discuss the convective turnover times themselves, and in particular the problematic stellar masses around the fully-convective limit.
Understanding stellar magnetic fields is crucial to understanding the structure and evolution of stars. While there are ways to observe the surface magnetic field of a star, it is more difficult to detect the structure of the magnetic field above the surface in the corona. In this talk I present the results of our multiwavelength study of the young (~40-50 Myr) solar-like star AB Doradus, where we find that it has an extended corona of ~8-10 stellar radii. I describe how we use the surface magnetic field map derived from spectropolarimetry to produce a 3D extrapolation of AB Dor's coronal magnetic field, from which we create synthetic radio images. I discuss how we compare our synthetic images with approximately simultaneous radio interferometry observations that show complex structures around AB Dor. I show that we are able to reproduce the observed radio flux and the morphology of the derived radio images, and discuss the implication of such a large coronal extent in young stars.
Stellar winds govern the lives of stellar systems, from dictating the evolution of the star itself to eroding the atmospheres of exoplanets. The impact of the wind on a stellar system is largely determined by the mass-loss rate -- which is notoriously difficult to measure on dwarf stars since the wind is so tenuous. Currently, mass-loss rates of cool stars have to be modelled or inferred indirectly, for example from astrospheric Ly$\alpha$ absorption. In this talk, I will present a more direct method to constrain the mass-loss rate of a star using detections of low-frequency coherent radio emission, exploiting the lack of free-free absorption to place upper limits on the stellar mass-loss rate. We apply this method to M dwarfs detected with LOFAR at 120 MHz and find upper limits down to 4 times the solar mass-loss rate, independent of distance. While these limits are already competitive with other methods, we expect to reach upper limits of less than the solar mass-loss rate in the near future.
Ultraviolet (UV) emission from stellar flares plays a crucial role in determining the habitability of exoplanetary systems. Flare UV emission may drive prebiotic chemistry, erode planetary atmospheres, or produce false biosignatures. In this talk, I will present the first statistical analysis of stellar flares with simultaneous observations in NUV and FUV. We identify 182 flares on 158 stars within 100 parsec of the Sun in both the near-ultraviolet (NUV: 1750-2750 \AA) and far-ultraviolet (FUV: 1350-1750 \AA) using high-cadence light curves from the Galaxy Evolution Explorer (GALEX). We show for the first time that uniformly selected stellar flares are far-ultraviolet luminous. We explore tentative trends of FUV/NUV energy ratio with respect to spectral type and total UV flare energy. We find that widely used assumptions for flare emission significantly underestimate the levels of FUV emission we observe. We construct empirical models for flare light curves in the NUV and FUV, and compare to the classical optical template for flares. I will discuss the implications for photochemical modeling of stellar activity and planetary atmospheric chemistry, the search for accurate biomarkers, and future missions to observe stellar flares in the ultraviolet.
The last three decades have seen the field of brown dwarfs grow from the discovery of the first examples by a relatively small group of active investigators to a vibrant area of research with >1000 examples being studied via scores of different groups. Once a unicorn, the brown dwarf has proven to be far more than just an inert by-product of star formation. Instead, it has much to inform us about the creation of low-mass star-like products, the growth of dust/grains in cold atmospheres, the broader interpretation of results from exoplanet studies, and much more. In this talk, I will provide a brief retrospective on the search for the first brown dwarfs, then rapidly pivot to a summary of our current understanding of their properties. I will also discuss pros and cons of the two definitions of brown dwarfs, along with an admonishment (somewhat self directed) for referring to such objects as "failed stars". Finally, I'll outline the major outstanding questions in the field and highlight a few current and upcoming surveys that should help us provide the answers.
Accurate evolution models are critical for tying observable properties (e.g., luminosity) of substellar objects to key properties like mass. The thermal evolution of brown dwarfs and planets is regulated by their atmospheres, which allow their heat of formation (and from the brief era of energy generation from deuterium fusion) to radiate to space for billions of years. We present a new generation of atmosphere and evolution models which include the effects of clouds for warm substellar objects. We show how silicate (plus iron and corundum) clouds change the spectra of exoplanets and brown dwarfs in objects from 900--2400 K at a range of surface gravities. We include, for the first time in our cloudy modeling framework, three metallicities including super-solar (+0.5, similar to Jupiter) and sub-solar (-0.5) and demonstrate that these can change the evolution models substantially at some ages. We show how the emergence and disappearance of clouds affects the evolution of planets, creating a new set of "hybrid" evolution models applicable for giant planets and brown dwarfs. Our models have key upgrades from prior generations, including updated chemistry and opacities, and we present medium-resolution spectra (applicable for JWST, and other, applications) and high-resolution spectra appropriate for echelle spectroscopy from the ground.
We present a metallicity classification system for T dwarfs based on previously identified metal-poor brown dwarfs and new discoveries made by the Backyard Worlds: Planet 9 project. We define three metallicity classes in the near-infrared to augment the current dwarf sequence: mild subdwarfs (d/sdT), subdwarfs (sdT), and extreme subdwarfs (esdT), with sets of spectral standards that define individual subtypes. We demonstrate how this sequence encodes both temperature and metallicity variations among the currently known sample of metal-poor brown dwarfs, and define a metallicity index analogous to the $\zeta$ index for M subdwarfs to quantify metallicity classification. We also use spectral model fits to infer metallicities for these classes, benchmarked to companions of stars with known compositions. This sequence is a necessary advance as more metal-poor brown dwarfs in the thick disk, halo, and globular clusters are identified in deep infrared imaging and spectroscopic fields by JWST, Euclid, and the Nancy Grace Roman Space Telescope.
Gl 229 B, the first T dwarf discovered, orbits a M1V star at 33 AU and has a dynamical mass of 71.4±0.6 MJup and Teff~850 K. From its bolometric luminosity, substellar evolutionary models predict masses 10σ lower than the dynamical mass, making Gl 229 B the touchstone of an emerging “over-massive” brown dwarf problem. Moreover, atmospheric retrievals using ground-based spectra have reported C/O>1 for the object, considerably higher than the C/O of Gl 229 A. We seek to unveil the mysteries behind Gl 229 B with a multi-instrument campaign. It is possible that Gl 229 “B” is an unresolved binary pair of brown dwarfs, which could explain its low luminosity and anomalous C/O measurements. To this end, we present VLTI/GRAVITY and CRIRES+ results that tentatively reveal Gl 229 B as a nearly equal-mass brown dwarf binary with a tight orbital separation of ~0.03 AU (~70 RJup). Armed with knowledge of the binary properties, we showcase atmospheric retrievals of JWST MIRI low-resolution spectrum (5-14 microns) for Gl 229 B, which contains absorption features from CO, CH4, H2O, and NH3. We fit the JWST spectrum as both a single source and a binary source to compare the quality of fit. From the JWST spectrum, we also provide an updated bolometric luminosity and quantify the vertical mixing efficiency in the brown dwarfs’ atmospheres by constraining the extent of carbon and nitrogen disequilibrium chemistry. Finally, we report revised C, O, and N elemental abundances for Gl 229 “B”.
In recent observations of the Trapezium Cluster in the Orion Nebula with the JWST, we have discovered and characterised a sample of over 500 planetary-mass candidates with masses down to 0.6 Jupiter masses. In an unexpected twist we find that a significant population of these planetary-mass objects are in wide binaries. The binary fraction of stars and brown dwarfs is well known to decrease monotonically with decreasing mass such that the binary fraction for the planetary-mass regime is expected to approach zero. The existence of substantial population of Jupiter Mass Binary Objects (JuMBOs) raises serious questions of our understanding of both star and planet formation. In this talk I will present the discovery of these JuMBOs, the 500+ free-floating planetary-mass candidates, and discuss the implications for our understanding of star and planet formation.
Since the discovery of brown dwarfs 30 years ago, gaining an in-depth understanding of their atmospheres has emerged as a major challenge for the field. Concurrently, the complementary field of exoplanets has highlighted the similarities between brown dwarfs and the small but growing sample of directly-imaged exoplanets. Without a bright host star, isolated brown dwarfs serve as critical analogs for bona fide giant exoplanets. Studies to date have revealed the complex nature of brown dwarf and giant planet atmospheres. In particular, atmospheric phenomena such as condensate clouds, atmospheric dynamics and aurorae play a dominant role in shaping the appearance of extrasolar atmospheres. In this talk I will describe recent and ongoing efforts to reveal the physics of these three critical atmospheric phenomena as well as prospects for future work with next-generation facilities.
We present state-of-the-art very long baseline interferometry (VLBI) radio observations of the ultracool dwarf (UCD) LSR J1835+3259, which resolve for the first time ever the extended radio emission of this nearby UCD. The radio morphology is consistent with the presence of a steady radiation belt powered by synchrotron emission, and aurora, powered by the coherent electron cyclotron maser mechanism. This is the first time a radiation belt is found beyond our solar system. Those results show that, similar to the Jupiter case, radio emitting UCDs possess dipole-ordered magnetic fields with radiation belt-like morphologies and aurorae. In this talk, we will present the latest results on very-long baseline interferometry (VLBI) efforts on this magnetic structure akin to the Van Allen belts. We will also take a sneak peek into novel VLBI detections showcasing distinctive radio-emitting behaviors in various UCDs, and will discuss the potential implications of those behaviors on existing models of radio emission from UCDs.
Substellar atmospheres exhibit a variety of chemical species and physical and chemical processes that determine their appearances. As substellar atmospheres cool, their chemistry becomes more complex. The JWST Cycle 1 program GO 2124 observed a sample of 12 of the coldest known extrasolar atmospheres of objects at the T/Y transition. With unprecedented precision using NIRSpec G395H spectra and MIRI long wavelength photometry, we present a detailed spectral sequence of the coldest brown dwarfs focusing on the diversity present in ultracool atmospheres. Particularly, we present: 1) prominent molecules and their state of chemical equilibrium/disequilibrium 2) the most complete SED of an extrasolar world and its retrieval and forward modeling analysis, 3) the detection and modeling of methane in emission in a substellar atmosphere indicative of auroral activity, 4) far/mid-infrared color-magnitude diagrams, and 4) progress on the retrieval and self-consistent modeling for the whole sample. Because of similarities between T/Y transition dwarfs and giant exoplanets, the observations and results presented here also allow us to constrain the aspects that may influence the appearance of exoplanets.
For two decades astronomers have been measuring weather on other worlds with the goal of understanding what atmospheric phenomena drive time-dependent brightness variations in brown dwarfs and gas giant exoplanets. Previous weather studies have been limited to broadband photometry or low resolution (R ∼100) spectroscopy. In the era of JWST, precise time-resolved medium-resolution spectroscopy of the coldest brown dwarfs is finally possible, allowing the effects of chemistry, temperature, and condensates to be disentangled. WISE 0855 (280K) is the coldest known brown dwarf and the best analog for studying processes that also occur on gas giant planets within our Solar System. We present high SNR (80 – 100), medium resolution (R ∼1000), time-series JWST/NIRSpec spectra of WISE 0855. Our observations span 11 hours with 15 minute pointings covering 2.87–5.27 microns. The dominant time-variable feature is carbon monoxide (CO) gas absorption, producing modulations in its band strength with peak-to-peak amplitudes of 8%. We discuss the changes in CO in the context of other expected disequilibrium species such as phosphine and carbon dioxide. Using atmospheric and structural models, we investigate the impact of water clouds based on observed water vapor features. Lastly, we discuss JWST’s potential to measure similar features on other cold brown dwarfs and widely-separated gas giant exoplanets.
JWST has opened the door to spectroscopy of directly imaged exoplanets beyond 3 microns, offering a new landscape for measuring their fundamental and atmospheric properties. Directly imaged exoplanets are Jupiter analogs that orbit at large separations (~10-100 AU) from their host stars. These planets, with masses of ~2-14MJup and temperatures of ~500-2000 K, remain a mystery for planet formation models—core accretion and gravitational instability. Observations that probe elemental abundances in these companions can shed light on their formation. We present results from cycle 1 programs that have pioneered the use of JWST’s NIRSpec IFU to obtain spectra of substellar companions close to sunlike stars. For HD 19467 B, NIRSpec spectra show detections of CO, CO2, CH4, and H2O. We forward model the R~2,700 spectra using custom PHOENIX atmospheric model grids to constrain the abundances, the C/O ratio, and non-equilibrium chemistry. For the multi planet system YSES-1, we have obtained one of the most comprehensive datasets of a multi-planet system with spectral coverage from 1-12 microns using NIRSpec and MIRI. The spectra allow direct spectral comparison of sibling planets in unprecedented detail. These spectra show a direct detection of silicate clouds in the 6 MJup exoplanet YSES-1 c. Ongoing atmospheric modeling will better constrain non-equilibrium chemistry and cloud composition to increase our understanding of substellar atmospheres and formation.