arXiv subjects

Malavika Vasist

Publications and source records attributed to Malavika Vasist.

7 recordsLinked to original sources

arXiv2025

Panchromatic characterization of the Y0 brown dwarf WISEP J173835.52+273258.9 using JWST/MIRI

Cold brown dwarf atmospheres are good training grounds for analyzing temperate giant planets. WISEP J173835.52+273258.9 (WISE 1738) is an isolated Y0 brown dwarf with a temperature between 350-400 K, at the T-Y transition. While its near-infrared spectrum has been studied, bulk properties and chemistry remain uncertain. We analyze new JWST MIRI medium-resolution spectra (5-18 micron), combined with near-infrared spectra (0.98-2.2 micron) from HST/WFC3 and Gemini/GNIRS, to better constrain WISE 1738's atmosphere and physical parameters. We use Neural Posterior Estimation (NPE) with a cloud-free petitRADTRANS model and evaluate results using posterior checks, coverage, and L-C2ST diagnostics. Our retrieval confirms previous constraints on H2O, CH4, and NH3, and for the first time constrains CO, CO2, and 15NH3. We find evidence of disequilibrium chemistry through CO and CO2 abundances not expected under equilibrium. Estimated properties are temperature 402 (+12,-9) K, log g 4.43 (+0.26,-0.34) cm/s2, mass 13 (+11,-7) M_Jup, radius 1.14 (+0.03,-0.03) R_Jup, and bolometric luminosity -6.52 (+0.05,-0.04) log L/L_sun. Evolutionary models suggest an age between 1 and 4 Gyr, consistent with a 6-hour rotation. We place an upper bound on 15NH3, implying a 3-sigma lower limit on the 14N/15N ratio of 275. We also derive a C/O ratio of 1.35 (+0.39,-0.31) and metallicity of 0.34 (+0.12,-0.11), without accounting for oxygen sequestration.

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arXiv2024

The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems II: A 1 to 20 Micron Spectrum of the Planetary-Mass Companion VHS 1256-1257 b

We present the highest fidelity spectrum to date of a planetary-mass object. VHS 1256 b is a $<$20 M$_\mathrm{Jup}$ widely separated ($\sim$8\arcsec, a = 150 au), young, planetary-mass companion that shares photometric colors and spectroscopic features with the directly imaged exoplanets HR 8799 c, d, and e. As an L-to-T transition object, VHS 1256 b exists along the region of the color-magnitude diagram where substellar atmospheres transition from cloudy to clear. We observed VHS 1256~b with \textit{JWST}'s NIRSpec IFU and MIRI MRS modes for coverage from 1 $μ$m to 20 $μ$m at resolutions of $\sim$1,000 - 3,700. Water, methane, carbon monoxide, carbon dioxide, sodium, and potassium are observed in several portions of the \textit{JWST} spectrum based on comparisons from template brown dwarf spectra, molecular opacities, and atmospheric models. The spectral shape of VHS 1256 b is influenced by disequilibrium chemistry and clouds. We directly detect silicate clouds, the first such detection reported for a planetary-mass companion.

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arXiv2023

15NH3 in the atmosphere of a cool brown dwarf

Brown dwarfs serve as ideal laboratories for studying the atmospheres of giant exoplanets on wide orbits as the governing physical and chemical processes in them are nearly identical. Understanding the formation of gas giant planets is challenging, often involving the endeavour to link atmospheric abundance ratios, such as the carbon-to-oxygen (C/O) ratio, to formation scenarios. However, the complexity of planet formation requires additional tracers, as the unambiguous interpretation of the measured C/O ratio is fraught with complexity. Isotope ratios, such as deuterium-to-hydrogen and 14N/15N, offer a promising avenue to gain further insight into this formation process, mirroring their utility within the solar system. For exoplanets only a handful of constraints on 12C/13C exist, pointing to the accretion of 13C-rich ice from beyond the disks' CO iceline. Here we report on the mid-infrared detection of the 14NH3 and 15NH3 isotopologues in the atmosphere of a cool brown dwarf with an effective temperature of 380 K in a spectrum taken with the Mid-InfraRed Instrument of the James Webb Space Telescope. As expected, our results reveal a 14N/15N value consistent with star-like formation by gravitational collapse, demonstrating that this ratio can be accurately constrained. Since young stars and their planets should be more strongly enriched in the 15N isotope, we expect that 15NH3 will be detectable in a number of cold, wide-separation exoplanets.

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arXiv2023

The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems I: High Contrast Imaging of the Exoplanet HIP 65426 b from 2-16 $μ$m

We present JWST Early Release Science (ERS) coronagraphic observations of the super-Jupiter exoplanet, HIP 65426 b, with the Near-Infrared Camera (NIRCam) from 2-5 $μ$m, and with the Mid-Infrared Instrument (MIRI) from 11-16 $μ$m. At a separation of $\sim$0.82" (86$^{+116}_{-31}$ au), HIP 65426 b is clearly detected in all seven of our observational filters, representing the first images of an exoplanet to be obtained by JWST, and the first ever direct detection of an exoplanet beyond 5 $μ$m. These observations demonstrate that JWST is exceeding its nominal predicted performance by up to a factor of 10, depending on separation and subtraction method, with measured 5$σ$ contrast limits of $\sim$1$\times10^{-5}$ and $\sim$2$\times10^{-4}$ at 1" for NIRCam at 4.4 $μ$m and MIRI at 11.3 $μ$m, respectively. These contrast limits provide sensitivity to sub-Jupiter companions with masses as low as 0.3$M_\mathrm{Jup}$ beyond separations of $\sim$100 au. Together with existing ground-based near-infrared data, the JWST photometry are well fit by a BT-SETTL atmospheric model from 1-16 $μ$m, and span $\sim$97% of HIP 65426 b's luminous range. Independent of the choice of model atmosphere we measure an empirical bolometric luminosity that is tightly constrained between $\mathrm{log}\!\left(L_\mathrm{bol}/L_{\odot}\right)$=-4.31 to $-$4.14, which in turn provides a robust mass constraint of 7.1$\pm$1.2 $M_\mathrm{Jup}$. In totality, these observations confirm that JWST presents a powerful and exciting opportunity to characterise the population of exoplanets amenable to high-contrast imaging in greater detail.

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arXiv2023

Neural posterior estimation for exoplanetary atmospheric retrieval

Retrieving the physical parameters from spectroscopic observations of exoplanets is key to understanding their atmospheric properties. Exoplanetary atmospheric retrievals are usually based on approximate Bayesian inference and rely on sampling-based approaches to compute parameter posterior distributions. Accurate or repeated retrievals, however, can result in very long computation times due to the sequential nature of sampling-based algorithms. We aim to amortize exoplanetary atmospheric retrieval using neural posterior estimation (NPE), a simulation-based inference algorithm based on variational inference and normalizing flows. In this way, we aim (i) to strongly reduce inference time, (ii) to scale inference to complex simulation models with many nuisance parameters or intractable likelihood functions, and (iii) to enable the statistical validation of the inference results. We evaluate NPE on a radiative transfer model for exoplanet spectra petitRADTRANS, including the effects of scattering and clouds. We train a neural autoregressive flow to quickly estimate posteriors and compare against retrievals computed with MultiNest. NPE produces accurate posterior approximations while reducing inference time down to a few seconds. We demonstrate the computational faithfulness of our posterior approximations using inference diagnostics including posterior predictive checks and coverage, taking advantage of the quasi-instantaneous inference time of NPE. Our analysis confirms the reliability of the approximate posteriors produced by NPE. The accuracy and reliability of the inference results produced by NPE establishes it as a promising approach for atmospheric retrievals. Amortization of the posterior inference makes repeated inference on several observations computationally inexpensive since it does not require on-the-fly simulations, making the retrieval efficient, scalable, and testable.

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arXiv2023

The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems: Best Practices for Data Collection in Cycle 2 and Beyond

We present a set of recommended best practices for JWST data collection for members of the community focussed on the direct imaging and spectroscopy of exoplanetary systems. These findings and recommendations are based on the early analysis of the JWST Early Release Science Program 1386, "High-Contrast Imaging of Exoplanets and Exoplanetary Systems with JWST." Our goal is for this information to be useful for observers in preparation of JWST proposals for Cycle 2 and beyond. In addition to compiling a set of best practices from our ERS program, in a few cases we also draw on the expertise gained within the instrument commissioning programs, as well as include a handful of data processing best practices. We anticipate that this document will be regularly updated and resubmitted to arXiv.org to ensure that we have distributed our knowledge of best-practices for data collection as widely and efficiently as possible.

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arXiv2022

The JWST Early Release Science Program for the Direct Imaging & Spectroscopy of Exoplanetary Systems

The direct characterization of exoplanetary systems with high contrast imaging is among the highest priorities for the broader exoplanet community. As large space missions will be necessary for detecting and characterizing exo-Earth twins, developing the techniques and technology for direct imaging of exoplanets is a driving focus for the community. For the first time, JWST will directly observe extrasolar planets at mid-infrared wavelengths beyond 5$μ$m, deliver detailed spectroscopy revealing much more precise chemical abundances and atmospheric conditions, and provide sensitivity to analogs of our solar system ice-giant planets at wide orbital separations, an entirely new class of exoplanet. However, in order to maximise the scientific output over the lifetime of the mission, an exquisite understanding of the instrumental performance of JWST is needed as early in the mission as possible. In this paper, we describe our 55-hour Early Release Science Program that will utilize all four JWST instruments to extend the characterisation of planetary mass companions to $\sim$15$μ$m as well as image a circumstellar disk in the mid-infrared with unprecedented sensitivity. Our program will also assess the performance of the observatory in the key modes expected to be commonly used for exoplanet direct imaging and spectroscopy, optimize data calibration and processing, and generate representative datasets that will enable a broad user base to effectively plan for general observing programs in future cycles.

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