Papers by Sagnick Mukherjee
Nature
The Saturn-mass exoplanet WASP-39b has been the subject of extensive efforts to determine its atm... more The Saturn-mass exoplanet WASP-39b has been the subject of extensive efforts to determine its atmospheric properties using transmission spectroscopy1–4. However, these efforts have been hampered by modelling degeneracies between composition and cloud properties that are caused by limited data quality5–9. Here we present the transmission spectrum of WASP-39b obtained using the Single-Object Slitless Spectroscopy (SOSS) mode of the Near Infrared Imager and Slitless Spectrograph (NIRISS) instrument on the JWST. This spectrum spans 0.6–2.8 μm in wavelength and shows several water-absorption bands, the potassium resonance doublet and signatures of clouds. The precision and broad wavelength coverage of NIRISS/SOSS allows us to break model degeneracies between cloud properties and the atmospheric composition of WASP-39b, favouring a heavy-element enhancement (‘metallicity’) of about 10–30 times the solar value, a sub-solar carbon-to-oxygen (C/O) ratio and a solar-to-super-solar potassium-t...
Early Release Science of the Exoplanet WASP-39b with JWST NIRSpec G395H
Nature
Early Release Science of the exoplanet WASP-39b with JWST NIRCam
Nature
Le Centre pour la Communication Scientifique Directe - HAL - Paris Observatory, Nov 8, 2022
HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific re... more HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires
Cornell University - arXiv, Nov 18, 2022
Transmission spectroscopy provides insight into the atmospheric properties and consequently the f... more Transmission spectroscopy provides insight into the atmospheric properties and consequently the formation history, physics, and chemistry of transiting exoplanets 1. However, obtaining precise inferences of atmospheric properties from transmission spectra requires simultaneously measuring the strength and shape of multiple spectral absorption features from a wide range of chemical species 2-4. This has been challenging given the precision and wavelength coverage of previous observatories 5. Here, we present the transmission spectrum of the Saturn-mass exoplanet WASP-39 b obtained using the SOSS mode of the NIRISS instrument on the JWST. This spectrum spans 0.6-2.8 m in µ wavelength and reveals multiple water absorption bands, the potassium resonance doublet, as well as signatures of clouds. The precision and broad wavelength coverage of NIRISS-SOSS allows us to break model degeneracies between cloud properties and the atmospheric composition of WASP-39 b, favouring a heavy element enhancement ("metallicity") of~10-30 the solar value, a sub-solar carbon-to-oxygen (C/O) ratio, and × * 1 2
Cornell University - arXiv, Aug 2, 2022
Direct imaging studies have mainly used low-resolution spectroscopy (R ∼ 20 − 100) to study the a... more Direct imaging studies have mainly used low-resolution spectroscopy (R ∼ 20 − 100) to study the atmospheres of giant exoplanets and brown dwarf companions, but the presence of clouds has often led to degeneracies in the retrieved atmospheric abundances (e.g. C/O, metallicity). This precludes clear insights into the formation mechanisms of these companions. The Keck Planet Imager and Characterizer (KPIC) uses adaptive optics and single-mode fibers to transport light into NIRSPEC (R ∼ 35, 000 in K band), and aims to address these challenges with high-resolution spectroscopy. Using an atmospheric retrieval framework based on petitRADTRANS, we analyze KPIC high-resolution spectrum (2.29 − 2.49 µm) and archival low-resolution spectrum (1 − 2.2 µm) of the benchmark brown dwarf HD 4747 B (m = 67.2 ± 1.8 M Jup , a = 10.0 ± 0.2 au, T eff ≈ 1400 K). We find that our measured C/O and metallicity for the companion from the KPIC high-resolution spectrum agree with that of its host star within 1 − 2σ. The retrieved parameters from the K band high-resolution spectrum are also independent of our choice of cloud model. In contrast, the retrieved parameters from the lowresolution spectrum are highly sensitive to our chosen cloud model. Finally, we detect CO, H 2 O, and CH 4 (volume mixing ratio of log(CH 4)=−4.82 ± 0.23) in this L/T transition companion with the KPIC data. The relative molecular abundances allow us to constrain the degree of chemical disequilibrium in the atmosphere of HD 4747 B, and infer a vertical diffusion coefficient that is at the upper limit predicted from mixing length theory.
Probing the Extent of Vertical Mixing in Brown Dwarf Atmospheres with Disequilibrium Chemistry
The Astrophysical Journal
Evidence of disequilibrium chemistry due to vertical mixing in the atmospheres of many T- and Y-d... more Evidence of disequilibrium chemistry due to vertical mixing in the atmospheres of many T- and Y-dwarfs has been inferred due to enhanced mixing ratios of CO and reduced NH3. Atmospheric models of planets and brown dwarfs typically parameterize this vertical mixing phenomenon with the vertical eddy diffusion coefficient, K zz . While K zz can perhaps be approximated in the convective regions in the atmosphere with mixing length theory, in radiative regions, the strength of vertical mixing is uncertain by many orders of magnitude. With a new grid of self-consistent 1D model atmospheres from T eff of 400–1000 K, computed with a new radiative-convective equilibrium python code PICASO 3.0, we aim to assess how molecular abundances and corresponding spectra can be used as a probe of depth-dependent K zz . At a given surface gravity, we find nonmonotonic behavior in the CO abundance as a function of T eff, as chemical abundances are sometimes quenched in either of two potential atmospheric...
Cornell University - arXiv, Aug 16, 2022
Upcoming James Webb Space Telescope (JWST) observations will allow us to study exoplanet and brow... more Upcoming James Webb Space Telescope (JWST) observations will allow us to study exoplanet and brown dwarf atmospheres in great detail. The physical interpretation of these upcoming high signalto-noise observations requires precise atmospheric models of exoplanets and brown dwarfs. While several one-dimensional and three-dimensional atmospheric models have been developed in the past three decades, these models have often relied on simplified assumptions like chemical equilibrium and are also often not open-source, which limits their usage and development by the wider community. We present a python-based one-dimensional atmospheric radiative-convective equilibrium model. This model has heritage from the Fortran-based code (Marley et al. 1996) which has been widely used to model the atmospheres of Solar System objects, brown dwarfs and exoplanets. In short, the basic capability of the original model is to compute the atmospheric state of the object under radiativeconvective equilibrium given its effective or internal temperature, gravity and host-star properties (if relevant). In the new model, which has been included within the well-utilized code-base PICASO, we have added these original features as well as the new capability of self-consistently treating disequilibrium chemistry. This code is widely applicable to Hydrogen-dominated atmospheres (e.g. brown dwarfs and giant planets).
Cornell University - arXiv, Aug 31, 2022
We present JWST Early Release Science (ERS) coronagraphic observations of the super-Jupiter exopl... more 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 µm, and with the Mid-Infrared Instrument (MIRI) from 11−16 µm. At a separation of ∼0.82 (87 +108 −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 µm. These observations demonstrate that JWST is exceeding its nominal predicted performance by up to a factor of 10, with measured 5σ contrast limits of ∼4×10 −6 (∼2.4 µJy) and ∼2×10 −4 (∼10 µJy) at 1 for NIRCam at 3.6 µm and MIRI at 11.3 µm, respectively. These contrast limits provide sensitivity to sub-Jupiter companions with masses as low as 0.3M Jup beyond separations of ∼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 µm, and span ∼97% of HIP 65426 b's luminous range. Independent of the choice of forward model atmosphere we measure an empirical bolometric luminosity that is tightly constrained between log(L bol /L)=−4.35 to −4.21, which in turn provides a robust mass constraint of 7.1±1.1 M Jup. In totality, these observations confirm that JWST presents a powerful and exciting opportunity to characterise the population of exoplanets amenable to direct imaging in greater detail.
A Clear View of a Cloudy Brown Dwarf Companion from High-resolution Spectroscopy
The Astrophysical Journal
Direct imaging studies have mainly used low-resolution spectroscopy (R ∼ 20–100) to study the atm... more Direct imaging studies have mainly used low-resolution spectroscopy (R ∼ 20–100) to study the atmospheres of giant exoplanets and brown dwarf companions, but the presence of clouds has often led to degeneracies in the retrieved atmospheric abundances (e.g., carbon-to-oxygen ratio, metallicity). This precludes clear insights into the formation mechanisms of these companions. The Keck Planet Imager and Characterizer (KPIC) uses adaptive optics and single-mode fibers to transport light into NIRSPEC (R ∼ 35,000 in the K band), and aims to address these challenges with high-resolution spectroscopy. Using an atmospheric retrieval framework based on petitRADTRANS, we analyze the KPIC high-resolution spectrum (2.29–2.49 μm) and the archival low-resolution spectrum (1–2.2 μm) of the benchmark brown dwarf HD 4747 B (m = 67.2 ± 1.8 M Jup, a = 10.0 ± 0.2 au, T eff ≈ 1400 K). We find that our measured C/O and metallicity for the companion from the KPIC high-resolution spectrum agree with those o...
Nature
Carbon dioxide (CO2) is a key chemical species that is found in a wide range of planetary atmosph... more Carbon dioxide (CO2) is a key chemical species that is found in a wide range of planetary atmospheres. In the context of exoplanets, CO2 is an indicator of the metal enrichment (that is, elements heavier than helium, also called ‘metallicity’)1–3, and thus the formation processes of the primary atmospheres of hot gas giants4–6. It is also one of the most promising species to detect in the secondary atmospheres of terrestrial exoplanets7–9. Previous photometric measurements of transiting planets with the Spitzer Space Telescope have given hints of the presence of CO2, but have not yielded definitive detections owing to the lack of unambiguous spectroscopic identification10–12. Here we present the detection of CO2 in the atmosphere of the gas giant exoplanet WASP-39b from transmission spectroscopy observations obtained with JWST as part of the Early Release Science programme13,14. The data used in this study span 3.0–5.5 micrometres in wavelength and show a prominent CO2 absorption fe...
The Astrophysical Journal, 2021
The detection of disk-integrated polarization from Luhman 16 A and B in the H band, and subsequen... more The detection of disk-integrated polarization from Luhman 16 A and B in the H band, and subsequent modeling, has been interpreted in the framework of zonal cloud bands on these bodies. Recently, Tan and Showman investigated the 3D atmospheric circulation and cloud structures of brown dwarfs with general circulation models (GCMs), and their simulations yielded complex cloud distributions showing some aspects of zonal jets, but also complex vortices that cannot be captured by a simple model. Here we use these 3D GCMs specific to Luhman 16 A and B, along with the 3D Monte Carlo radiative transfer code ARTES, to calculate their polarization signals. We adopt the 3D temperature–pressure and cloud profiles from the GCMs as our input atmospheric structures. Our polarization calculations at 1.6 μm agree well with the measured degree of linear polarization from both Luhman 16 A and B. Our calculations reproduce the measured polarization for both objects with cloud particle sizes between 0.5 ...
The Astrophysical Journal, 2021
Future space-based direct imaging missions will perform low-resolution (R < 100) optical (0.3–... more Future space-based direct imaging missions will perform low-resolution (R < 100) optical (0.3–1 μm) spectroscopy of planets, thus enabling reflected spectroscopy of cool giants. Reflected light spectroscopy is encoded with rich information about the scattering and absorbing properties of planet atmospheres. Given the diversity of clouds and hazes expected in exoplanets, it is imperative that we solidify the methodology to accurately and precisely retrieve these scattering and absorbing properties that are agnostic to cloud species. In particular, we focus on determining how different cloud parameterizations affect resultant inferences of both cloud and atmospheric composition. We simulate mock observations of the reflected spectra from three top-priority direct imaging cool giant targets with different effective temperatures, ranging from 135 to 533 K. We perform retrievals of cloud structure and molecular abundances on these three planets using four different parameterizations, ...
The power spectral density (PSD) of the X-ray emission variability from the accretion disccorona ... more The power spectral density (PSD) of the X-ray emission variability from the accretion disccorona region of black hole X-ray binaries and active galactic nuclei has a broken power-law shape with a characteristic break time-scale TB. If the disc and the jet are connected, the jet variability may also contain a characteristic time-scale related to that of the disc-corona. Recent observations of the blazar Mrk 421 have confirmed the broken power-law shape of the PSD of its jet X-ray variability. We model the time variability of a blazar, in which emitting particles are assumed to be accelerated by successive shock waves flowing down the jet with a varying inter-shock time-scale (TIS). We investigate the possible relation between the characteristic time-scales in the disc and jet variability based on the above model, along with mathematically and physically simulated disc variability. We find that both the PSD of the jet and disc variability may have a broken power-law shape but the brea...
Techniques and Instrumentation for Detection of Exoplanets X, 2021
We present an exposure time calculator (ETC) under development for polarimetric observing modes f... more We present an exposure time calculator (ETC) under development for polarimetric observing modes for two high-contrast direct imaging instruments planned for the W. M. Keck Observatory: a future polarimetric observing mode for NIRC2, the near-infrared AO-supported high-contrast direct imager currently on Keck II and SCALES (Santa Cruz Array of Lenslets for Exoplanet Spectroscopy), a low-resolution mid-infrared integral field spectrograph. In addition to producing estimates of polarized companion signal-to-noise ratios (SNR), the ETC will integrate a radiative transfer modeling code (ARTES) in order to calculate expected companion polarization. The ETC includes a graphical user interface and will incorporate the PSI-Red thermal imaging module of PSISIM, an instrument performance simulation tool originally developed for the Planetary Systems Imager-a modular suite of adaptive optics (AO) supported instruments planned for the Thirty Meter Telescope.
Cornell University - arXiv, Nov 18, 2022
Photochemistry is a fundamental process of planetary atmospheres that is integral to habitability... more Photochemistry is a fundamental process of planetary atmospheres that is integral to habitability, atmospheric composition and stability, and aerosol formation [1]. However, no unambiguous photochemical products have been detected in exoplanet atmospheres to date. Here we show that photochemically produced sulphur dioxide (SO2) is present in the atmosphere of the hot, giant exoplanet WASP-39b, as constrained by data from the JWST Transiting Exoplanet Early Release Science Program [2, 3] and informed by a suite of photochemical models. We find that SO2 is produced by successive oxidation of sulphur radicals freed when hydrogen sulphide (H2S) is destroyed. The SO2 distribution computed by the photochemical models robustly explains the 4.05 µm spectral feature seen in JWST transmission spectra [4] [Rustamkulov et al.(submitted), Alderson et al.(submitted)] and leads to observable features at ultraviolet and thermal infrared wavelengths not available from the current observations. The sensitivity of the SO2 feature to the enrichment of heavy elements in the atmosphere ("metallicity") suggests that it can be used as a powerful tracer of atmospheric properties, with our results implying a metallicity of ∼10× solar for WASP-39b. Through providing improved constraints on bulk metallicity and sulphur abundance, the detection of SO2 opens a new avenue for the investigation of giant-planet formation. Our work demonstrates that sulphur photochemistry may be readily observable for exoplanets with super-solar metallicity and equilibrium temperatures 750 K. The confirmation of photochemistry through the agreement between theoretical predictions and observational data is pivotal for further atmospheric characterisation studies.
The Accretion Disk-Jet Connection in Blazars
Monthly Notices of the Royal Astronomical Society
X-Ray Surface Brightness Profiles of Optically Selected Active Galactic Nuclei: Comparison with X-Ray AGNs
The Astrophysical Journal
X-Ray Surface Brightness Profiles of Optically Selected Active Galactic Nuclei: Comparison with X-Ray AGNs
The Astrophysical Journal
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Papers by Sagnick Mukherjee