Fingering convection and cloudless models for cool brown dwarf atmospheres

This work aims to improve the current understanding of the atmospheres of brown dwarfs, especially cold ones with spectral type T and Y, whose modeling is a current challenge. Silicate and iron clouds are believed to disappear at the photosphere at the L/T transition, but cloudless models fail to reproduce correctly the spectra of T dwarfs, advocating for the addition of more physics, e.g. other types of clouds or internal energy transport mechanisms. We use a one-dimensional (1D) radiative/convective equilibrium code ATMO to investigate this issue. This code includes both equilibrium and out-of-equilibrium chemistry and solves consistently the PT structure. Included opacity sources are H2-H2, H2-He, H2O, CO, CO2, CH4, NH3, K, Na, and TiO, VO if they are present in the atmosphere. We show that the spectra of Y dwarfs can be accurately reproduced with a cloudless model if vertical mixing and NH3 quenching are taken into account. T dwarf spectra still have some reddening in e.g. J - H compared to cloudless models. This reddening can be reproduced by slightly reducing the temperature gradient in the atmosphere. We propose that this reduction of the stabilizing temperature gradient in these layers, leading to cooler structures, is due to the onset of fingering convection, triggered by the destabilizing impact of condensation of very thin dust.Comment: Accepted in ApJ

Near-Infrared Spectroscopy of the Y0 WISEP J173835.52+273258.9 and the Y1 WISE J035000.32-565830.2: the Importance of Non-Equilibrium Chemistry

We present new near-infrared spectra, obtained at Gemini Observatory, for two Y dwarfs: WISE J035000.32-565830.2 (W0350) and WISEP J173835.52+273258.9 (W1738). A FLAMINGOS-2 R=540 spectrum was obtained for W0350, covering 1.0 < lambda um < 1.7, and a cross-dispersed GNIRS R=2800 spectrum was obtained for W1738, covering 0.993-1.087 um, 1.191-1.305 um, 1.589-1.631 um, and 1.985-2.175 um, in four orders. We also present revised YJH photometry for W1738, using new NIRI Y and J imaging, and a re-analysis of the previously published NIRI H band images. We compare these data, together with previously published data for late-T and Y dwarfs, to cloud-free models of solar metallicity, calculated both in chemical equilibrium and with disequilibrium driven by vertical transport. We find that for the Y dwarfs the non-equilibrium models reproduce the near-infrared data better than the equilibrium models. The remaining discrepancies suggest that fine-tuning the CH_4/CO and NH_3/N_2 balance is needed. Improved trigonometric parallaxes would improve the analysis. Despite the uncertainties and discrepancies, the models reproduce the observed near-infrared spectra well. We find that for the Y0, W1738, T_eff = 425 +/- 25 K and log g = 4.0 +/- 0.25, and for the Y1, W0350, T_eff = 350 +/- 25 K and log g = 4.0 +/- 0.25. W1738 may be metal-rich. Based on evolutionary models, these temperatures and gravities correspond to a mass range for both Y dwarfs of 3-9 Jupiter masses, with W0350 being a cooler, slightly older, version of W1738; the age of W0350 is 0.3-3 Gyr, and the age of W1738 is 0.15-1 Gyr.Comment: Accepted on March 30 2016 for publication in Ap

Near-Infrared Spectroscopy of the Y0 WISEP J173835.52+273258.9 and the Y1 WISE J035000.32-565830.2: the Importance of Non-Equilibrium Chemistry

This is the author accepted manuscript. The final version is available from American Astronomical Society / IOP Publishing via the DOI in this record.We present new near-infrared spectra, obtained at Gemini Observatory, for two Y dwarfs: WISE J035000.32-565830.2 (W0350) and WISEP J173835.52+273258.9 (W1738). A FLAMINGOS-2 R=540 spectrum was obtained for W0350, covering 1.0 < lambda um < 1.7, and a cross-dispersed GNIRS R=2800 spectrum was obtained for W1738, covering 0.993-1.087 um, 1.191-1.305 um, 1.589-1.631 um, and 1.985-2.175 um, in four orders. We also present revised YJH photometry for W1738, using new NIRI Y and J imaging, and a re-analysis of the previously published NIRI H band images. We compare these data, together with previously published data for late-T and Y dwarfs, to cloud-free models of solar metallicity, calculated both in chemical equilibrium and with disequilibrium driven by vertical transport. We find that for the Y dwarfs the non-equilibrium models reproduce the near-infrared data better than the equilibrium models. The remaining discrepancies suggest that fine-tuning the CH_4/CO and NH_3/N_2 balance is needed. Improved trigonometric parallaxes would improve the analysis. Despite the uncertainties and discrepancies, the models reproduce the observed near-infrared spectra well. We find that for the Y0, W1738, T_eff = 425 +/- 25 K and log g = 4.0 +/- 0.25, and for the Y1, W0350, T_eff = 350 +/- 25 K and log g = 4.0 +/- 0.25. W1738 may be metal-rich. Based on evolutionary models, these temperatures and gravities correspond to a mass range for both Y dwarfs of 3-9 Jupiter masses, with W0350 being a cooler, slightly older, version of W1738; the age of W0350 is 0.3-3 Gyr, and the age of W1738 is 0.15-1 Gyr.Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Minist´erio da Ciˆencia, Tecnologia e Inova¸c˜ao (Brazil) and Ministerio de Ciencia, Tecnolog´ıa e Innovaci´on Productiva (Argentina). S. L.’s research is supported by Gemini Observatory. D.S.’ work was supported in part by NASA grant NNH12AT89I from Astrophysics Theory. I. B.’s work is supported by the European Research Council through grant ERC-AdG No.– 17 – 320478-TOFU.This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. This research has made use of the NASA/ IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration

STARBENCH: the D-type expansion of an HII region

STARBENCH is a project focused on benchmarking and validating different star formation and stellar feedback codes. In this first STARBENCH paper we perform a comparison study of the D-type expansion of an H II region. The aim of this work is to understand the differences observed between the 12 participating numerical codes against the various analytical expressions examining the D-type phase of H II region expansion. To do this, we propose two well-defined tests which are tackled by 1D and 3D grid- and smoothed particle hydrodynamics-based codes. The first test examines the ‘early phase’ D-type scenario during which the mechanical pressure driving the expansion is significantly larger than the thermal pressure of the neutral medium. The second test examines the ‘late phase’ D-type scenario during which the system relaxes to pressure equilibrium with the external medium. Although they are mutually in excellent agreement, all 12 participating codes follow a modified expansion law that deviates significantly from the classical Spitzer solution in both scenarios. We present a semi-empirical formula combining the two different solutions appropriate to both early and late phases that agrees with high-resolution simulations to ≲ 2 per cent. This formula provides a much better benchmark solution for code validation than the Spitzer solution. The present comparison has validated the participating codes and through this project we provide a data set for calibrating the treatment of ionizing radiation hydrodynamics codes

The resonance lines of sodium and potassium in brown dwarf spectra

This is the final version. Available from the publisher via the DOI in this record.Accurate pressure broadened line profiles of alkali resonance dou-blets are needed for the modelling of atmospheres of cool stars and for generat-ing their synthetic spectra in the region 400 - 900 nm. When the lines utterly dominate their region of the spectrum, it becomes important to represent the profiles accurately over the whole range from the line centre to the far line wings. In this paper we examine the theories of spectral line shapes that have been used and carry out new calculations of the line shapes for the resonance lines of sodium and potassium broadened by helium.Science and Technology Facilities Counci

JWST/NIRSpec Observations of the Coldest Known Brown Dwarf

We present 1-5um spectroscopy of the coldest known brown dwarf, WISE J085510.83-071442.5 (WISE 0855), performed with the Near-Infrared Spectrograph (NIRSpec) on board the James Webb Space Telescope (JWST). NIRSpec has dramatically improved the measurement of spectral energy distribution of WISE 0855 in terms of wavelength coverage, signal-to-noise ratios, and spectral resolution. We have performed preliminary modeling of the NIRSpec data using the ATMO 2020 models of cloudless atmospheres, arriving at a best fitting model that has T_eff=285 K. That temperature is ~20 K higher than the value derived by combining our luminosity estimate with evolutionary models (i.e., the radius in the model fit to the SED is somewhat smaller than expected from evolutionary models). Through comparisons to the model spectra, we detect absorption in the fundamental band of CO, which is consistent with an earlier detection in a ground-based spectrum and indicates the presence of vertical mixing. Although PH_3 is expected in Y dwarfs that experience vertical mixing, it is not detected in WISE 0855. Previous ground-based M-band spectroscopy of WISE 0855 has been cited for evidence of H_2O ice clouds, but we find that the NIRSpec data in that wavelength range are matched well by our cloudless model. Thus, clear evidence of H_2O ice clouds in WISE 0855 has not been identified yet, but it may still be present in the NIRSpec data. The physical properties of WISE 0855, including the presence of H_2O clouds, can be better constrained by more detailed fitting with both cloudless and cloudy models and the incorporation of unpublished 5-28um data from the Mid-infrared Instrument on JWST.Comment: Astronomical Journal, in pres

Globules and pillars seen in the [CII] 158 micron line with SOFIA

Molecular globules and pillars are spectacular features, found only in the interface region between a molecular cloud and an HII-region. Impacting Far-ultraviolet (FUV) radiation creates photon dominated regions (PDRs) on their surfaces that can be traced by typical cooling lines. With the GREAT receiver onboard SOFIA we mapped and spectrally resolved the [CII] 158 micron atomic fine-structure line and the highly excited 12CO J=11-10 molecular line from three objects in Cygnus X (a pillar, a globule, and a strong IRAS source). We focus here on the globule and compare our data with existing Spitzer data and recent Herschel Open-Time PACS data. Extended [CII] emission and more compact CO-emission was found in the globule. We ascribe this emission mainly to an internal PDR, created by a possibly embedded star-cluster with at least one early B-star. However, external PDR emission caused by the excitation by the Cyg OB2 association cannot be fully excluded. The velocity-resolved [CII] emission traces the emission of PDR surfaces, possible rotation of the globule, and high-velocity outflowing gas. The globule shows a velocity shift of ~2 km/s with respect to the expanding HII-region, which can be understood as the residual turbulence of the molecular cloud from which the globule arose. This scenario is compatible with recent numerical simulations that emphazise the effect of turbulence. It is remarkable that an isolated globule shows these strong dynamical features traced by the [CII]-line, but it demands more observational studies to verify if there is indeed an embedded cluster of B-stars.Comment: Letter accepted by A&A (SOFIA special issue

Globules and pillars in Cygnus X. I. <i>Herschel</i> far-infrared imaging of the Cygnus OB2 environment

The radiative feedback of massive stars on molecular clouds creates pillars, globules and other features at the interface between the H II region and molecular cloud. Optical and near-infrared observations from the ground as well as with the Hubble or Spitzer satellites have revealed numerous examples of such cloud structures. We present here Herschel far-infrared observations between 70 μm and 500 μm of the immediate environment of the rich Cygnus OB2 association, performed within the Herschel imaging survey of OB Young Stellar objects (HOBYS) program. All of the observed irradiated structures were detected based on their appearance at 70 μm, and have been classified as pillars, globules, evaporating gasous globules (EGGs), proplyd-like objects, and condensations. From the 70 μm and 160 μm flux maps, we derive the local far-ultraviolet (FUV) field on the photon dominated surfaces. In parallel, we use a census of the O-stars to estimate the overall FUV-field, that is 103-104 G0 (Habing field) close to the central OB cluster (within 10 pc) and decreases down to a few tens G0, in a distance of 50 pc. From a spectral energy distribution (SED) fit to the four longest Herschel wavelengths, we determine column density and temperature maps and derive masses, volume densities and surface densities for these structures. We find that the morphological classification corresponds to distinct physical properties. Pillars and globules are massive (~500 M⊙) and large (equivalent radius r ~ 0.6 pc) structures, corresponding to what is defined as "clumps" for molecular clouds. EGGs and proplyd-likeobjects are smaller (r ~ 0.1 and 0.2 pc) and less massive (~10 and ~30 M⊙). Cloud condensations are small (~0.1 pc), have an average mass of 35 M⊙, are dense (~6 × 104 cm-3), and can thus be described as molecular cloud "cores". All pillars and globules are oriented toward the Cyg OB2 association center and have the longest estimated photoevaporation lifetimes, a few million years, while all other features should survive less than a million years. These lifetimes are consistent with that found in simulations of turbulent, UV-illuminated clouds. We propose a tentative evolutionary scheme in which pillars can evolve into globules, which in turn then evolve into EGGs, condensations and proplyd-like objects

Erratum: A library of ATMO forward model transmission spectra for hot Jupiter exoplanets

This is the final version. Available from OUP via the DOI in this recordThe article to which this is the erratum is in ORE at http://hdl.handle.net/10871/30324The paper ‘A library of ATMO forward model transmission spectra for hot Jupiter exoplanets’ was published in MNRAS 474, 4, 5158–5185. In the original manuscript (Goyal et al. 2018), we presented a grid of forward model transmission spectra for hot Jupiter exoplanets. However, we recently identified an error in the treatment of rainout in our 1D atmosphere model ATMO. The correction of this error led to changes in the equilibrium chemical abundances using rainout condensation and thereby the transmission spectra. We note that this error only affects the online library2,3 that includes rainout condensation, the library with local condensation (without rainout) is unaffected. We further note that the gas phase equilibrium scheme used in ATMO has been compared by Drummond et al. (2016) with the analytical schemes of Burrows & Sharp (1999) and Heng & Tsai (2016). The gas phase chemistry with and without local condensation has also been verified in Baudino et al. (2017) against the petitCODE (Mollière et al. 2015, 2017) and Exo-REM (Baudino et al. 2015) models. Therefore, issues with the previous version of the grid were confined to the implementation of rainout

JWST/NIRSpec Observations of the Planetary Mass Companion TWA 27B

We present 1-5um spectroscopy of the young planetary mass companion TWA 27B (2M1207B) performed with NIRSpec on board the James Webb Space Telescope. In these data, the fundamental band of CH_4 is absent and the fundamental band of CO is weak. The nondetection of CH_4 reinforces a previously observed trend of weaker CH_4 with younger ages among L dwarfs, which has been attributed to enhanced non-equilibrium chemistry among young objects. The weakness of CO may reflect an additional atmospheric property that varies with age, such as the temperature gradient or cloud thickness. We are able to reproduce the broad shape of the spectrum with an ATMO cloudless model that has T=1300 K, non-equilibrium chemistry, and a temperature gradient reduction caused by fingering convection. However, the fundamental bands of CH_4 and CO are somewhat stronger in the model. In addition, the model temperature of 1300 K is higher than expected from evolutionary models given the luminosity and age of TWA 27B (T=1200 K). Previous models of young L-type objects suggest that the inclusion of clouds could potentially resolve these issues; it remains to be seen whether cloudy models can provide a good fit to the 1-5um data from NIRSpec. TWA 27B exhibits emission in Paschen transitions and the He I triplet at 1.083um, which are signatures of accretion that provide the first evidence of a circumstellar disk. We have used the NIRSpec data to estimate the bolometric luminosity of TWA 27B (log L/L_sun=-4.466+/-0.014), which implies a mass of 5-6 MJup according to evolutionary models.Comment: Astrophysical Journal Letters, in pres