arXiv subjects

T. K. Greathouse

Publications and source records attributed to T. K. Greathouse.

At least 19 recordsLinked to original sources

arXiv2025

Spatial variability of CH4 and C2H2 absorptions in Jupiter's auroral regions from Juno-UVS observations

Color ratios derived from molecular hydrogen emissions provide valuable diagnostics for the energy of precipitating electrons and the structure of the auroral atmosphere. We aim to characterize the horizontal and vertical variability of hydrocarbon absorption in Jupiter's auroral atmosphere using ultraviolet data from the Juno-UVS spectrograph and to investigate potential departures from the expected structure. We constructed color ratio maps sensitive to CH$_4$ and C$_2$H$_2$ absorptions for perijoves (PJs) 6 and 10, two of Juno's close approaches to Jupiter, by integrating auroral H$_2$ emission over hydrocarbon-sensitive spectral intervals. For CH$_4$, we redefined the absorbed spectral band, replacing the traditionally used 125-130 nm interval with 135-140 nm, in order to mitigate higher-order calibration issues. In regions of intense auroral brightness, we developed a correction method to account for spectral distortion due to detector non-linearities at high fluxes. CH$_4$ and C$_2$H$_2$ absorptions generally follow the expected vertical distribution, with the CH$_4$ density extending to higher altitudes than C$_2$H$_2$. However, several localized regions show unexpected spatial distribution of the absorption. In PJ6, such anomalies are attributed to instrumental non-linearities. After correction, the CR distributions become consistent with standard hydrocarbon vertical distributions. In PJ10, however, some anomalous patterns persist despite correction. Spectral modeling indicates that these can be explained by modifying the relative abundances of CH$_4$ and C$_2$H$_2$, suggesting horizontal compositional variability and possible deviations in homopause altitude between species. These findings underscore the importance of accounting for local atmospheric composition when interpreting ultraviolet auroral spectra and retrieving electron energy distributions.

astro-ph.EP↗

arXiv2025

Jupiter's ultraviolet auroral bridge: the influence of the solar wind on polar auroral morphology

Jupiters ultraviolet aurora frequently shows a number of arcs between the dusk-side polar region and the main emission, which are denoted as bridges. This work presents a largely automated detection and statistical analysis of bridges over 248 Hubble-Space-Telescope observations, alongside a multi-instrument study of crossings of magnetic field lines connected to bridges by the Juno spacecraft during its first 30 perijoves. Bridges are observed to arise on timescales of around 2 hours, can persist over a full Jupiter rotation, and are conjugate between hemispheres. The appearance of bridges is associated with compression of the magnetosphere, likely by the solar wind. Low-altitude bridge crossings are associated with upward-dominated, broadband electron distributions, consistent with Zone-II aurorae, as well as with plasma-wave emission observed by Juno-Waves, in agreement with existing theoretical models for the generation of polar-region aurorae. Main-emission crossings where no bridges are visible also show characteristics associated with bridges (more upward electron flux, plasma-wave emission), which is not the case for main-emission crossings with visible bridges, as though bridges remain present but spatially indistinguishable from the main emission in the former case. In all, compression of the magnetosphere may work to spatially separate the Zone-I and Zone-II regions of the main emission, in the form of Zone-II bridges.

physics.space-ph↗

arXiv2025

Jupiter's UV auroral response to a magnetospheric compression event

The highly elliptical polar orbit of the Juno mission provides a unique opportunity to simultaneously measure the compression state of Jupiter's magnetosphere and the total power emitted by the planet's ultraviolet aurora, using a single spacecraft. This allows us to study how Jupiter's aurora respond to a compression event. In this paper, we present a case study of an extreme compression event that occurred on December 6-7 2022 when Juno was a distance of 70 R$_{J}$ from Jupiter. This extreme compression was accompanied by a very large increase in the ultraviolet auroral emissions to 12 TW, a factor of six higher than the baseline level. This event coincided with the predicted arrival of a powerful interplanetary shock, which was expected to cause the largest increase in the solar wind dynamic pressure seen thus far during the Juno mission. The simultaneous occurrence of the interplanetary shock, the extreme compression and the bright ultraviolet aurora suggests that in this case, the auroral brightening was caused by the solar wind shock compressing the magnetosphere.

astro-ph.EP↗

arXiv2024

Properties of electrons accelerated by the Ganymede-magnetosphere interaction: survey of Juno high-latitude observations

The encounter between the Jovian co-rotating plasma and Ganymede gives rise to electromagnetic waves that propagate along the magnetic field lines and accelerate particles by resonant or non-resonant wave-particle interaction. They ultimately precipitate into Jupiter's atmosphere and trigger auroral emissions. In this study, we use Juno/JADE, Juno/UVS data, and magnetic field line tracing to characterize the properties of electrons accelerated by the Ganymede-magnetosphere interaction in the far-field region. We show that the precipitating energy flux exhibits an exponential decay as a function of downtail distance from the moon, with an e-folding value of 29°, consistent with previous UV observations from the Hubble Space Telescope (HST). We characterize the electron energy distributions and show that two distributions exist. Electrons creating the Main Alfvén Wing (MAW) spot and the auroral tail always have broadband distribution and a mean characteristic energy of 2.2 keV while in the region connected to the Transhemispheric Electron Beam (TEB) spot the electrons are distributed non-monotonically, with a higher characteristic energy above 10 keV. Based on the observation of bidirectional electron beams, we suggest that Juno was located within the acceleration region during the 11 observations reported. We thus estimate that the acceleration region is extended, at least, between an altitude of 0.5 and 1.3 Jupiter radius above the 1-bar surface. Finally, we estimate the size of the interaction region in the Ganymede orbital plane using far-field measurements. These observations provide important insights for the study of particle acceleration processes involved in moon-magnetosphere interactions.

astro-ph.EP↗

arXiv2023

Catalog of Ultraviolet Bright Stars (CUBS): Strategies for UV occultation measurements, planetary illumination modeling, and sky map analyses using hybrid IUE-Kurucz spectra

Ultraviolet spectroscopy is a powerful method to study planetary surface composition through reflectance measurements and atmospheric composition through stellar/solar occultations, transits of other planetary bodies, and direct imaging of airglow and auroral emissions. The next generation of ultraviolet spectrographs (UVS) on board ESA's JUICE (Jupiter Icy Moons Explorer) and NASA's Europa Clipper missions will perform such measurements of Jupiter and its moons in the early 2030's. This work presents a compilation of a detailed UV stellar catalog, named CUBS, of targets with high intensity in the 50-210 nm wavelength range with applications relevant to planetary spectroscopy. These applications include: 1) Planning and simulating occultations, including calibration measurements; 2) Modeling starlight illumination of dark, nightside planetary surfaces primarily lit by the sky; and 3) Studying the origin of diffuse galactic UV light as mapped by existing datasets from Juno-UVS and others. CUBS includes information drawn from resources such as the International Ultraviolet Explorer (IUE) catalog and SIMBAD. We have constructed model spectra at 0.1 nm resolution for almost 90,000 targets using Kurucz models and, when available, IUE spectra. CUBS also includes robust checks for agreement between the Kurucz models and the IUE data. We also present a tool for which our catalog can be used to identify the best candidates for stellar occultation observations, with applications for any UV instrument. We report on our methods for producing CUBS and discuss plans for its implementation during ongoing and upcoming planetary missions.

astro-ph.EP↗

arXiv2022

Alternating north-south brightness ratio of Ganymede's auroral ovals: Hubble Space Telescope observations around the Juno PJ34 flyby

We report results of Hubble Space Telescope observations from Ganymede's orbitally trailing side which were taken around the flyby of the Juno spacecraft on June 7, 2021. We find that Ganymede's northern and southern auroral ovals alternate in brightness such that the oval facing Jupiter's magnetospheric plasma sheet is brighter than the other one. This suggests that the generator that powers Ganymede's aurora is the momentum of the Jovian plasma sheet north and south of Ganymede's magnetosphere. Magnetic coupling of Ganymede to the plasma sheet above and below the moon causes asymmetric magnetic stresses and electromagnetic energy fluxes ultimately powering the auroral acceleration process. No clear statistically significant time variability of the auroral emission on short time scales of 100s could be resolved. We show that electron energy fluxes of several tens of mW m$^{-2}$ are required for its OI 1356 Å$\;$ emission making Ganymede a very poor auroral emitter.

astro-ph.EP↗

arXiv2021

Mapping the zonal winds of Jupiter's stratospheric equatorial oscillation

Since the 1950s, quasi-periodic oscillations have been studied in the terrestrial equatorial stratosphere. Other planets of the solar system present (or are expected to present) such oscillations, like the Jupiter Equatorial Oscillation(JEO) and the Saturn Semi-Annual Oscillation (SSAO). In Jupiter's stratosphere, the equatorial oscillation of its relative temperature structure about the equator, is characterized by a quasi-period of 4.4 years. The stratospheric wind field in Jupiter's equatorial zone has never been directly observed. In this paper, we aim at mapping the absolute wind speeds in Jupiter's equatorial stratosphere to quantify vertical and horizontal wind and temperature shear. Assuming geostrophic equilibrium, we apply the thermal wind balance using nearly simultaneous stratospheric temperature measurements between 0.1 and 30 mbar performed with Gemini/TEXES and direct zonal wind measurements derived at 1 mbar from ALMA observations, all carried out between March 14th and 22nd, 2017. We are thus able to calculate self-consistently the zonal wind field in Jupiter's stratosphere where the JEO occurs. We obtain stratospheric map of the zonal wind speeds as a function of latitude and pressure about Jupiter's equator for the first time. The winds are vertically layered with successive eastward and westward jets. We find a 200 m/s westward jet at 4 mbar at the equator, with a typical longitudinal variability on the order of ~50 m/s. By extending our wind calculations to the upper troposphere, we find a wind structure qualitatively close to the wind observed using cloud-tracking techniques. Nearly simultaneous temperature and wind measurements, both in the stratosphere, are a powerful tool for future investigations of the JEO (and other planetary equatorial oscillations) and its temporal evolution.

astro-ph.EP↗

arXiv2021

Multi-frequency high spectral resolution observations of HCN toward the circumstellar envelope of Y CVn

High spectral resolution observations toward the low mass-loss rate C-rich, J-type AGB star Y CVn have been carried out at 7.5, 13.1 and 14.0 um with SOFIA/EXES and IRTF/TEXES. Around 130 HCN and H13CN lines of bands v2, 2v2, 2v2-v2, 3v2-2v2, 3v2-v2, and 4v2-2v2 have been identified involving lower levels with energies up to ~3900 K. These lines have been complemented with the pure rotational lines J=1-0 and 3-2 of the vibrational states up to 2v2 acquired with the IRAM 30 m telescope, and with the continuum taken with ISO. We have analyzed the data with a ro-vibrational diagram and a code which models the absorption and emission of the circumstellar envelope of an AGB star. The continuum is produced by the star with a small contribution from dust grains comprising warm to hot SiC and cold amorphous carbon. The HCN abundance distribution seems to be anisotropic. The ejected gas is accelerated up to the terminal velocity (~8 km/s) from the photosphere to ~3R* but there is evidence of higher velocities (>9-10 km/s) beyond this region. In the vicinity of Y CVn, the line widths are as high as ~10 km/s, which implies a maximum turbulent velocity of 6 km/s or the existence of other physical mechanisms probably related to matter ejection that involve higher gas expansion velocities than expected. HCN is rotationally and vibrationally out of LTE throughout the whole envelope. A difference of about 1500 K in the rotational temperature at the photosphere is needed to explain the observations at 7.5 and 13-14 um. Our analysis finds a total HCN column density that ranges from ~2.1E+18 to 3.5E+18 cm^{-2}, an abundance with respect to H2 of 3.5E-05 to 1.3E-04, and a 12C/13C isotopic ratio of ~2.5 throughout the whole envelope.

astro-ph.SR↗

arXiv2021

First direct measurement of auroral and equatorial jets in the stratosphere of Jupiter

Context. The tropospheric wind pattern in Jupiter consists of alternating prograde and retrograde zonal jets with typical velocities of up to 100 m/s around the equator. At much higher altitudes, in the ionosphere, strong auroral jets have been discovered with velocities of 1-2 km/s. There is no such direct measurement in the stratosphere of the planet. Aims. In this paper, we bridge the altitude gap between these measurements by directly measuring the wind speeds in Jupiter's stratosphere. Methods. We use the Atacama Large Millimeter/submillimeter Array's very high spectral and angular resolution imaging of the stratosphere of Jupiter to retrieve the wind speeds as a function of latitude by fitting the Doppler shifts induced by the winds on the spectral lines. Results. We detect for the first time equatorial zonal jets that reside at 1 mbar, i.e. above the altitudes where Jupiter's Quasi-Quadrennial Oscillation occurs. Most noticeably, we find 300-400 m/s non-zonal winds at 0.1 mbar over the polar regions underneath the main auroral ovals. They are in counter-rotation and lie several hundreds of kilometers below the ionospheric auroral winds. We suspect them to be the lower tail of the ionospheric auroral winds. Conclusions. We detect directly and for the first time strong winds in Jupiter's stratosphere. They are zonal at low-to-mid latitudes and non-zonal at polar latitudes. The wind system found at polar latitudes may help increase the effciency of chemical complexification by confining the photochemical products in a region of large energetic electron precipitation.

astro-ph.EP↗

arXiv2020

A stringent upper limit of the PH$_3$ abundance at the cloud top of Venus

Following the announcement of the detection of phosphine (PH$_3$) in the cloud deck of Venus at millimeter wavelengths, we have searched for other possible signatures of this molecule in the infrared range. Since 2012, we have been observing Venus in the thermal infrared at various wavelengths to monitor the behavior of SO$_2$ and H$_2$O at the cloud top. We have identified a spectral interval recorded in March 2015 around 950 cm$^{-1}$ where a PH$_3$ transition is present. From the absence of any feature at this frequency, we derive, on the disk-integrated spectrum, a 3-$σ$ upper limit of 5 ppbv for the PH$_3$ mixing ratio, assumed to be constant throughout the atmosphere. This limit is 4 times lower than the disk-integrated mixing ratio derived at millimeter wavelengths. Our result brings a strong constraint on the maximum PH$_3$ abundance at the cloud top and in the lower mesosphere of Venus.

astro-ph.EP↗

arXiv2020

Jupiter's Equatorial Plumes and Hot Spots: Spectral Mapping from Gemini/TEXES and Juno/MWR

We present multi-wavelength measurements of the thermal, chemical, and cloud contrasts associated with the visibly dark formations (also known as 5-$μ$m hot spots) and intervening bright plumes on the boundary between Jupiter's Equatorial Zone (EZ) and North Equatorial Belt (NEB). Observations made by the TEXES 5-20 $μ$m spectrometer at the Gemini North Telescope in March 2017 reveal the upper-tropospheric properties of 12 hot spots, which are directly compared to measurements by Juno using the Microwave Radiometer (MWR), JIRAM at 5 $μ$m, and JunoCam visible images. MWR and thermal-infrared spectroscopic results are consistent near 0.7 bar. Mid-infrared-derived aerosol opacity is consistent with that inferred from visible-albedo and 5-$μ$m opacity maps. Aerosol contrasts, the defining characteristics of the cloudy plumes and aerosol-depleted hot spots, are not a good proxy for microwave brightness. The hot spots are neither uniformly warmer nor ammonia-depleted compared to their surroundings at $p<1$ bar. At 0.7 bar, the microwave brightness at the edges of hot spots is comparable to other features within the NEB. Conversely, hot spots are brighter at 1.5 bar, signifying either warm temperatures and/or depleted NH$_3$ at depth. Temperatures and ammonia are spatially variable within the hot spots, so the precise location of the observations matters to their interpretation. Reflective plumes sometimes have enhanced NH$_3$, cold temperatures, and elevated aerosol opacity, but each plume appears different. Neither plumes nor hot spots had microwave signatures in channels sensing $p>10$ bars, suggesting that the hot-spot/plume wave is a relatively shallow feature.

astro-ph.EP↗

arXiv2017

Circumstellar ammonia in oxygen-rich evolved stars

The circumstellar ammonia (NH$_3$) chemistry in evolved stars is poorly understood. Previous observations and modelling showed that NH$_3$ abundance in oxygen-rich stars is several orders of magnitude above that predicted by equilibrium chemistry. In this article, we characterise the spatial distribution and excitation of NH$_3$ in the O-rich circumstellar envelopes (CSEs) of four diverse targets: IK Tau, VY CMa, OH 231.8+4.2, and IRC +10420 with multi-wavelength observations. We observed the 1.3-cm inversion line emission with the Very Large Array (VLA) and submillimetre rotational line emission with the Heterodyne Instrument for the Far-Infrared (HIFI) aboard Herschel from all four targets. For IK Tau and VY CMa, we observed the rovibrational absorption lines in the $ν_2$ band near 10.5 $μ$m with the Texas Echelon Cross Echelle Spectrograph (TEXES) at the NASA Infrared Telescope Facility (IRTF). We also attempted to search for the rotational transition within the $v_2=1$ state near 2 mm with the IRAM 30m Telescope towards IK Tau. Non-LTE radiative transfer modelling, including radiative pumping to the vibrational state, was carried out to derive the radial distribution of NH$_3$ in these CSEs. Our modelling shows that the NH$_3$ abundance relative to molecular hydrogen is generally of the order of $10^{-7}$, which is a few times lower than previous estimates that were made without considering radiative pumping and is at least 10 times higher than that in the C-rich CSE of IRC +10216. Incidentally, we also derived a new period of IK Tau from its $V$-band light curve. NH$_3$ is again detected in very high abundance in O-rich CSEs. Its emission mainly arises from localised spatial-kinematic structures that are probably denser than the ambient gas. Circumstellar shocks in the accelerated wind may contribute to the production of NH$_3$. (Abridged abstract)

astro-ph.SR↗

arXiv2017

Jupiter's North Equatorial Belt expansion and thermal wave activity ahead of Juno's arrival

The dark colors of Jupiter's North Equatorial Belt (NEB, $7-17^\circ$N) appeared to expand northward into the neighboring zone in 2015, consistent with a 3-5 year cycle of activity in the NEB. Inversions of thermal-IR imaging from the Very Large Telescope revealed a moderate warming and reduction of aerosol opacity at the cloud tops at $17-20^\circ$N, suggesting subsidence and drying in the expanded sector. Two new thermal waves were identified during this period: (i) an upper tropospheric thermal wave (wavenumber 16-17, amplitude 2.5 K at 170 mbar) in the mid-NEB that was anti-correlated with haze reflectivity; and (ii) a stratospheric wave (wavenumber 13-14, amplitude 7.3 K at 5 mbar) at $20-30^\circ$N. Both were quasi-stationary, confined to regions of eastward zonal flow, and are morphologically similar to waves observed during previous expansion events.

astro-ph.EP↗

arXiv2017

Mid-Infrared Mapping of Jupiter's Temperatures, Aerosol Opacity and Chemical Distributions with IRTF/TEXES

Global maps of Jupiter's atmospheric temperatures, gaseous composition and aerosol opacity are derived from a programme of 5-20 $μ$m mid-infrared spectroscopic observations using the Texas Echelon Cross Echelle Spectrograph (TEXES) on NASA's Infrared Telescope Facility (IRTF). Image cubes from December 2014 in eight spectral channels, with spectral resolutions of $R\sim2000-12000$ and spatial resolutions of $2-4^\circ$ latitude, are inverted to generate 3D maps of tropospheric and stratospheric temperatures, 2D maps of upper tropospheric aerosols, phosphine and ammonia, and 2D maps of stratospheric ethane and acetylene. The results are compared to a re-analysis of Cassini Composite Infrared Spectrometer (CIRS) observations acquired during Cassini's closest approach to Jupiter in December 2000, demonstrating that this new archive of ground-based mapping spectroscopy can match and surpass the quality of previous investigations, and will permit future studies of Jupiter's evolving atmosphere. We identify mid-infrared signatures of Jupiter's 5-$μ$m hotspots via simultaneous M, N and Q-band observations, which are interpreted as temperature and ammonia variations in the northern Equatorial Zone and on the edge of the North Equatorial Belt (NEB). Equatorial plumes enriched in NH$_3$ gas are located south-east of NH$_3$-desiccated `hotspots' on the edge of the NEB. Comparison of the hotspot locations in several channels across the 5-20 $μ$m range indicate that these anomalous regions tilt westward with altitude. Aerosols and PH$_3$ are both enriched at the equator but are not co-located with the NH$_3$ plumes. We find hemispheric asymmetries in the distribution of tropospheric PH$_3$, stratospheric hydrocarbons and the 2D wind field. Jupiter's stratosphere is notably warmer at northern mid-latitudes than in the south in both 2000 and 2014. [Abridged]

astro-ph.EP↗

arXiv2015

2D photochemical modeling of Saturn's stratosphere. Part II: Feedback between composition and temperature

Saturn's axial tilt produces seasons in a similar way as on Earth. Both the stratospheric temperature and composition are affected by this latitudinally varying insolation along the seasons. The thermal structure is controlled and regulated by the amount of hydrocarbons in the stratosphere, which act as absorbers and coolants from the UV to the far-IR spectral range, and this structure influences the amount of hydrocarbons. We study here the feedback between the chemical composition and the thermal structure by coupling a latitudinal and seasonal photochemical model with a radiative seasonal model. Our results show that the seasonal temperature peak in the higher stratosphere, associated with the seasonal increase of insolation, is shifted earlier than the maximum insolation peak. This shift is increased with increasing latitudes and is caused by the low amount of stratospheric coolants in the spring season. At 80$^{\circ}$ in both hemispheres, the temperature peak at 1d-2mbar is seen to occur half a season earlier than was previously predicted by radiative seasonal models that assumed spatially and temporally uniform distribution of coolants. This shift progressively decreases with increasing pressure, up to around the 0.5mbar pressure level where it vanishes. However, the thermal field has a small feedback on the abundance distributions. This feedback modifies the predicted equator-to-pole temperature gradient. The meridional gradients of temperature at the mbar pressure levels are better reproduced when this feedback is accounted for. At lower pressure levels, the thermal structure seems to depart from pure radiative seasonal equilibrium as previously suggested by Guerlet et al. (2014). Although the agreement with the absolute value of the stratospheric temperature observed by Cassini is moderate, it is a mandatory step toward a fully coupled GCM-photochemical model.

astro-ph.EP↗

arXiv2014

Systematic trend of water vapour absorption in red giant atmospheres revealed by high resolution TEXES 12 micron spectra

The structures of the outer atmospheres of red giants are very complex. The notion of large optically thick molecular spheres around the stars (MOLspheres) has been invoked in order to explain e.g. spectro-interferometric observations. However, high-resolution spectra in the mid-IR do not easily fit into this picture. They rule out any large sphere of water vapour in LTE surrounding red giants. Our aim here is to investigate high-resolution, mid-infrared spectra for a range of red giants, from early-K to mid M. We have recorded 12 microns spectra of 10 well-studied bright red giants, with TEXES on the IRTF. We find that all giants in our study cooler than 4300 K, spanning a range of effective temperatures, show water absorption lines stronger than expected. The strengths of the lines vary smoothly with spectral type. We identify several spectral features in the wavelength region that undoubtedly are formed in the photosphere. From a study of water-line ratios of the stars, we find that the excitation temperatures, in the line-forming regions, are several hundred Kelvin lower than expected from a classical photospheric model. This could either be due to an actually lower temperature structure in the outer regions of the photospheres caused by, for example, extra cooling, or due to non-LTE level populations, affecting the source function and line opacities. We have demonstrated that these diagnostically interesting water lines are a general feature of red giants across spectral types, and we argue for a general explanation of their formation rather than explanations requiring specific properties. Since the water lines are neither weak (filled in by emission) nor appear in emission, as predicted by LTE MOLsphere models in their simplest forms, the evidence for the existence of such large optically-thick, molecular spheres enshrouding the stars is weakened. (abbreviated)

astro-ph.SR↗

arXiv2009

TEXES Observations of M Supergiants: Dynamics and Thermodynamics of Wind Acceleration

We have detected [Fe II] 17.94 um and 24.52 um emission from a sample of M supergiants using TEXES on the IRTF. These low opacity emission lines are resolved at R = 50, 000 and provide new diagnostics of the dynamics and thermodynamics of the stellar wind acceleration zone. The [Fe II] lines, from the first excited term, are sensitive to the warm plasma where energy is deposited into the extended atmosphere to form the chromosphere and wind outflow. These diagnostics complement previous KAO and ISO observations which were sensitive to the cooler and more extended circumstellar envelopes. The turbulent velocities, Vturb is about 12 to 13 km/s, observed in the [Fe II] forbidden lines are found to be a common property of our sample, and are less than that derived from the hotter chromospheric C II] 2325 Angstrom lines observed in alpha Ori, where Vturb is about 17 to 19 km/s. For the first time, we have dynamically resolved the motions of the dominant cool atmospheric component discovered in alpha Ori from multi-wavelength radio interferometry by Lim et al. (1998). Surprisingly, the emission centroids are quite Gaussian and at rest with respect to the M supergiants. These constraints combined with model calculations of the infrared emission line fluxes for alpha Ori imply that the warm material has a low outflow velocity and is located close to the star. We have also detected narrow [Fe I] 24.04 um emission that confirms that Fe II is the dominant ionization state in alpha Ori's extended atmosphere.

astro-ph.SR↗

arXiv2008

The TEXES Survey For H2 Emission From Protoplanetary Disks

We report the results of a search for pure rotational molecular hydrogen emission from the circumstellar environments of young stellar objects with disks using the Texas Echelon Cross Echelle Spectrograph (TEXES) on the NASA Infrared Telescope Facility and the Gemini North Observatory. We searched for mid-infrared H2 emission in the S(1), S(2), and S(4) transitions. Keck/NIRSPEC observations of the H2 S(9) transition were included for some sources as an additional constraint on the gas temperature. We detected H2 emission from 6 of 29 sources observed: AB Aur, DoAr 21, Elias 29, GSS 30 IRS 1, GV Tau N, and HL Tau. Four of the six targets with detected emission are class I sources that show evidence for surrounding material in an envelope in addition to a circumstellar disk. In these cases, we show that accretion shock heating is a plausible excitation mechanism. The detected emission lines are narrow (~10 km/s), centered at the stellar velocity, and spatially unresolved at scales of 0.4 arcsec, which is consistent with origin from a disk at radii 10-50 AU from the star. In cases where we detect multiple emission lines, we derive temperatures > 500 K from ~1 M_earth of gas. Our upper limits for the non-detections place upper limits on the amount of H2 gas with T > 500 K of less than a few Earth masses. Such warm gas temperatures are significantly higher than the equilibrium dust temperatures at these radii, suggesting that the gas is decoupled from the dust in the regions we are studying and that processes such as UV, X-ray, and accretion heating may be important.

astro-ph↗