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Context. Highly excited molecular hydrogen H2 has been observed in many regions of shocked molecular gas. A recently published $K$-band spectrum of Herbig-Haro 7 (HH7) contains several vibration-rotation lines of H2 from highly excited energy levels that have not been detected elsewhere, including a line at 2.179 $\mu$m identified as arising from the $v$=2 $J$=29 level, which lies above the dissociation limit of H2. One emission line at 2.104 $\mu$m in this spectrum was unidentified. Aims. We aim to complete the analysis of the spectrum of HH7 by including previously missing molecular data that have been recently computed. Methods. We re-analysed the $K$-band spectrum, emphasising the physics of quasi-bound upper levels that can produce infrared emission lines in the $K$ band. Results. We confirm the identification of the $2-1$ $S$(27) line at 2.1785 $\mu$m and identify the line at 2.1042 $\mu$m as due to the 1-0 $S$(29) transition of H2, whose upper level energy is also higher than the dissociation limit. This latter identification, its column density, and the energy of its upper level further substantiate the existence of a hot thermal component at 5000 K in the HH7 environment.} Conclusion. The presence of the newly identified $1-0$ $S$(29) line, whose quasi-bound upper level ($v$=1, $J$=31) has a significant spontaneous dissociation probability, shows that dissociation of H2 is occurring. The mechanism by which virtually all of the H2 levels with energies from 20,000 K to 53,000 K is maintained in local thermodynamic equilibrium at a single temperature of $\sim$5,000 K remains to be understood., Comment: 5 pages, 3 figures, accepted in Astronomy and Astrophysics Letters 22/12/2022

We present infrared spectroscopy of the 2019 eruption of the recurrent nova V3890 Sgr, obtained over the period 5.1-46.3 days after the eruption. The spectrum of the red giant became more prominent as the flux declined, and by day 46.3 dominated the spectrum. Hydrogen and helium emission lines consisted of a narrow component superposed on a broad pedestal. The full width at half maximum of the narrow components declined with time $t$ as the eruption progressed, as ${t}^{-0.74}$, whereas those of the broad components remained essentially constant. Conversely, the line fluxes of the narrow components of Pa,$\beta$ remained roughly constant, while those of the broad components declined by a factor $\sim30$ over a period of $\lesssim25$~days. The behaviour of the broad components is consistent with them arising in unencumbered fast-flowing ejecta perpendicular to the binary plane, in material that was ejected in a short $\sim3.3$-day burst. The narrow components arise in material that encounters the accumulated circumstellar material. The outburst spectra were rich in coronal lines. There were two coronal line phases, one that originated in gas ionised by supersoft X-ray source, the other in shocked gas. From the relative fluxes of silicon and sulphur coronal lines on day 23.4 - when the emitting gas was shocked - we deduce that the temperature of the coronal gas was $9.3\times10^5$~K, and that the abundances are approximately solar., Comment: Accepted for publication in MNRAS on 2022 August 1. 19 pages, 10 figures

R Coronae Borealis (RCB) and dustless Hydrogen-deficient Carbon (dLHdC) stars are believed to be remnants of low mass white dwarf mergers. These supergiant stars have peculiar hydrogen-deficient carbon-rich chemistries and stark overabundances of $^{18}$O. RCB stars undergo dust formation episodes resulting in large-amplitude photometric variations that are not seen in dLHdC stars. Recently, the sample of known dLHdC stars in the Milky Way has more than quintupled with the discovery of 27 new dLHdC stars. We present medium resolution (R$\approx3000$) near-infrared spectra of 20 newly discovered dLHdC stars. We confirm that unlike RCB stars, dLHdC stars do not show strong blueshifted ($>200$ km s$^{-1}$) He I 1.0833 $μ$m absorption, suggesting the absence of strong, dust-driven winds around them. We also present medium resolution (R$\approx3000-8000$) $K$-band spectra for 47 RCB stars. We measure the $^{16}$O/$^{18}$O ratios of 7 dLHdC and 31 RCB stars that show $^{12}$C$^{16}$O and $^{12}$C$^{18}$O absorption bands, and present the largest sample of values of $^{16}$O/$^{18}$O for dLHdC and RCB stars to date. We find that six of the seven dLHdC stars have $^{16}$O/$^{18}$O $1$. We conclude that most dLHdC stars have lower $^{16}$O/$^{18}$O than most RCB stars. This confirms one of the first spectroscopic differences between RCB and dLHdC stars. Our results rule out the existing picture that RCB stars represent an evolved stage of dLHdC stars. Instead, we suggest that whether the white dwarf merger remnant is a dLHdC or RCB star depends on the mass ratios, masses and compositions of the merging white dwarfs., Submitted to A&A, 9 pages, 4 figures

We present a detailed study of the 2019 outburst of the cataclysmic variable V1047~Cen, which hosted a classical nova eruption in 2005. The peculiar outburst occurred 14 years after the classical nova event and lasted for more than 400 days, reaching an amplitude of around 6 magnitudes in the optical. Early spectral follow-up revealed what could be a dwarf nova (accretion disk instability) outburst. However, the outburst duration, high velocity ($>$2000\,km\,s$^{-1}$) features in the optical line profiles, luminous optical emission, and presence of prominent long-lasting radio emission together suggest a phenomenon more exotic and energetic than a dwarf nova outburst. The outburst amplitude, radiated energy, and spectral evolution are also not consistent with a classical nova eruption. There are similarities between V1047~Cen's 2019 outburst and those of classical symbiotic stars, but pre-2005 images of the field of V1047~Cen indicate that the system likely hosts a dwarf companion, implying a typical cataclysmic variable system. Based on our multi-wavelength observations, we suggest that the outburst may have started with a brightening of the disk due to enhanced mass transfer or disk instability, possibly leading to enhanced nuclear shell burning on the white dwarf, which was already experiencing some level of quasi-steady shell burning. This eventually led to the generation of a wind and/or bipolar, collimated outflows. The 2019 outburst of V1047~Cen appears to be unique, and nothing similar has been observed in a typical cataclysmic variable system before, hinting at a potentially new astrophysical phenomenon., 36 pages, 24 figures, 9 tables. Accepted in ApJ

We describe and discuss remarkable infrared spectra, covering key portions of the $2-5$ $\mu$m wavelength interval, of the probable OH/IR supergiant 2MASS J17470898$-$2829561 (2M1747), located in direction of the Sgr B molecular cloud complex within the Central Molecular Zone (CMZ) of the Galaxy. This star was originally singled out for examination based on its suitability for spectroscopy of lines of H$_3^+$ in the CMZ. Analysis of the spectra shows that 2M1747 is deeply embedded within Sgr B1, with A$_V$ $\gtrsim$ 100 mag, making it the only star within Sgr B for which infrared spectra have been obtained at present, and thereby a unique infrared probe of the dense interstellar medium within the CMZ. Despite the high extinction, spectra of 2M1747 reveal a veiled photosphere in the $K$ band and circumstellar gas in the $M$ band, giving clues as to its nature. Its $ 3.5-4.0$ $\mu$m spectrum contains the strongest absorption lines of H$_3^+$ observed toward any object to date. The $4.5-4.8$ $\mu$m spectrum has impressively deep and wide absorption lines of interstellar CO, most of which arise in dense gas within Sgr B1. The $3-5$ $\mu$m spectrum also contains several solid state absorption features, which are characteristic of both dense and diffuse clouds, and which raise questions about the identifications of some of these features. We discuss the nature of the star, the extinction to it, the extinction law for dust in the CMZ, and the identifications of the various solid-state features and where they are produced along this complex line of sight., Comment: 17 pages, 10 figures; accepted by ApJ 2021 March 10

We present near infrared spectroscopy of Nova Herculis 2021 (V1674 Her), obtained over the first 70 days of its evolution. This fastest nova on record displays a rich emission line spectrum, including strong coronal line emission with complex structures. The hydrogen line fluxes, combined with a distance of 4.7 (+1.3 / -1.0) kpc, give an upper limit to the hydrogen ejected mass of 1.4 (+0.8 / -1.2) 10^{-3} solar masses. The coronal lines appeared at day 11.5, the earliest onset yet observed for any classical nova, before there was an obvious source of ionizing radiation. We argue that the gas cannot be photoionized, at least in the earliest phase, and must be shocked. Its temperature is estimated to be 10^{5.57 +/- 0.05} K on day 11.5. Tentative analysis indicates a solar abundance of aluminum and an underabundance of calcium, relative to silicon, with respect to solar values in the ejecta. Further, we show that the vexing problem of whether collisional or photoionization is responsible for coronal emission in classical novae can be resolved by correlating the temporal sequence in which the X-ray supersoft phase and the near-infrared coronal line emission appear., 8 pages, 3 figures, 2 tables

The molecular gas in the Central Molecular Zone (CMZ) of the Galaxy has been studied using infrared absorption spectra of H$_3^+$ lines at 3.5-4.0 $\mu$m and CO lines near 2.34 $\mu$m. In addition to the previously reported spectra of these lines toward 8 stars located within 30 pc of Sgr A$^\ast$, there are now spectra toward $\sim$30 bright stars located from 140 pc west to 120 pc east of Sgr A$^\ast$. The spectra show the presence of warm ($T\sim 200$ K) and diffuse ($n < 100 $cm$^{-3}$) gas with $N$(H$_3^+$) $\sim 3 \times 10^{15} $cm$^{-2}$ on majority of sightlines. Instead of our previous analysis in which only electrons from photoionization of carbon atoms were considered, we have developed a simple model calculation in which the cosmic ray ionization of H$_2$ and H are also taken into account. We conclude: (1) Warm and diffuse gas dominates the volume of the CMZ. The volume filling factor of dense gas must be much less than 0.1 and the CMZ is not as opaque as previously considered. The X-ray emitting ultra-hot 10$^8 $K plasma, which some thought to dominate the CMZ, does not exist over extended regions. (2) The cosmic ray ionization rate is $\zeta \sim 2 \times 10^{-14} $s$^{-1}$, higher than in Galactic dense clouds and diffuse clouds by factors of $\sim$1000 and $\sim$100, respectively. If the equipartition law stands, this suggests a pervading magnetic field on the order of $\sim$100 $\mu$G., Comment: 39 pages, 7 tables, 10 figures, published at the ApJ

Spectroscopy of absorption lines of H

We have developed a method for determining elemental Fe-group abundances in planetary nebulae using an infrared emission line of Zn, the least refractory Fe-group species. Many planetary nebulae, particularly those of the Milky Way’s thick disk and bulge, display subsolar [Fe/H] (as inferred from Zn) although their abundances of α elements such as O, S, and Ar are nearly solar. We discuss the implications for determining enhancements of species synthesized by the progenitor star during the AGB (e.g.,s-process products), and for galactic chemical evolution in view of the metallicity dependence of AGB nucleosynthetic yields.

Velocity profiles of a line of H$_3^+$ at 3.7 $\mu$m produced in warm diffuse gas have been observed toward 18 stars in the Central Molecular Zone (CMZ) of the Galaxy. Their longitude-velocity diagram indicates that the gas is radially expanding within the CMZ at speeds up to a maximum of $\sim$150 km s$^{-1}$. The current momentum and energy in the gas are $\sim 5 \times 10^8 M_\odot$ km s$^{-1}$ and $\sim 5\times 10^{53}$ erg. The motion is similar to that of the Expanding Molecular Ring (EMR) discovered in 1972 by Kaifu et al. and by Scoville. We propose that the expanding gas seen in H$_3^+$ is part of the same phenomenon, in spite of differences in estimates of density, morphology, and degree of rotation. The outward motion suggests that one or more ejection events occurred near the center of the CMZ (0.5$-$1) $\times$ 10$^6$ years ago, which may be related to creation of the recently observed microwave bubble. These observations revive the circular face-on view of the CMZ proposed in 1972, which fell out of favor after 1991 when Binney et al. proposed that a face-on view of the CMZ would show it to have an elliptical shape, with high eccentricity. While that model may apply on kiloparsec scales, we argue that it is incorrect to apply it to the much smaller CMZ. We discuss the fate of the expanding gas, which appears to be eventual infall into the center, leading to episodes of star formation and violent events associated with accretion onto Sgr~A$^\ast$., Comment: 21 pages incl. 12 figures and 1 table; accepted for publication in The Astrophysical Journal. Minor typos corrected in this version

We briefy describe the results of a K-band spectroscopic survey of over 500 highly reddened point-like objects on sightlines toward the Central Molecular Zone (CMZ) of the Galaxy. The goal was to find stars with featureless or nearly featureless spectra suitable for near- and mid-infrared absorption spectroscopy of the Galactic center's interstellar gas on sightlines spread across the CMZ. Until recently only a few such stars have been known outside of very localized sightlines in the vicinity of the Quintuplet and Central clusters. We have used Spitzer Space Telescope (GLIMPSE) and 2MASS photometry to select promising candidates, and over the last ten years have been acquiring low-resolution K-band spectra of them. As expected, the vast majority are cool and/or highly reddened red giants with complex photospheric spectra unsuitable for measuring faint interstellar lines. Approximately ten percent of them, whose observations are reported here, have featureless or nearly featureless spectra. Although not evenly distributed in Galactic longitude, these stars are scattered across the CMZ. Many of them are luminous stars that are deeply embedded in warm dust cocoons, and have K-band continua rising steeply to longer wavelengths. A significant fraction of them are hot stars of a variety of spectral types, including at least five newly discovered Wolf-Rayet stars. All of them should be suitable for spectroscopy of interstellar absorption lines at infrared wavelengths greater than 3 microns and many are also suitable at shorter wavelengths., 24 pages, 6 figures; ApJ, in press

Fourteen years after its eruption as a classical nova (CN), V1047 Cen (Nova Cen 2005) began an unusual re-brightening in 2019 April. The amplitude of the brightening suggests that this is a dwarf nova (DN) eruption in a CN system. Very few CNe have had DN eruptions within decades of the main CN outburst. The 14 years separating the CN and DN eruptions of V1047 Cen is the shortest of all instances recorded thus far. Explaining this rapid succession of CN and DN outbursts in V1047 Cen may be challenging within the framework of standard theories for DN outbursts. Following a CN eruption, the mass accretion rate is believed to remain high $(\dot{M}\sim10^{-8}$M$_\odot$yr$^{-1})$ for a few centuries, due to the irradiation of the secondary star by the still-hot surface of the white dwarf. Thus a DN eruption is not expected to occur during this high mass accretion phase as DN outbursts, which result from thermal instabilities in the accretion disk, and arise during a regime of low mass accretion rate $(\dot{M}\sim10^{-10}$M$_\odot$yr$^{-1})$. Here we present near-infrared spectroscopy to show that the present outburst is most likely a DN eruption, and discuss the possible reasons for its early occurrence. Even if the present re-brightening is later shown to be due to a cause other than a DN outburst, the present study provides invaluable documentation of this unusual event., Astrophysical Journal Letters, in press. Data in Figure 2 will be available as "Data behind the Figure" in the published version of the paper

Spectroscopy of absorption lines of H 3 + in the central molecular zone (CMZ) of the Galaxy show that a previously largely unknown component of the interstellar medium there, warm ( T ∼200 K) and diffuse ( n ≲ 10 2 cm −3 ) gas, makes up a large fraction of the volume of the CMZ, and that this gas is moving radially outward from the centre. These discoveries upend the generally accepted understanding that the interstellar environment of the CMZ comprises almost entirely an ultra-hot plasma and dense molecular clouds. The radial momentum associated with the diffuse gas in the CMZ exceeds that of the ejecta of thousands of core-collapse supernovae and implies some extraordinary past activity in the centre, possibly associated with the supermassive black hole, Sgr A*. We speculate that within approximately 10 6 years, gravity could halt the expansion of the diffuse gas and that contraction towards the centre could then commence. This article is part of a discussion meeting issue ‘Advances in hydrogen molecular ions: H 3 + , H 5 + and beyond’.

This paper updates the recent discovery of over a dozen new diffuse interstellar bands (DIBs), first in H-band spectra of stars in the Galactic center (GC) and toward stars in the Cygnus OB2 Association. The H-band DIBs, which currently number 15, are the longest wavelength DIBs reported to date and are the first found on sightlines toward the Galactic center. K-band (2.0-2.5 μm) spectra of the GC stars do not reveal additional DIBs. Comparison of the velocity profile of the strongest of the new DIBs in the sightline toward GCS3-2 (in the GC) with that toward Cygnus OB2 No. 9 and also with the broad velocity profiles of H3+ lines toward GCS3-2 confirm that a significant fraction of the diffuse material producing the DIB absorptions on sightlines to the GC is located within the central few hundred parsecs of the Galaxy.

We report the detection of a number of emission lines in the 1.0--2.4~$\mu$m spectra of four of the five bright infrared dust-embedded stars at the center of the Galactic center's Quintuplet Cluster. Spectroscopy of the central stars of these objects is hampered not only by the large interstellar extinction that obscures all objects in the Galactic center, but also by the large amounts of warm circumstellar dust surrounding each of the five. The pinwheel morphologies of the dust observed previously around two of them are indicative of Wolf-Rayet colliding wind binaries; however, infrared spectra of each of the five have until now revealed only dust continua steeply rising to long wavelengths and absorption lines and bands from interstellar gas and dust. The emission lines detected, from ionized carbon and from helium, are broad and confirm that the objects are dusty late-type carbon Wolf-Rayet stars., Comment: 12 pages, 7 figures, 3 tables. Accepted for publication in ApJ

We have obtained K-band spectra at R ~ 5000 and an angular resolution of 03 of a section of the Herbig–Haro 7 (HH7) bow shock, using the Near-Infrared Integral Field Spectrograph at Gemini North. Present in the portion of the data cube corresponding to the brightest part of the bow shock are emission lines of H2 with upper state energies ranging from ~6000 K to the dissociation energy of H2, ~50,000 K. Because of low signal-to-noise ratios, the highest excitation lines cannot be easily seen elsewhere in the observed region. However, excitation temperatures, measured throughout much of the observed region using lines from levels as high as 25,000 K, are a strong function of upper level energy, indicating that the very highest levels are populated throughout. The level populations in the brightest region are well fit by a two-temperature model, with 98.5% of the emitting gas at T = 1800 K and 1.5% at T = 5200 K. The bulk of the H2 line emission in HH7, from the 1800 K gas, has previously been well-modeled by a continuous shock, but the 5200 K cozmponent is inconsistent with standalone standard continuous shock models. We discuss various possible origins for the hot component and suggest that this component is H2 newly reformed on dust grains and then ejected from them, presumably following dissociation of some of the H2 by the shock.

The central 400 parsecs of the Milky Way, a region known as the central molecular zone (CMZ), contains interstellar gas in a wide range of physical environments, from ultra-hot, rarified and highly ionized to warm, dense and molecular. The combination of infrared spectroscopy of and CO is a powerful way to determine the basic properties of molecular interstellar gas, because the abundance ratio of to CO in ‘dense’ clouds is quite different from that in ‘diffuse’ clouds. Moreover, the energy-level structure and the radiative properties of combined with the unusually warm temperatures of molecular gas in the CMZ make a unique probe of the physical conditions there. This paper describes how, using infrared absorption spectroscopy of and CO, it has been discovered that a large fraction of the volume of the CMZ is filled with warm, diffuse and partially molecular gas moving at speeds of up to approximately 200 km s −1 and that the mean cosmic ray ionization rate in the CMZ exceeds by roughly an order of magnitude values found in diffuse molecular clouds elsewhere in the Galaxy.

We present a measurement of metallicity in the Galactic center Quintuplet Cluster made using quantitative spectral analysis of two Luminous Blue Variables (LBVs). The analysis employs line-blanketed NLTE wind/atmosphere models fit to high-resolution near-infrared spectra containing lines of H, HeI, SiII, MgII, and FeII. We are able to break the H/He ratio vs. mass-loss rate degeneracy found in other LBVs and to obtain robust estimates of the He content of both objects. Our results indicate solar iron abundance and roughly twice solar abundance in the alpha-elements. These results are discussed within the framework of recent measurements of oxygen and carbon composition in the nearby Arches Cluster and iron abundances in red giants and supergiants within the central 30 pc of the Galaxy. The relatively large enrichment of alpha-elements with respect to iron is consistent with a history of more nucleosynthesis in high mass stars than the Galactic disk.

We present infrared spectroscopy of the recurrent nova RS Ophiuchi, obtained 11.81, 20.75 and 55.71 days following its 2006 eruption. The spectra are dominated by hydrogen recombination lines, together with HeI, OI and OII lines; the electron temperature of ~10^4 K implied by the recombination spectrum suggests that we are seeing primarily the wind of the red giant, ionized by the ultraviolet flash when RS Oph erupted. However, strong coronal emission lines (i.e. emission from fine structure transitions in ions having high ionization potential) are present in the last spectrum. These imply a temperature of 930000K for the coronal gas; this is in line with x-ray observations of the 2006 eruption. The emission line widths decrease with time in a way that is consistent with the shock model for the x-ray emission., 5 pages, 4 figures, 3 tables. Accepted for publication in MNRAS

We analyze dust features present in the mid-infrared (Spitzer) and recently published L-band (UKIRT) spectra of the infrared galaxy IRAS 08572+3915. The line of sight toward the AGN nucleus crosses a high column density of carbonaceous dust whose characteristic absorption features appear clearly. They provide a real insight into the chemical environment of the diffuse interstellar medium. Thanks to the moderate redshift of IRAS 08572+3915, the wavelength of the aromatic CH stretching mode is free of major telluric lines, and a strong observational constraint of H sp2 /H sp3 ≤0.08 has been determined. This limit clearly shows that the bonding of hydrogen atoms in interstellar hydrogenated amorphous carbon is highly aliphatic. The presence of a broad absorption feature centered at 6.2 μm, probably arising from olefinic/aromatic structures, corresponds to the backbone of this carbonaceous material, which is the major carbon-containing component of the interstellar medium along this line of sight.

Using adaptive optics at the Gemini North telescope we have obtained a K-band spectrum of the star near the center of the luminous Galactic center bowshock IRS8, as well as a spectrum of the bowshock itself. The stellar spectrum contains emission and absorption lines characteristic of an O5-O6 giant or supergiant. The wind from such a star is fully capable of producing the observed bowshock. However, both the early spectral type and the apparently young age of the star, if it is single, mark it as unique among hot stars within one parsec of the center., 17 pages incl. 4 figures, to be published in ApJ 10 Nov. 2006

International audience; We report the detection of numerous absorption lines of the nu1 band of gaseous formaldehyde (H2CO) near 3.6 mum towards the high-mass protostar W33A. This is the first infrared detection of gaseous H2CO in an interstellar cloud. An upper limit toward RAFGL 7009S is also reported. The column density of H2CO detected at 3.6 mum toward W33A is much higher than that measured in large beam millimeter wave observations toward the same source, suggesting that it is concentrated close to the infrared continuum source. The mean temperature of the H2CO, derived from LTE spectral modelling, is ~100 K, which is close to the values found for most other gas phase species observed in absorption and is the expected temperature of the cloud at the dust mantle sublimation interface, implying a large jump in the abundance of gaseous H2CO from the surrounding cold cloud to the warm gas. The ratio of gaseous and solid H2CO column densities is about 0.03 for W33A and less than 0.02 for RAFGL 7009S. We discuss this detection in the context of the two most likely mechanisms for H2CO formation: grain mantle evaporation, and gas phase reactions between species produced by photodissociation of grain mantle evaporants.

The Trifid nebula (M20) is a well-known prominent optical HII region trisected by bands of obscuring dust lanes and excited by an O7.5 star HD 164492A. Previous near-IR, mid-IR and radio continuum observations of the cluster of stars at the center of the Trifid nebula indicated circumstellar disks associated with hot stars with envelopes that are photoionized externally by the UV radiation from the hot central star, HD 164492A. Using WFPC2 of the HST, we present evidence of a resolved proplyd in H$\alpha$ and [SII] line emission from a stellar source emitting cool dust emission. Using UKIRT, an infrared observation of the stellar source with a proplyd indicates a late F -- mid G spectral type. We also note a remarkable complex of filamentary and sheet-like structures which appear to arise from the edge of a protostellar condensation. These observations are consistent with a picture in which the bright massive star HD 164492A is responsible for the photoevaporation of protoplanetary disks of other less-massive members of the cluster as well as the closest protostellar condensation facing the central cluster. Using the evidence for a proplyd, we argue that the massive and intermediate mass members of the cluster, HD 164492C (B6 star) and HD 164492 (Herbig Be star) have disks associated with them.

We report the discovery of a uniquely large excess of 18O in the hydrogen-deficient carbon (HdC) star, HD 137613, based on a spectrum of the first overtone bands of CO at 2.3-2.4 micron in which three strong absorption bands of 12C18O are clearly present. Bands of 12C16O also are present but no bands of 13C16O or 12C17O are seen. We estimate an isotopic ratio 16O/18O ~1. The Solar value of this ratio is ~500. Neither He-core burning nor He-shell flash burning can produce the isotopic ratios of oxygen and carbon observed in HD 137613. However, a remarkable similarity exists between the observed abundances and those found in the outer layers of the broad He-shell of early-AGB stars, soon after the end of He-core burning. It is not known how the outer envelope down to the He-shell could be lost but some mechanism of enhanced mass loss must be involved. HD 137613 may be a post-early-AGB star with the outer layers of the former He-burning shell as its photosphere. The unusual elemental abundances of the HdC stars resemble those of the R Coronae Borealis (RCB) stars, but HdC stars do not produce clouds of dust that produce declines in brightness. None of the other RCB or HdC stars observed shows significant 18O., Comment: 12 pages, 2 figures, ApJ Letters, in press

High-resolution images in the H and K bands obtained by the Gemini North Telescope of the peculiar Galactic center source, IRS 8, reveal a central pointlike object enveloped in a remarkable bow shock, whose apex is located 02 to the northeast. The H-K' color of the bow shock is considerably redder than that of the central star. A United Kingdom Infrared Telescope (UKIRT) K-band spectrum reveals that the combined spectrum of the point source and bow shock is nearly featureless and that no shocked line emission (e.g., from H2) is physically associated with the bow. We interpret the bow as resulting from the interaction of the envelope or wind of the central star of IRS 8 with the extension of the northern arm of Sgr A West and/or the circumnuclear disk, and its emission as coming from radiatively heated, and possibly shock heated, dust.

The growing numbers of brown dwarfs have largely been divided into two new spectral classes, L and T. Subclassification systems for L, based on optical (0.6–1.0 μm) spectra, have existed since 1999, but there is a need for infrared systems for both spectral classes. The differences in the infrared (1.0–2.5 μm) characteristics of L spectra and T spectra observed prior to 2000 are huge, all then-known T dwarfs showing strong methane absorption bands, and all L dwarfs devoid of these bands. However, in the last 2–3 years a significant number of brown dwarfs with infrared spectra in transition between the L and first observed T types have been discovered. The spectra of late L dwarfs and these “transition objects” can be put into a well ordered sequence of increasing H2O and CH4 band strengths and decreasing CO band strengths. Specific infrared spectral indicators have been found that allow the L and T sequences to be linked and the boundary between L and T to be precisely defined. In current T classification schemes the transition objects are defined as early T dwarfs. We recommend that the boundary between the L and T classes be defined as the first appearance of methane absorption just longward of 1.60 μm, in the H band.

Infrared spectroscopy of H3+ toward the Galactic center uncovered the presence of vast amount of diffuse (100 cm−3) and warm gas (250 K) filling the Central Molecular Zone of our Galaxy (CMZ; R 200 K) and moderately dense gas (200 cm−3) local in the Galactic nucleus. The absorption profiles of H3+ R(1,1)l consists of two components. The one is sharp absorption lines in the negative velocity, that represents the diffuse clouds in the foreground Galactic Arms. The other is the trough absorption spanning from −150 to +90 km s−1. The absorption profile of the trough component has detailed structure with several absorption minima, which was not noticed in the previous lower resolution ...

Spectra of the archetypal Type II Seyfert galaxy NGC 1068 in a narrow wavelength interval near 3.7 microns have revealed a weak absorption feature due to two lines of the molecular ion H3+. The observed wavelength of the feature corresponds to velocity of -70 km/s relative to the systemic velocity of the galaxy, implying an outward flow from the nucleus along the line of sight. The absorption by H3+ along with the previously known broad hydrocarbon absorption at 3.4~microns probably are formed in diffuse gas that is in close proximity to the continuum source, i.e. within a few tens of parsecs of the central engine. Based on that conclusion and the measured H3+ absorption velocity and with the assumption of a spherically symmetric wind we estimate a rate of mass outflow from the AGN of ~1 Msun/yr., Comment: Accepted by ApJ 2 Sep 2015

The 2.46um v=1-0 R(5) line of deuterated molecular hydrogen, HD, has been detected in the Orion Peak~1 shock emission region, at a surface brightness of (8.5 +/- 2.1) x 10^(-9) W m^(-2)sr^(-1) over a 6 arcsec^2 area. Comparison of the column density of HD(v=1,J=6) with the column density of HD(v=0,J=6) previously observed from ISO and the H_2 level column densities toward the same region implies that the excitation of HD is similar to that of H_2 for these energy levels, despite much higher spontaneous transition rates for HD. We suggest that this rough equality is caused by the coupling of the HD levels to those of H_2, due to strong reactive collisions, HD + H -> H_2 + D, in warm, partially dissociated gas. The deuterium abundance implied by the combined ISO and UKIRT measurements toward Orion Peak 1 is [D]/[H] = (5.1 +/- 1.9)x10^(-6)., 15 pages, 2 figures, accepted for publication in ApJ

We present 0.6-2.5um, R~400 spectra of twenty-seven cool, low luminosity stars and substellar objects. Based on these and previously published spectra we develop a preliminary spectral classification system for L and T dwarfs. For late L and T types the classification system is based entirely on four spectral indices in the 1-2.5um interval. Two of these indices are derived from water absorption bands at 1.15um and 1.4um, the latter of which shows a smooth increase in depth through the L and T sequences and can be used to classify both spectral types. The other two indices make use of methane absorption features in the H and K bands, with the K band index also applicable to mid to late L dwarfs. Continuum indices shortward of 1um used by previous authors to classify L dwarfs are found to be useful only through mid L subclasses. We employ the 1.5um water index and the 2.2um methane index to complete the L classification through L9.5 and to link the new system with a modified version of the 2MASS ``Color-d'' index. By correlating the depths of the methane and water absorption features, we establish a T spectral sequence from types T0 to T8, based on all four indices, which is a smooth continuation of the L sequence. We reclassify two 2MASS L8 dwarfs as L9 and L9.5 and identify one SDSS object as L9. In the proposed system methane absorption appears in the K band approximately at L8, two subclasses earlier than its appearance in the H band. The L and T spectral classes are distinguished by the absence and presence, respectively, of H band methane absorption., Comment: 40 pages, 14 figures, to be published in Ap.J., Jan 1, 2002

We present ZJHKL'M' photometry of a sample of 58 late-M, L, and T dwarfs, most of which are identified from the Sloan Digital Sky Survey and the Two Micron All-Sky Survey. Near-infrared spectra and spectral classifications for most of this sample are presented in a companion paper by Geballe et al. We derive the luminosities of 18 dwarfs in the sample and the results imply that the effective temperature range for the L dwarfs in our sample is approximately 2200-1300 K and for the T dwarfs 1300-800 K. We obtained new photometric data at the United Kingdom Infrared Telescope for: 42 dwarfs at Z, 34 dwarfs at JHK, 21 dwarfs at L', as well as M' data for two L dwarfs and two T dwarfs. The M' data provide the first accurate photometry for L and T dwarfs in this bandpass - for a T2 and a T5 dwarf, we find K-M'=1.2 and 1.6, respectively. These colors are much bluer than predicted by models suggesting that CO may be more abundant in these objects than expected, as has been found for the T6 dwarf Gl 229B. We also find that K-L' increases monotonically through most of the M, L, and T subclasses, but it is almost constant between types L6 and T5. The degeneracy is probably due to the onset of methane absorption at the blue edge of the L' bandpass. The JHK colors of L dwarfs show significant scatter, suggesting that the fluxes in these bandpasses are sensitive to variations in photospheric dust properties. The H-K colors of the later T dwarfs also show some scatter which we suggest is due to variations in pressure-induced molecular hydrogen opacity, which is sensitive to gravity and metallicity., 33 pages incl. 6 figures, accepted by ApJ August 2001

We have identified a line at 3.625 μm in the planetary nebulae (PNs) NGC 7027 and IC 5117 as the 2D3/2-2D5/2 fine-structure transition of the 3d9 ion Zn+3. Zinc (N = 30) is believed to be made primarily by explosive nucleosynthesis and r-process neutron capture reactions, and because Zn/H closely tracks Fe/H in metal-poor stars and is relatively undepleted into dust in the ISM, Zn/H has been used as a surrogate for Fe/H in the high-redshift universe. However, some isotopes of Zn are also produced as part of the s-process in asymptotic giant branch stars, the progenitors of PNs. Although there is currently no published calculation of the collision strength for [Zn IV] 3.625 μm, for plausible values the observed line fluxes do not require Zn/H greater than solar, suggesting that these two PNs, which have elevated abundances of other s-process elements and C, are not significantly self-enriched in Zn. Consequently, Zn/H determined from emission lines should be a good indicator of elemental Fe/H in PNs, where gas-phase Fe/H values cannot be used directly because of depletion effects, enabling comparisons between nebular and stellar Fe/H abundances. We also report an upper limit on the strength of the 1-0 P(2) line of HeH+ at 3.608 μm in NGC 7027.

We report the discovery of three cool brown dwarfs which fall in the effective temperature gap between the latest L dwarfs currently known, with no methane absorption bands in the 1-2.5um range, and the previously known methane (T) dwarfs, whose spectra are dominated by methane and water. The newly discovered objects were detected as very red objects in the Sloan Digital Sky Survey imaging data and have JHK colors between the red L dwarfs and the blue Gl229B-like T dwarfs. They show both CO and methane absorption in their near-infrared spectra in addition to water, with weaker methane absorption features in the H and K bands than those in all other methane dwarfs reported to date. Due to the presence of methane in these bands, we propose that these objects are early T dwarfs. The three form part of the brown dwarf spectral sequence and fill in the large gap in the overall spectral sequence from the hottest main sequence stars to the coolest methane dwarfs currently known., One 5-page PS file incl. 1 table and 4 figures; accepted by ApJ Letters

Photodissociation regions (PDRs) and shocks give rise to conspicuous emission from rotationally and vibrationally excited molecular hydrogen. This line emission has now been studied with ISO and from the ground in great detail. A remarkable discovery has been that toward the Orion outflow and other shock-excited regions, the H2 level populations show a very high excitation component. We suggest that these high-excitation populations may arise from non-thermal pumping processes, such as H2 formation and high-velocity ion-molecule collision in partially dissociative shocks. In PDRs such as NGC 7023 however, formation pumping is always less important than fluorescent pumping.We furthermore present two HD emission line detections toward Orion Peak 1. This enables the first comparison of the H2 and the HD excitation, which surprisingly turn out to be identical.

We present new NICMOS/HST infrared images and CGS4/UKIRT Br-alpha (4.05 um) spectroscopy of the Pistol Star and its associated nebula, finding strong evidence to support the hypothesis that the Pistol Nebula was ejected from the Pistol Star. The Pa-alpha NICMOS image shows that the nebula completely surrounds the Pistol Star, although the line intensity is much stronger on its northern and western edges. The Br-alpha spectra show the classical ring-like signature of quasi-spherical expansion, with weak blueshifted emission (V_max approx -60 km/s) and strong redshifted emission (V_max approx +10 km/s), where the velocities are with respect to the velocity of the Pistol Star; further, the redshifted emission appears to be "flattened" in the position-velocity diagram. These data suggest that the nebula was ejected from the star several thousand years ago, with a velocity between the current terminal velocity of the stellar wind (95 km/s) and the present expansion velocity of gas in the outer shell of the nebula (60 km/s). The Pa-alpha image reveals several emission-line stars in the region, including two newly-identified emission-line stars north of the Pistol Star with spectral types earlier than WC8 (T_eff > 50,000 K). The presence of these stars, the morphology of the Pa-alpha emission, and the velocity field in the gas suggest that the side of the nebula furthest from us is approaching, and being ionized by, the hot stars of the Quintuplet, and that the highest velocity redshifted gas has been decelerated by winds from the Quintuplet stars. We also discuss the possibility that the nebular gas might be magnetically confined by the ambient magnetic field delineated by the nearby nonthermal filaments., Figure 1 is included as a JPG file. Figure 1 and 2 also available at ftp://quintup.astro.ucla.edu/pistol2/

We report the discovery of the coolest field dwarf yet known, selected as a stellar object with extremely red colors from commissioning imaging data of the Sloan Digital Sky Survey. Its spectrum from 0.8 to 2.5 microns is dominated by strong bands of H_2 O and CH_4. Its spectrum and colors over this range are very similar to those of Gliese 229B, the only other known example of a methane dwarf. It is roughly 1.2 mag fainter than Gliese 229B, implying that it lies at a distance of roughly 10 pc. Such a cool object must have a mass well below the hydrogen-burning limit of 0.08 solar masses, and therefore is a genuine brown dwarf, with a probable mass in the range 0.015-0.06 solar masses for an age range of 0.3-5 Gyr., Submitted to ApJ Letters. 14 pages Latex, with 3 embedded postscript figures

We present new groundbased 3 $\mu$m spectra of 14 young stellar objects with H_2O ice absorption bands. The broad absorption feature at 3.47 $\mu$m was detected toward all objects and its optical depth is correlated with the optical depth of H_2O ice, strengthening an earlier finding. The broad absorption feature at 3.25 $\mu$m was detected toward two more sources and an upper limit is given for a third source. The optical depths of the 3.25 $\mu$m feature obtained to date are better correlated with the optical depth of the refractory silicate dust than with that of H_2O ice. If this trend is confirmed, this would support our proposed identification of the feature as the C--H stretch of aromatic hydrocarbons at low temperature. An absorption feature at 3.53 $\mu$m due to solid methanol was detected for the first time toward MonR2/IRS2, as well as toward W33A and GL 2136. The wavelengths of the CH_3OH features toward W33A, GL 2136, and NGC7538/IRS9 can be fit by CH_3OH-rich ices, while the wavelength of the feature toward MonR2/IRS2 suggests an H_2O-rich ice environment. Solid methanol abundances toward GL 2136, NGC7538/IRS9, and MonR2/IRS2 are 3-5 % relative to H_2O ice. There is an additional narrow absorption feature near 3.47 $\mu$m toward W33A. For the object W51/IRS2, spatially resolved spectra from 2 to 4 $\mu$m indicate that the H_2O ice is located predominantly in front of the eastern component and that the H_2O ice extinction is much deeper than previously estimated. For the object RNO 91, spectra from 2 to 4 $\mu$m reveal stellar (or circumstellar) CO gas absorption and deeper H_2O ice extinction than previously estimated.

New multi-epoch, mid-infrared (8-13 microns) spectrophotometric observations are presented for 30~late-type stars. The observations were collected over a four year period (1994-1997), permitting an investigation of the mid-infrared spectral shape as a function of the pulsation cycle (typically 1-2 years). The spectra of stars with little excess infrared emission and those with carbon-rich dust show the least spectral variability, while stars with evidence for dusty, oxygen-rich envelopes are most likely to show discernible variations in their spectral profile. Most significantly, a large fraction of variable stars with strong 9.7 micron emission features show clear spectral profile changes which repeat from one cycle to the next. The significant sharpening of the silicate feature near maximum light can not be fully explained by heating and cooling of the circumstellar dust shell during the pulsational cycle, suggesting that the dust optical properties themselves must also be varying. In addition, the appearance of a narrow emission feature near the silicate peak for a few stars may require the production of especially ``pure'' silicate dust near maximum light. The general narrowing of the silicate feature observed may reflect the evolution of the pre-existing dirty grains whose surface impurities have been evaporated off when the grain temperature rises preceding maximum light. An improved theory of dust formation which can explain the observed changes in the grain properties around a single, pulsating star may lead to a definitive explanation for the diversity of silicate emission profiles observed amongst oxygen-rich, late-type stars.