3 Micron Ice‐Band Absorption in Young Stellar Objects

We present the results of a mid-infrared (5-16.5m) imaging spectroscopy survey of Young Stellar Objects (YSOs) and their surrounding environment in four low-mass star formation regions: RCrA, Ophiuchi, Serpens and Chamaeleon I. This survey was performed using ISOCAM and its Circular Variable Filters (CVF) and observed 42 YSO candidates: we were able to obtain complete 5-16.5m spectra for 40 of these with a spectral resolving power of=' 40. A number of spectral features were measured, most notably the 9.7m silicate feature, the bending modes of both water and CO2 ices at 6.0 and 15.2m respectively and the well-known unidentified feature at 6.8 m. The strength of the unidentified feature was observed to correlate very well with that of the water ice bending mode and far less strongly with the CO2 ice bending mode. This suggests, in a manner consistent with previous observations, that the carrier of the unidentified feature is a strongly polar ice. Absorption profiles of the bending mode of CO 2 ice are observed to show a significant long wavelength wing, which suggests that a significant fraction of the CO 2 ice observed exists in a polar (H2O-rich) phase. The sources observed in RCrA, Oph and Serpens show similar spectral characteristics, whilst the sources observed in Cha I are somewhat anomalous, predominantly showing silicate emission and little or no absorption due to volatile ices. However this is consistent with previous studies of this region of the Cha I cloud, which contains an unusual cluster of YSOs. From comparisons of the strengths of the water ice and silicate bands we detect an apparent under-abundance of water ice towards the sources in Oph, relative to both RCrA and Serpens. This may be indicative of dierences in chemical composition between the dierent clouds, or may be due to evaporation. Finally the CO2:H2O ice ratios observed towards the sources in Oph show significantly greater scatter than in the other regions, possibly due to varying local conditions around the YSOs in Oph.

We present near-infrared spectropolarimetric data between 0.9 and 4.2 μm for three prototypes of low-mass young stellar objects (YSOs), L1551 IRS 5, HL Tau, and T Tau in the Taurus dark cloud. These sources are in different classes in the standard spectral classification scheme of low-mass YSOs by Lada. The polarization curves of the observed sources show distinct differences. The class I protostar L1551 IRS 5 shows a flat polarization curve with high polarization through the observed wavelengths. It also shows a polarization excess at the 3.1 μm ice band absorption feature. The flat-spectrum HL Tau, which is thought to be in a transient phase from class I to class II, shows a steep decrease of polarization with increasing wavelengths from 1.0 to 2.5 μm, while it shows a flat polarization curve with high polarization in optical wavelengths and a slowly decreasing slope with small polarization in 3-4 μm. The class II source T Tau displays small polarization no more than 2% through the observed wavelengths; the polarization in the shorter wavelengths from optical to 1.3 μm decreases with increasing wavelengths. T Tau also shows an increasing polarization curve in the longer wavelengths over 1.6 μm, which is most likely to come from the infrared companion T Tau S (Kobayashi et al.). The prominent differences of the observed near-infrared polarization curves can be clearly understood in terms of the standard spectral classification scheme of low-mass YSOs. Thus near-infrared spectropolarimetry could serve as a potentially powerful diagnostic of circumstellar material, complementary to the standard spectral classification.

We have obtained low-resolution (λ/Δλ ∼ 160) 3-μm spectra of a selection of field stars lying behind the Taurus dark cloud. Several of these stars have abnormal extinction laws, which are best characterized by A K ≤ 0.27E(J−K) and A V ≤ 5.4 E(J−K). We find a very good correlation between A V and τ 3.05 (the H 2 O ice feature), confirming a previous result, and between A V and τ 3.45 (the long-wavelength wing), yielding the ratios τ 3.05 /A V = 0.059 ± 0.003 and τ 3.45 /A V = 0.0077 ± 0.0005. The threshold extinctions for the H 2 O ice band and the long-wavelength wing are found to be the same, within the uncertainties, at A v0 = 2.6. The fact that these thresholds are the same means that the long-wavelength wing cannot be a result of a highly refractory hydrocarbon residue

We have obtained CO absorption profiles of several young stellar objects (YSOs), spanning a range of mass and luminosity, in order to investigate their ice mantle composition. We present the first detection of CO toward the class I YSO L1489 IRS in the Taurus dark cloud. In general, the CO profiles for YSOs show evidence for both processed and pristine ices in the same line of sight, strong indirect evidence for CO, is suggested in R CrA IRS 7, L1489 IRS, Elias 18, and GL 961E. Toward other sources (R CrA IRS 1, IRS 2, W33A, NGC 7538 IRS 9, Mon R2 IRS 2) CO is present in (nearly) pure form. We propose an evolutionary scenario to explain the chemical diversity of the icy mantles toward these objects.

An absorption feature at 2040 cm-1 (4.90 μm), previously observed only in W33A, has been detected toward two deeply embedded young stellar objects (YSOs), AFGL 989 and Mon R2 IRS 2. Upper limits are reported for several other YSOs and for one object located behind the Taurus Dark Cloud. We attribute this interstellar feature to solid carbonyl sulfide (OCS) embedded in icy grain mantles along the line of sight. As in the case of W33A, the best match of the newly observed features with laboratory spectra of astrophysically relevant mixtures is obtained for traces of OCS in a methanol-rich matrix. This, again, suggests the presence of independent grain mantle populations and, in particular, of a minor fraction of methanol-rich icy grain mantles in which OCS is embedded. From the strength of the absorption feature we deduce OCS column densities and OCS/H2O ratios toward the observed objects. Taking into consideration sulfur chemistry and the origins of solid OCS, we conclude that a major fraction of the elemental sulfur is presently unaccounted for in dense molecular clouds.

Context. HD 85567 is an enigmatic object exhibiting the B[e] phenomenon, i.e. an infrared excess and forbidden emission lines in the optical. The object's evolutionary status is uncertain and there are conflicting claims that it is either a young stellar object or an evolved, interacting binary. Aims. To elucidate the reason for the B[e] behaviour of HD 85567, we have observed it with the VLTI and AMBER. Methods. Our observations were conducted in the K-band with moderate spectral resolution (R~1500, i.e. 200 km/s). The spectrum of HD 85567 exhibits Br gamma and CO overtone bandhead emission. The interferometric data obtained consist of spectrally dispersed visibilities, closure phases and differential phases across these spectral features and the K-band continuum. Results. The closure phase observations do not reveal evidence of asymmetry. The apparent size of HD 85567 in the K-band was determined by fitting the visibilities with a ring model. The best fitting radius, 0.8 +/- 0.3 AU, is relatively small making HD 85567 undersized in comparison to the size-luminosity relationship based on YSOs of low and intermediate luminosity. This has previously been found to be the case for luminous YSOs, and it has been proposed that this is due to the presence of an optically thick gaseous disc. We demonstrate that the differential phase observations over the CO bandhead emission are indeed consistent with the presence of a compact (~1 AU) gaseous disc interior to the dust sublimation radius. Conclusions. The observations reveal no sign of binarity. However, the data do indicate the presence of a gaseous disc interior to the dust sublimation radius. We conclude that the data are consistent with the hypothesis that HD 85567 is a YSO with an optically thick gaseous disc within a larger dust disc that is being photo-evaporated from the outer edge.

A hyperspectral camera (HSC-type Specim IQ) has been applied at the linear plasma device PSI-2 under steady-state conditions. The camera has the capacity of hyperspectral imaging (HSI) with the dimension of a data array 512 × 512 × 204 (x, y, λ) covering the spectral span from 400 to 1000nm with moderate average spectral resolution (FWHM ∼7nm). After radiometric calibration and background/continuum emission subtraction, two main applications of the camera, (i) plasma diagnostics in helium (He) plasmas and (ii) plasma-material interaction studies with tungsten (W) targets in neon (Ne) plasmas, have been carried out. The measurements were complemented by a movable Langmuir double probe system (LP) measuring electron temperature (Te) and electron density (ne) in radial direction r and a fiber-coupled cross-dispersion spectrometer with high spectral resolution (Spectrelle) recording neutral He, W, and Ne emission lines over the full plasma column. (i) Two-dimensional (2D) imaging of Te and ne radial profiles in axial direction z of the He plasma column were for the first time obtained by the regression analysis of Te and ne (from LP) and six He I line ratios (from HSC). The spatially resolved plasma parameters covered in these studies range between Te ∼ 0.8-13.4eV and ne ∼ 0.2 × 1018-3.9 × 1018m-3 and permit a reconstruction of the plasma conditions in PSI-2 in 2D without LP perturbation. (ii) W sputtering was studied in situ in Ne plasmas exposing W target samples (negatively biased at 100V) under perpendicular Ne plasma impact. Simultaneously, the 2D distributions of W (W I line at 429.5nm) in front of the target and the 2D Ne plasma distribution (Ne I line at 703.2nm) were recorded with complete spectral separation as confirmed by the Spectrelle spectrometer. This permits the simultaneous measurement of the neutral W penetration and its angular distribution induced in the sputtering process and of the impinging plasma distribution. The HSI technique offers, despite a few technical drawbacks, such as the moderate spectral resolution and poor time resolution, a new possibility to distinguish multiple emission lines from plasma and impurities and complements the portfolio of existing Optical Emission Spectroscopy techniques, providing a good compromise regarding spectral, spatial, and temporal resolution.

The G333 giant molecular cloud contains a few star clusters and H II regions, plus a number of condensations currently forming stars. We have mapped 13 of these sources with the appearance of young stellar objects (YSOs) with the Spitzer Infrared Spectrograph in the SL, SH, and LH modules (5-36 micron). We use these spectra plus available photometry and images to characterize the YSOs. The spectral energy distributions (SEDs) of all sources peak between 35 and 110 micron, thereby showing their young age. The objects are divided into two groups: YSOs associated with extended emission in IRAC band 2 at 4.5 micron (`outflow sources') and YSOs that have extended emission in all IRAC bands peaking at the longest wavelengths (`red sources'). The two groups of objects have distinctly different spectra: All the YSOs associated with outflows show evidence of massive envelopes surrounding the protostar because the spectra show deep silicate absorption features and absorption by ices at 6.0, 6.8, and 15.2 micron. We identify these YSOs with massive envelopes cool enough to contain ice-coated grains as the `bloated' protostars in the models of Hosokawa et al. All spectral maps show ionized forbidden lines and PAH emission features. For four of the red sources, these lines are concentrated to the centres of the maps, from which we infer that these YSOs are the source of ionizing photons. Both types of objects show evidence of shocks, with most of the outflow sources showing a line of [S I] in the outflows and two of the red sources showing the more highly excited [Ne III] and [S IV] lines in outflow regions at some distance from the YSOs. The 4.5 micron emission seen in the IRAC band 2 images of the outflow sources is not due to H2 lines, which are too faint in the 5-10 micron wavelength region to be as strong as is needed to account for the IRAC band 2 emission.

Normal field stars located behind dense clouds are a valuable resource in interstellar astrophysics, as they provide continua in which to study phenomena such as gas-phase and solid-state absorption features, interstellar extinction, and polarization. This paper reports the results of a search for highly reddened stars behind the Taurus Dark Cloud complex. We use the Two Micron All Sky Survey (2MASS) Point Source Catalog to survey a ~50 deg2 area of the cloud to a limiting magnitude of Ks = 10.0. Photometry in the 1.2-2.2 μm passbands from 2MASS is combined with photometry at longer infrared wavelengths (3.6-12 μm) from the Spitzer Space Telescope and the Infrared Astronomical Satellite to provide effective discrimination between reddened field stars and young stellar objects (YSOs) embedded in the cloud. Our final catalog contains 248 confirmed or probable background field stars, together with estimates of their total visual extinctions, which span the range 2 < AV < 29 mag. We also identify the 2MASS source J04292083+2742074 (IRAS 04262+2735) as a previously unrecognized candidate YSO, based on the presence of infrared emission greatly in excess of that predicted for a normal reddened photosphere at wavelengths >5 μm.

This paper describes a novel optical design for a multi-channel narrow-band imager observing at the Lyman ultraviolet wavelengths. As one of the key instruments onboard the Census of Warm-Hot Intergalactic Medium, Accretion, and Feedback Explorer space mission, the imager is designed to map the neutral hydrogen Lyman-α line emissions from the intergalactic medium and nearby galaxies. The optical design features an instantaneous field of view (FOV) of 20arc min in diameter and a moderate and adjustable spectral resolution of 500-2000. A highly dispersive monochromator system is used to split the incident light into several wavelength channels, each with a full-field imaging capability and adjustable bandwidth, and the ensuing imaging optics form the individual channel images on different areas of the same microchannel plate detector. Channel wavelengths are tunable and a continuous wavelength scanning across the entire band of interest is proposed, which yields spectrally resolved 3D images with moderate spectral resolutions.

We present spectroscopic observations of a sample of 15 embedded young stellar objects (YSOs) in the Large Magellanic Cloud (LMC). These observations were obtained with the Spitzer Infrared Spectrograph (IRS) as part of the SAGE-Spec Legacy program. We analyze the two prominent ice bands in the IRS spectral range: the bending mode of CO2 ice at 15.2 μm and the ice band between 5 and 7 μm that includes contributions from the bending mode of water ice at 6 μm among other ice species. The 5–7 μm band is difficult to identify in our LMC sample due to the conspicuous presence of polycyclic aromatic hydrocarbon emission superimposed onto the ice spectra. We identify water ice in the spectra of two sources; the spectrum of one of those sources also exhibits the 6.8 μm ice feature attributed in the literature to ammonium and methanol. We model the CO2 band in detail, using the combination of laboratory ice profiles available in the literature. We find that a significant fraction (≳50%) of CO2 ice is locked in a water-rich component, consistent with what is observed for Galactic sources. The majority of the sources in the LMC also require a pure-CO2 contribution to the ice profile, evidence of thermal processing. There is a suggestion that CO2 production might be enhanced in the LMC, but the size of the available sample precludes firmer conclusions. We place our results in the context of the star formation environment in the LMC.

An important tracer of the origin and evolution of cometary ices is the comparison with ices found in dense clouds and toward young stellar objects (YSOs). We present a survey of ices in the 2–5 μm spectra of 23 massive YSOs, taken with the NASA InfraRed Telescope Facility SpeX spectrometer. The 4.90 μm absorption band of OCS ice is detected in 20 sight lines, more than 5 times the previously known detections. The absorption profile shows little variation and is consistent with OCS embedded in CH3OH-rich ices, and proton-irradiated H2S or SO2-containing ices. The OCS column densities correlate well with those of CH3OH and OCN−, but not with H2O and apolar CO ice. This association of OCS with CH3OH and OCN− firmly establishes their formation location deep inside dense clouds or protostellar envelopes. The median composition of this ice phase toward massive YSOs, as a percentage of H2O, is CO:CH3OH:OCN−:OCS = 24:20:1.53:0.15. CS, due to its low abundance, is likely not the main precursor to OCS. Sulfurization of CO is likely needed, although the source of this sulfur is not well constrained. Compared to massive YSOs, low-mass YSOs and dense clouds have similar or somewhat lower CO and CH3OH ice abundances, but less OCN− and more apolar CO, while OCS awaits detection. Comets tend to be underabundant in carbon-bearing species, but this does not appear to be the case for OCS, perhaps signalling OCS production in protoplanetary disks.

ABSTRACTMid-infrared (mid-IR) variability in young stellar objects (YSOs) is driven by several physical mechanisms, which produce a variety of amplitudes and light-curve shapes. One of these mechanisms, variable disc accretion, is predicted by models of episodic accretion to drive secular variability, including in the mid-IR. Because the largest accretion bursts are rare, adding new objects to the YSO eruptive variable class aids our understanding of the episodic accretion phenomenon and its possible impact on stellar and planetary formation. A previous analysis of 6.5 yr of NeoWISE light curves (3–5 μm) of ∼7000 nearby YSOs found an increase in the fraction of variability and variability amplitude for objects at younger stages of evolution. To help interpret these light curves, we have obtained low- and high-resolution near-IR spectra of 78 objects from this sample of YSOs. In this work, we present the analysis of nine nearby YSOs (d&amp;lt;1 kpc) that show the characteristics of known classes of eruptive variable YSOs. We find one FU Orionis (FUor)-like source, one EX Lupi type object, and six YSOs with mixed characteristics or V1647 Ori like objects. The varied characteristics observed in our sample are consistent with recent discoveries of eruptive YSOs. We discuss how a wide range in YSO outburst parameters (central mass, maximum accretion rate during outburst, evolutionary stage, and/or instability leading to the outburst) may play a significant role in the observed spectrophotometric properties of YSO outbursts.

Context. The James Webb Space Telescope provides unprecedented information about ices in protoplanetary disks. However, the saturation of ice bands in highly inclined disks hinders the measurement of ice abundances via classical spectroscopy. This is unfortunate as the presence and, more importantly, the abundance of ices play a key role in, for example, the evolution of dust (because it modifies the sticking properties) and the composition of planetesimals and exoplanetary atmospheres. Aims. To overcome this issue and quantify the ice abundance within disks, we introduce a new method based on measuring the changes in the apparent disk thickness as a function of wavelength, which is directly and quantitatively related to the grain opacity. Specifically, we expect (i) that the increased opacity within ice bands results in a thicker disk than in the adjacent continuum, and (ii) the thickness variations to be proportional to the abundance of ice. Methods. We extracted the disk thickness from model images of edge-on disks containing different abundances of water ice as well as from James Webb Space Telescope spectral imaging of four edge-on disks. Results. For both models and observations, the disk thickness decreases toward longer wavelengths except at the positions of ice absorption features, where the thickness is greater across the band. In the model images, we demonstrate that this effect increases with ice abundance without any hint of saturation. This demonstrates that ice species are present within each disk and confirms our expectation that this method can be applied to estimate ice abundances. Conclusions. Thanks to this method, it will thus be possible to constrain the abundance of ice in highly inclined disks with disk model fitting. Unlike spectroscopic analyses, this method is not subject to saturation and should therefore be more robust and applicable to all disks for which the two surfaces can be resolved.

We present Spitzer Space Telescope IRAC and MIPS observations of a 0.85 deg^2 field including the Corona Australis (CrA) star-forming region. At a distance of 130 pc, CrA is one of the closest regions known to be actively forming stars, particularly within its embedded association, the Coronet. Using the Spitzer data, we identify 51 young stellar objects (YSOs) in CrA which include sources in the well-studied Coronet cluster as well as distributed throughout the molecular cloud. Twelve of the YSOs discussed are new candidates, one of which is located in the Coronet. Known YSOs retrieved from the literature are also added to the list, and a total of 116 candidate YSOs in CrA are compiled. Based on these YSO candidates, the star formation rate is computed to be 12 M_o Myr^-1, similar to that of the Lupus clouds. A clustering analysis was also performed, finding that the main cluster core, consisting of 68 members, is elongated (having an aspect ratio of 2.36), with a circular radius of 0.59 pc and mean surface density of 150 pc^-2. In addition, we analyze outflows and jets in CrA by means of new CO and H_2 data. We present 1.3 mm interferometric continuum observations made with the Submillimeter Array (SMA) covering R CrA, IRS 5, IRS 7, and IRAS 18595-3712 (IRAS 32). We also present multi-epoch H_2 maps and detect jets and outflows, study their proper motions, and identify exciting sources. The Spitzer and ISAAC/VLT observations of IRAS 32 show a bipolar precessing jet, which drives a CO (2-1) outflow detected in the SMA observations. There is also clear evidence for a parsec-scale precessing outflow, E-W oriented, and originating in the SMA 2 region, likely driven by SMA 2 or IRS 7A.