Abundant Methanol Ice toward a Massive Young Stellar Object in the Central Molecular Zone

Previous radio observations revealed widespread gas-phase methanol (CH_3OH) in the Central Molecular Zone (CMZ) at the Galactic center (GC), but its origin remains unclear. Here, we report the discovery of CH_3OH ice toward a star in the CMZ, based on a Subaru 3.4–4.0 μm spectrum, aided by NASA/IRTF L’ imaging and 2–4 μm spectra. The star lies ~8000 au away in projection from a massive young stellar object (MYSO). Its observed high CH_3OH ice abundance (17% ± 3% relative to H_2O ice) suggests that the 3.535 μm CH_3OH ice absorption likely arises in the MYSO's extended envelope. However, it is also possible that CH_3OH ice forms with a higher abundance in dense clouds within the CMZ, compared to within the disk. Either way, our result implies that gas-phase CH_3OH in the CMZ can be largely produced by desorption from icy grains. The high solid CH_3OH abundance confirms the prominent 15.4 μm shoulder absorption observed toward GC MYSOs arises from CO_2 ice mixed with CH_3OH

The c2d Spitzer spectroscopy survey of ices around low-mass young stellar objects, III: CH4

CH4 is proposed to be the starting point of a rich organic chemistry. Solid CH4 abundances have previously been determined mostly toward high mass star forming regions. Spitzer/IRS now provides a unique opportunity to probe solid CH4 toward low mass star forming regions as well. Infrared spectra from the Spitzer Space Telescope are presented to determine the solid CH4 abundance toward a large sample of low mass young stellar objects. 25 out of 52 ice sources in the $c2d$ (cores to disks) legacy have an absorption feature at 7.7 um, attributed to the bending mode of solid CH4. The solid CH4 / H2O abundances are 2-8%, except for three sources with abundances as high as 11-13%. These latter sources have relatively large uncertainties due to small total ice column densities. Toward sources with H2O column densities above 2E18 cm-2, the CH4 abundances (20 out of 25) are nearly constant at 4.7+/-1.6%. Correlation plots with solid H2O, CH3OH, CO2 and CO column densities and abundances relative to H2O reveal a closer relationship of solid CH4 with CO2 and H2O than with solid CO and CH3OH. The inferred solid CH4 abundances are consistent with models where CH4 is formed through sequential hydrogenation of C on grain surfaces. Finally the equal or higher abundances toward low mass young stellar objects compared with high mass objects and the correlation studies support this formation pathway as well, but not the two competing theories: formation from CH3OH and formation in gas phase with subsequent freeze-out.Comment: 27 pages, 7 figures, accepted by Ap

Massive Young Stellar Objects in the Galactic Center. II. Seeing Through the Ice-rich Envelopes

To study the demographics of interstellar ices in the Central Molecular Zone (CMZ) of the Milky Way, we obtain near-infrared spectra of $109$ red point sources using NASA IRTF/SpeX at Maunakea. We select the sample from near- and mid-infrared photometry, including $12$ objects in the previous paper of this series, to ensure that these sources trace a large amount of absorption through clouds in each line of sight. We find that most of the sample ($100$ objects) show CO band-head absorption at $2.3\ \mu$m, tagging them as red (super-) giants. Despite the photospheric signature, however, a fraction of the sample with $L$-band spectra ($9/82=0.11$) exhibit large H$_2$O ice column densities ($N > 2\times10^{18}\ {\rm cm}^{-2}$), and six of them also reveal CH$_3$OH ice absorption. As one of such objects is identified as a young stellar object (YSO) in our previous work, these ice-rich sight lines are likely associated with background stars in projection to an extended envelope of a YSO or a dense cloud core. The low frequency of such objects in the early stage of stellar evolution implies a low star-formation rate ($<0.02\ M_\odot$ yr$^{-1}$), reinforcing the previous claim on the suppressed star-formation activity in the CMZ. Our data also indicate that the strong "shoulder" CO$_2$ ice absorption at $15.4\ \mu$m observed in YSO candidates in the previous paper arises from CH$_3$OH-rich ice grains having a large CO$_2$ concentration [$N {\rm (CO_2)} / N {\rm (CH_3OH)} \approx 1/3$].Comment: 28 pages, 12 figures, 3 tables. Accepted for publication in the Astrophysical Journa

Modeling Spitzer observations of VV Ser. I. The circumstellar disk of a UX Orionis star

We present mid-infrared Spitzer-IRS spectra of the well-known UX Orionis star VV Ser. We combine the Spitzer data with interferometric and spectroscopic data from the literature covering UV to submillimeter wavelengths. The full set of data are modeled by a two-dimensional axisymmetric Monte Carlo radiative transfer code. The model is used to test the prediction of (Dullemond et al. 2003) that disks around UX Orionis stars must have a self-shadowed shape, and that these disks are seen nearly edge-on, looking just over the edge of a puffed-up inner rim, formed roughly at the dust sublimation radius. We find that a single, relatively simple model is consistent with all the available observational constraints spanning 4 orders of magnitude in wavelength and spatial scales, providing strong support for this interpretation of UX Orionis stars. The grains in the upper layers of the puffed-up inner rim must be small (0.01-0.4 micron) to reproduce the colors (R_V ~ 3.6) of the extinction events, while the shape and strength of the mid-infrared silicate emission features indicate that grains in the outer disk (> 1-2 AU) are somewhat larger (0.3-3.0 micron). From the model fit, the location of the puffed-up inner rim is estimated to be at a dust temperature of 1500 K or at 0.7-0.8 AU for small grains. This is almost twice the rim radius estimated from near-infrared interferometry. A best fitting model for the inner rim in which large grains in the disk mid-plane reach to within 0.25 AU of the star, while small grains in the disk surface create a puffed-up inner rim at ~0.7-0.8 AU, is able to reproduce all the data, including the near-infrared visibilities. [Abstract abridged]Comment: 12 pages, accepted for publication in Ap

Spitzer Space Telescope Spectroscopy of Ices toward Low-Mass Embedded Protostars

Sensitive 5-38 μm Spitzer Space Telescope and ground-based 3-5 μm spectra of the embedded low-mass protostars B5 IRS1 and HH 46 IRS show deep ice absorption bands superposed on steeply rising mid-infrared continua. The ices likely originate in the circumstellar envelopes. The CO_2 bending mode at 15 μm is a particularly powerful tracer of the ice composition and processing history. Toward these protostars, this band shows little evidence for thermal processing at temperatures above 50 K. Signatures of lower temperature processing are present in the CO and OCN^- bands, however. The observed CO2 profile indicates an intimate mixture with H_(2)O, but not necessarily with CH_(3)OH, in contrast to some high-mass protostars. This is consistent with the low CH_(3)OH abundance derived from the ground-based L-band spectra. The CO_2 : H_(2)O column density ratios are high in both B5 IRS1 and HH 46 IRS (~35%). Clearly, the Spitzer spectra are essential for studying ice evolution in low-mass protostellar environments and for eventually determining the relation between interstellar and solar system ices

Massive Young Stellar Objects in the Galactic Center. I. Spectroscopic Identification from Spitzer/IRS Observations

We present results from our spectroscopic study, using the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope, designed to identify massive young stellar objects (YSOs) in the Galactic Center (GC). Our sample of 107 YSO candidates was selected based on IRAC colors from the high spatial resolution, high sensitivity Spitzer/IRAC images in the Central Molecular Zone (CMZ), which spans the central ~300 pc region of the Milky Way Galaxy. We obtained IRS spectra over 5um to 35um using both high- and low-resolution IRS modules. We spectroscopically identify massive YSOs by the presence of a 15.4um shoulder on the absorption profile of 15um CO2 ice, suggestive of CO2 ice mixed with CH3OH ice on grains. This 15.4um shoulder is clearly observed in 16 sources and possibly observed in an additional 19 sources. We show that 9 massive YSOs also reveal molecular gas-phase absorption from CO2, C2H2, and/or HCN, which traces warm and dense gas in YSOs. Our results provide the first spectroscopic census of the massive YSO population in the GC. We fit YSO models to the observed spectral energy distributions and find YSO masses of 8 - 23 Msun, which generally agree with the masses derived from observed radio continuum emission. We find that about 50% of photometrically identified YSOs are confirmed with our spectroscopic study. This implies a preliminary star formation rate of ~0.07 Msun/yr at the GC.Comment: Accepted for publication in Ap

The c2d Spitzer Spectroscopic Survey of Ices Around Low-Mass Young Stellar Objects. IV. NH3 and CH3OH

NH3 and CH3OH are key molecules in astrochemical networks leading to the formation of more complex N- and O-bearing molecules, such as CH3CN and HCOOCH3. Despite a number of recent studies, little is known about their abundances in the solid state. (...) In this work, we investigate the ~ 8-10 micron region in the Spitzer IRS (InfraRed Spectrograph) spectra of 41 low-mass young stellar objects (YSOs). These data are part of a survey of interstellar ices in a sample of low-mass YSOs studied in earlier papers in this series. We used both an empirical and a local continuum method to correct for the contribution from the 10 micron silicate absorption in the recorded spectra. In addition, we conducted a systematic laboratory study of NH3- and CH3OH-containing ices to help interpret the astronomical spectra. We clearly detect a feature at ~9 micron in 24 low-mass YSOs. Within the uncertainty in continuum determination, we identify this feature with the NH3 nu_2 umbrella mode, and derive abundances with respect to water between ~2 and 15%. Simultaneously, we also revisited the case of CH3OH ice by studying the nu_4 C-O stretch mode of this molecule at ~9.7 micron in 16 objects, yielding abundances consistent with those derived by Boogert et al. 2008 (hereafter paper I) based on a simultaneous 9.75 and 3.53 micron data analysis. Our study indicates that NH3 is present primarily in H2O-rich ices, but that in some cases, such ices are insufficient to explain the observed narrow FWHM. The laboratory data point to CH3OH being in an almost pure methanol ice, or mixed mainly with CO or CO2, consistent with its formation through hydrogenation on grains. Finally, we use our derived NH3 abundances in combination with previously published abundances of other solid N-bearing species to find that up to 10-20 % of nitrogen is locked up in known ices.Comment: 31 pages, 15 figures, accepted for publication in Ap

Protostellar holes: Spitzer Space Telescope observations of the protostellar binary IRAS16293-2422

Mid-infrared (23-35 micron) emission from the deeply embedded "Class 0" protostar IRAS16293-2422 is detected with the Spitzer Space Telescope infrared spectrograph. A detailed radiative transfer model reproducing the full spectral energy distribution (SED) from 23 micron to 1.3 mm requires a large inner cavity of radius 600 AU in the envelope to avoid quenching the emission from the central sources. This is consistent with a previous suggestion based on high angular resolution millimeter interferometric data. An alternative interpretation using a 2D model of the envelope with an outflow cavity can reproduce the SED but not the interferometer visibilities. The cavity size is comparable to the centrifugal radius of the envelope and therefore appears to be a natural consequence of the rotation of the protostellar core, which has also caused the fragmentation leading to the central protostellar binary. With a large cavity such as required by the data, the average temperature at a given radius does not increase above 60-80 K and although hot spots with higher temperatures may be present close to each protostar, these constitute a small fraction of the material in the inner envelope. The proposed cavity will also have consequences for the interpretation of molecular line data, especially of complex species probing high temperatures in the inner regions of the envelope.Comment: Accepted for publication in ApJ Letter

First Spectroscopic Identification of Massive Young Stellar Objects in the Galactic Center

We report the detection of several molecular gas-phase and ice absorption features in three photometrically-selected young stellar object (YSO) candidates in the central 280 pc of the Milky Way. Our spectra, obtained with the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope, reveal gas- phase absorption from CO_2 (15.0 μm), C_2H_2 (13.7 μm) and HCN (14.0 μm). We attribute this absorption to warm, dense gas in massive YSOs. We also detect strong and broad 15 μm CO_2 ice absorption features, with a remarkable double- peaked structure. The prominent long-wavelength peak is due to CH_3OH-rich ice grains, and is similar to those found in other known massive YSOs. Our IRS observations demonstrate the youth of these objects, and provide the first spectroscopic identification of massive YSOs in the Galactic Center