Ronak Yogendra Shah

The Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey

The Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey is a new survey of Galactic 13CO J = 1 → 0 emission. The survey used the SEQUOIA multipixel array on the Five College Radio Astronomy Observatory 14 m telescope to cover a longitude range of l = 18°-557 and a latitude range of |b| 40°. At the velocity resolution of 0.21 km s-1, the typical rms sensitivity is σ(T) ~ 0.13 K. The survey comprises a total of 1,993,522 spectra. We show integrated intensity images (zeroth moment maps), channel maps, position-velocity diagrams, and an average spectrum of the completed survey data set. We also discuss the telescope and instrumental parameters, the observing modes, the data reduction processes, and the emission and noise characteristics of the data set. The Galactic Ring Survey data are available to the community online or in DVD form by request.

The Characterization and Galactic Distribution of Infrared Dark Clouds

Using 13CO J = 1 ? 0 molecular line emission from the Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey (BU-FCRAO GRS), we have established kinematic distances to 313 infrared dark clouds (IRDCs) by matching the morphology of the molecular line emission in distinct velocity channels to their mid-infrared extinction. The Galactic distribution of IRDCs shows an enhancement toward the Galaxys most massive and active star-forming structure, the so-called 5 kpc ring. IRDCs have typical sizes of ~5 pc, peak column densities of ~1022 cm-2, LTE masses of ~5 ? 103 M?, and volume-averaged H2 densities of ~2 ? 103 cm-3. The similarity of these properties to those of molecular clumps associated with active star formation suggests that IRDCs represent the densest clumps within giant molecular clouds where clusters may eventually form.

The Spitzer Survey of the Small Magellanic Cloud: S3MC Imaging and Photometry in the Mid- and Far-Infrared Wave Bands

We present the initial results from the Spitzer Survey of the Small Magellanic Cloud (S^3MC), which imaged the star-forming body of the SMC in all seven MIPS and IRAC wave bands. We find that the F_8/F_(24) ratio (an estimate of PAH abundance) has large spatial variations and takes a wide range of values that are unrelated to metallicity but anticorrelated with 24 μm brightness and F_(24)/F_(70) ratio. This suggests that photodestruction is primarily responsible for the low abundance of PAHs observed in star-forming low-metallicity galaxies. We use the S3MC images to compile a photometric catalog of ~400,000 mid- and far-infrared point sources in the SMC. The sources detected at the longest wavelengths fall into four main categories: (1) bright 5.8 μm sources with very faint optical counterparts and very red mid-infrared colors ([5.8] - [8.0] > 1.2), which we identify as YSOs; (2) bright mid-infrared sources with mildly red colors (0.16 ≾ [5.8] - [8.0] < 0.6), identified as carbon stars; (3) bright mid-infrared sources with neutral colors and bright optical counterparts, corresponding to oxygen-rich evolved stars; and (4) unreddened early B stars (B3-O9) with a large 24 μm excess. This excess is reminiscent of debris disks and is detected in only a small fraction of these stars (≾5%). The majority of the brightest infrared point sources in the SMC fall into groups 1-3. We use this photometric information to produce a catalog of 282 bright YSOs in the SMC with a very low level of contamination (~7%).

A Survey of Acetic Acid toward Hot Molecular Cores

We have surveyed 12 Galactic hot molecular cores for interstellar acetic acid (CH3COOH). This is the most extensive search for acetic acid to date. We have detected a new source of acetic acid toward the high-mass hot molecular core source G34.3+0.2. Using a temperature range between 70 and 185 K, we find a CH3COOH column density range of (0.77-1.64) ? 1015 cm-2 toward G34.3+0.2. This gives a relative CH3COOH/HCOOCH3 abundance ratio of ~3.3 ? 10-2, which is comparable to the abundance ratio of (3-6) ? 10-2 found toward Sgr B2(N-LMH) and W51e2 by Remijan and colleagues. All currently known acetic acid sources are within 7 kpc of the Galactic center. Furthermore, our survey suggests that hot molecular cores that have a mass range between 200 and 2000 M? and do not show a distinct differentiation between O and N chemistry may be the best places to search for acetic acid and the structurally similar biologically important molecule glycine.

The Spitzer Survey of the Small Magellanic Cloud: Discovery of Embedded Protostars in the H II Region NGC 346

We use Spitzer Space Telescope observations from the Spitzer Survey of the Small Magellanic Cloud (S^(3)MC) to study the young stellar content of N66, the largest and brightest H II region in the SMC. In addition to large numbers of normal stars, we detect a significant population of bright, red infrared sources that we identify as likely to be young stellar objects (YSOs). We use spectral energy distribution (SED) fits to classify objects as ordinary (main-sequence or red giant) stars, asymptotic giant branch stars, background galaxies, and YSOs. This represents the first large-scale attempt at blind source classification based on Spitzer SEDs in another galaxy. We firmly identify at least 61 YSOs, with another 50 probable YSOs; only one embedded protostar in the SMC was reported in the literature prior to the S^(3)MC. We present color selection criteria that can be used to identify a relatively clean sample of YSOs with IRAC photometry. Our fitted SEDs indicate that the infrared-bright YSOs in N66 have stellar masses ranging from 2 to 17 M_☉, and that approximately half of the objects are stage II protostars, with the remaining YSOs roughly evenly divided between stage I and stage III sources. We find evidence for primordial mass segregation in the H II region, with the most massive YSOs being preferentially closer to the center than lower mass objects. Despite the low metallicity and dust content of the SMC, the observable properties of the YSOs appear consistent with those in the Milky Way. Although the YSOs are heavily concentrated within the optically bright central region of N66, there is ongoing star formation throughout the complex, and we place a lower limit on the star formation rate of 3.2 × 10^(-3) M_☉ yr^(-1) over the last ~1 Myr.

Spitzer Space Telescope Detection of the Young Supernova Remnant 1E 0102.2–7219

We present infrared observations of the young, oxygen-rich supernova remnant 1E 0102.2-7219 (E0102) in the Small Magellanic Cloud, obtained with the Spitzer Space Telescope. The remnant is detected at 24 μm but not at 8 or 70 μm and has a filled morphology with two prominent filaments. We find evidence for the existence of up to 8 × 10-4 M☉ of hot dust (Td ~ 120 K) associated with the remnant. Most of the hot dust is located in the central region of E0102, which appears significantly enhanced in infrared and radio continuum emission relative to the X-ray emission. Even if all of the hot dust was formed in the explosion of E0102, the estimated mass of dust is at least 100 times lower than what is predicted by some recent theoretical models.

DISCOVERY OF A DISTANT STAR FORMATION REGION USING GLIMPSE

Examination of early, in-orbit checkout (IOC) images of a portion of the Galactic plane obtained by the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope revealed the presence of an extended emission nebula with internal structure. The Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) data show this nebula, located at l � 42 � and b � 0N5, contains bright point sources and two nonstellar regions. Ancillary data sets were used to help reveal the nature of this nebula and its exciting objects. In particular, 13 CO J ¼ 1 ! 0 line emission mapped by the Galactic Ring Survey (GRS) shows molecular gas associated with the infrared nebula. The 13 CO radial velocity yields a far-kinematic distance of 11.1 kpc to the nebula, since there is no evidence for H i self-absorption. At 11.1 kpc, the far-infrared luminosity of the nebula is 4:8;10 4 L� , and the mass of its molecular cloud is 1:1;10 4 M� . The spectral energy distribution rises steeply from 2.2 to 100 � m with an absorption feature at 10 � m, exhibiting the shape of a late Class 0 young stellar object (YSO). The radio continuum flux observed toward the nebula is consistent with the free-free emission from one or more massive YSOs (MYSOs) with spectral types in the range O9 to B0. This analysis demonstrates one technique the GLIMPSE team will use for revealing thousands of Galactic star formation regions.

The BU-FCRAO Galactic Ring Survey

Using the SEQUOIA multi-pixel array receiver on the Five College Radio Astronomy Observatory (FCRAO) 14 m telescope, we are conducting a new molecular line survey of the inner Galaxy. The Boston University (BU)-FCRAO Galactic Ring Survey (GRS) will map 13CO (1-0) emission from l = 15° to 52° and b = -1° to 1°. Compared with previous molecular line surveys of the inner Galaxy, the GRS offers excellent sensitivity, better spectral resolution, the same or better angular resolution, Nyquist sampling, and the use of 13CO (1-0), a better column density tracer than 12CO.