Spitzer Observations of Bok Globule B335: Isolated Star Formation Efficiency and Cloud Structure
Amelia M. Stutz, Mark Rubin, Michael W. Werner, George H. Rieke, John H. Bieging, Jocelyn Keene, Miju Kang, Yancy L. Shirley, K. Y. L. Su, Thangasamy Velusamy, David J. Wilner
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15 pages, 17 figures, emulateapj format, accepted for publication in ApJ
We present infrared and millimeter observations of Barnard 335, the prototypical isolated Bok globule with an embedded protostar. Using Spitzer data we measure the source luminosity accurately; we also constrain the density profile of the innermost globule material near the protostar using the observation of an 8.0 um shadow. HHT observations of 12CO 2 --> 1 confirm the detection of a flattened molecular core with diameter ~10000 AU and the same orientation as the circumstellar disk (~100 to 200 AU in diameter). This structure is probably the same as that generating the 8.0 um shadow and is expected from theoretical simulations of collapsing embedded protostars. We estimate the mass of the protostar to be only ~5% of the mass of the parent globule.
We report the discovery of small, isolated dust clouds in the Large Magellanic Cloud, which are excellent candidates for counterparts to the Bok globules observed in the Galaxy. We detect these small clouds silhoutted against diffuse H-alpha emission, based on parallel imaging with the WFPC-2 on HST. The clouds we identify as Bok globule candidates have typical sizes of approximately one arcsecond, corresponding to about 0.25 parsec linear diameter at the distance of the LMC. We derive lower limits to the optical depth within the dark clouds, and masses assuming that the clouds have density di…
Using the Submillimeter Common-User Bolometer Array (SCUBA) at the James Clerk Maxwell Telescope (JCMT), we obtained submillimeter polarization maps of the Bok globules B335, CB230, and CB244 at 850micron. We find strongly aligned polarization vectors in the case of B335 and CB230, indicating a strong coupling of the magnetic field to the dust grains. Based on the distribution of the orientation and strength of the linear polarization we derive the magnetic field strengths in the envelopes of the globules. In agreement with previous submillimeter polarization measurements of Bok globules we fi…
The shapes of isolated Bok globules and embedded dense cores of molecular clouds are analyzed using a nonparametric method, under the alternate hypotheses that they are randomly oriented prolate objects or that they are randomly oriented oblate objects. In all cases, the prolate hypothesis gives a better fit to the data. If Bok globules are oblate, they must be very flat; the average axis ratio is b/a = 0.3, and few or no globules can have b/a > 0.7. If Bok globules are prolate, then the mean axis ratio is b/a = 0.5. For most data samples of dense cores, the randomly-oriented oblate hypothesis…
We used near-infrared 2MASS data to construct visual extinction maps of a sample of Southern Bok globules utilizing the NICE method. We derived radial extinction profiles of dense cores identified in the globules and analyzed their stability against gravitational collapse with isothermal Bonnor-Ebert spheres. The frequency distribution of the stability parameter xi_max of these cores shows that a large number of them are located in stable states, followed by an abrupt decrease of cores in unstable states. This decrease is steeper for globules with associated IRAS point sources than for starles…
We present sub-arcsecond resolution millimeter-wave images of a circumstellar disk in the Bok globule CB26. The presence of an edge-on disk is confirmed by the dust continuum morphology and the velocity field of 13CO emission, which displays a Keplerian rotation pattern about an axis perpendicular to the long axis of the dust emission. We deduce a mass ~0.3 Msun for the obscured central star. The disk is optically thick at mm wavelengths inside 120 AU, has a symmetric 20 degree warp beyond 120 AU, an outer radius of ~200 AU, and a mass of at least 0.1 Msun. We suggest that the CB26 system is i…
We present new sub-arcsecond-resolution near-infrared polarimetric imaging and millimetre interferometry data on the circumstellar disk system in the Bok globule CB26. The data imply the presence of a M > 0.01 M_sun edge-on disk of >400 AU in diameter, being in Keplerian rotation around a young ~0.35 M_sun star. The mm dust emission from the inner 200 AU is highly optically thick, but the outer parts are optically thin and made of small dust grains. Planetesimal growth in the inner disk could neither be comfirmed nor excluded. The outer optically thin part of the disk is strongly warped. We ar…
We study the influence and structure of the magnetic field in the early phases of low-mass star formation using polarization maps of Bok globules at a wavelength of 850micron, obtained with the Submillimeter Common-User Bolometer Array (SCUBA) at the James Clerk Maxwell Telescope (JCMT). We discuss observations of the following sources: CB26 - a small globule with a nearly dispersed dense core and a young and large circumstellar disk, CB54 - a large globule with a massive dense core and a deeply embedded young stellar cluster, and B335, CB230, and CB244 - three nearby, relatively isolated smal…
On the basis of near-infrared imaging observations, we derived visual extinction (Av) distribution toward ten Bok globules through measurements of both the color excess (E_{H-K}) and the stellar density at J, H, and Ks (star count). Radial column density profiles for each globule were analyzed with the Bonnor-Ebert sphere model. Using the data of our ten globules and four globules in the literature, we investigated the stability of globules on the basis of xi_max, which characterizes the Bonnor-Ebert sphere as well as the stability of the equilibrium state against the gravitational collapse. W…
This paper is intended as part of a more extensive molecular line survey in star forming regions along the evolutionary track of a collapsing cloud toward a young stellar object. We have studied a sample of seven small dark clouds (Bok globules) and eight Herbig Ae/Be stars in the J=1->0 transition of HCO$^{+}$, H$^{13}$CO$^{+}$, HCN and H$^{13}$CN. The choice of these molecules is determined by the simple chemistry and the predicted high abundance of the reactants leading to their formation. The isotopically substituted species (isotopomers), H$^{13}$CO$^{+}$ and H$^{13}$CN, were observed in …
We present Very Large Array observations at 1.3 cm of several water maser detections obtained by previous single-dish studies of Bok globules in the Clemens & Barvainis (1988; CB) catalog. We report water maser emission in CB 3 (CB3-mm), CB 54 (IRAS 07020-1618), CB 101 (IRAS 17503-0833), and CB 232 (IRAS 21352+4307), and non-detection towards CB 65 (IRAS 16277-2332) and CB 205 (IRAS 19433+2743). These are the first reported interferometric observations of water masers in Bok globules of the CB catalog. We also present single-dish observations of millimeter and centimeter spectral lines towards…
We present mid-infrared (10.4 \micron, 11.7 \micron, and 18.3 \micron) imaging intended to locate and characterize the suspected protostellar components within the Bok globule CB54. We detect and confirm the protostellar status for the near-infrared source CB54YC1-II. The mid-infrared luminosity for CB54YC1-II was found to be $L_{midir} \approx 8 L_\sun$, and we estimate a central source mass of $M_* \approx 0.8 M_\sun$ (for a mass accretion rate of ${\dot M}=10^{-6} M_\sun yr^{-1}$). CB54 harbors another near-infrared source (CB54YC1-I), which was not detected by our observations. The non-det…
We report the most sensitive water maser survey towards Bok globules to date, using NASA's 70m antenna in Robledo de Chavela (Spain). We observed 207 positions within the CB catalog with a higher probability of harboring a young star, using as selection criteria the presence of radio continuum emission (from submillimeter to centimeter wavelengths), geometrical centers of molecular outflows, peaks in maps of high-density gas tracers (NH3 or CS), and IRAS point sources. We have obtained 7 maser detections, 6 of which (in CB 34, CB 54, CB 65, CB 101, CB 199, and CB 232) are reported for the firs…
We present Spitzer observations of the dark globule CB190 (L771). We observe a roughly circular 24 micron shadow with a 70 arcsec radius. The extinction profile of this shadow matches the profile derived from 2MASS photometry at the outer edges of the globule and reaches a maximum of ~32 visual magnitudes at the center. The corresponding mass of CB190 is ~10 Msun. Our 12CO and 13CO J = 2-1 data over a 10 arcmin X 10 arcmin region centered on the shadow show a temperature ~10 K. The thermal continuum indicates a similar temperature for the dust. The molecular data also show evidence of freezeou…
We report on results of near-infrared and optical observations of the mm disk embedded in the Bok globule CB 26 (Launhardt & Sargent 2001). The near-infrared images show a bipolar reflection nebula with a central extinction lane which coincides with the mm disk. Imaging polarimetry of this object yielded a polarization pattern which is typical for a young stellar object surrounded by a large circumstellar disk and an envelope, seen almost edge-on. The strong linear polarization in the bipolar lobes is caused by single scattering at dust grains and allowed to locate the illuminating source whic…
We describe a near-infrared imaging survey of Globule 2 in the Coalsack. This Bok globule is the highest density region of this southern hemisphere molecular cloud and is the most likely location for young stars in this complex. The survey is complete for K < 14.0, H < 14.5, and J < 15.5, several magnitudes more sensitive than previous observations of this globule. From the large number of background stars, we derive an accurate near-infrared extinction law for the cloud. Our result, E_{J-H}/E_{H-K} = 2.08 \pm 0.03, is significantly steeper than results for other southern clouds. We use the J-…
BHR 71 is a well isolated Bok globule located at ~200 pc, which harbours a highly collimated bipolar outflow. The outflow is driven by a very young Class 0 protostar with a luminosity of ~9 L_sun. It is one of a very small number that show enhanced abundances of a number of molecular species, notably SiO and CH3OH, due to shock processing of the ambient medium. In this paper the properties of the globule and outflow are discussed.
We construct spherical, hydrostatic models of dense molecular cores and Bok globules consisting of two distinct, spatially separate gas components: a central, isothermal region surrounded by a negative-index, polytropic envelope. The clouds are supported against their own self-gravity by a combination of thermal, mean magnetic, and turbulent wave pressure. The latter two are included by allowing for locally adiabatic, non-isentropic pressure components. Such models are meant to represent, in a schematic manner, the velocity and density structure of cores and globules, as inferred from molecula…
We report the discovery of four dusty cometary tails around low mass stars in two young clusters belonging to the W5 star forming region. Fits to the observed emission profiles from 24 micron observations with the Spitzer Space Telescope give tail lifetimes < 30 Myr, but more likely < 5 Myr. This result suggests that the cometary phase is a short lived phenomenon, occurring after photoevaporation by a nearby O star has removed gas from the outer disk of a young low mass star (see also Balog et al. 2006; Balog et al. 2008).
New molecular line observations of the Bok globule Barnard 68 in HCO+ irrefutably confirm the complex pattern of red and blue asymmetric line profiles seen across the face of the cloud in previous observations of CS. The new observations thus strengthen the previous interpretation that Barnard 68 is undergoing peculiar oscillations. Furthermore, the physical chemistry of B68 indicates that the object is much older than the sound crossing time and is therefore long-lived. A model is presented for the globule in which a modest external pressure perturbation is shown to lead to oscillations about…
Using Spitzer Space Telescope and Hubble Space Telescope observations of the superbubble N51D, we have identified three young stellar objects (YSOs) in dust globules, and made the first detection of a Herbig-Haro object outside the Galaxy. The spectral energy distributions of these YSOs suggest young massive stars with disk, envelope, and outflow cavities. The interstellar conditions are used to assess whether the star formation was spontaneous or induced by external pressure.
We present a new infrared extinction study of Globule 2, the most opaque molecular cloud core in the Coalsack complex. Using deep near-infrared imaging observations obtained with the ESO NTT we are able to examine the structure of the globule in significantly greater detail than previously possible. We find the most prominent structural feature of this globule to be a strong central ring of dust column density which was not evident in lower resolution studies of this cloud. This ring represents a region of high density and pressure that is likely a transient structure. For a spherical cloud ge…
In an effort to shed more light on the formation process of binary stars, we have started a program to study multiplicity among nearby low- and intermediate-mass protostars using the OVRO Millimeter Array. Here, we describe the project and present the first results on the protostellar core in the Bok globule CB230 (L1177). At 10 arcsec resolution, the molecular core is resolved into two components separated by 5000 AU. The morphology and kinematics of the double core suggest that it formed from a single cloud core due to rotational fragmentation.
In a previous paper we demonstrated that non-radial hydrodynamic oscillations of a thermally-supported (Bonnor-Ebert) sphere embedded in a low-density, high-temperature medium persist for many periods. The predicted column density variations and molecular spectral line profiles are similar to those observed in the Bok globule B68 suggesting that the motions in some starless cores may be oscillating perturbations on a thermally supported equilibrium structure. Such oscillations can produce molecular line maps which mimic rotation, collapse or expansion, and thus could make determining the dynam…
Extremely red objects, identified in the early Spitzer Space Telescope observations of the bright-rimmed globule IC 1396A and photometrically classified as Class I protostars Class II T Tauri stars based on their mid-infrared colors, were observed spectroscopically at 5.5 to 38 microns (IRS), at the 22 GHz water maser frequency (GBT), and in the optical (Palomar). The sources photometrically identified as Class I are confirmed as objects dominated by accretion luminosity from dense envelopes, with accretion rates 1e-5 to 1e-6 Msun/yr. The ice/silicate absorption ratio in the envelope is except…
The IC 1396N cometary globule within the large nearby HII region IC 1396 has been observed with the ACIS detector on board the Chandra X-ray Observatory. We detect 117 X-ray sources, of which ~50-60 are likely members of the young open cluster Trumpler~37 dispersed throughout the HII region, and 25 are associated with young stars formed within the globule. Spitzer/2MASS photometry shows the X-ray population is very young: 3 older Class III stars, 16 classical T Tauri stars, 6 protostars including a Class 0/I system. We infer a total T Tauri population of ~30 stars in the globule, including the…
We propose self-similar shocked flow models for certain dynamical evolution phases of young stellar objects (YSOs), `champagne flows' of H {\sevenrm II} regions surrounding OB stars and shaping processes of planetary nebulae (PNe). We analyze an isothermal fluid of spherical symmetry and construct families of similarity shocked flow solutions featured by: 1. either a core expansion with a finite central density or a core accretion at constant rate with a density scaling $\propto r^{-3/2}$; 2. a shock moving outward at a constant speed; 3. a preshock gas approaching a constant speed at large $r…
The optically-dark globule IC 1396A is revealed using Spitzer images at 3.6, 4.5, 5.8, 8, and 24 microns to be infrared-bright and to contain a set of previously unknown protostars. The mid-infrared colors of the 24 microns detected sources indicate several very young (Class I or 0) protostars and a dozen Class II stars. Three of the new sources (IC 1396A: gamma, delta, and epsilon) emit over 90% of their bolometric luminosities at wavelengths greater than 3 microns, and they are located within ~0.02 pc of the ionization front at the edge of the globule. Many of the sources have spectra that a…
We have obtained maps of the 1.25mm thermal dust emission and the molecular gas emission over a region of 20' by 10' arcmin around the Trifid Nebula (M20), with the IRAM 30m and the CSO telescopes as well as in the mid-infrared wavelength with ISO and SPITZER. Our survey is sensitive to features down to N(H2) \sim 10^{22} cm-2 in column density. The cloud material is distributed in fragmented dense gas filaments (n(H2) \sim 1000 cm-3) with sizes ranging from 1 to 10 pc. A massive filament, WF, with properties typical of Infra Red Dark Clouds, connects M20 to the W28 supernova remnant. These fi…
The polarization observed for stars background to dark clouds (Bok Globules) is often used as diagnostic to study the ongoing star formation processes in these clouds. Such polarization in the optical have been reported for eight nearby clouds CB3, CB25, CB39, CB52, CB54, CB58, CB62 and CB246 in one of our previous work (Sen et. al. 2000). With a view to understand the origin of this polarization, the the present work attempts are made to look for any possible relation between this observed polarization and other physical parameters in the cloud (like temperature, turbulence etc.). The observe…
InfraRed Dark Clouds (IRDCs) are extinction features against the Galactic infrared background, mainly in mid-infrared band. Recently they were proposed to be potential sites of massive star formation. In this work we have made 12CO, 13CO, and C18O (J=1-0) survey of 61 IRDCs, 52 of which are in the first Galactic quadrant, selected from a catalog given by Simon et al. (2006), while the others are in the outer Galaxy, selected by visually inspecting the MSX images. Detection rates in the three CO lines are 90%, 71%, and 62% respectively. The distribution of IRDCs in the first Galactic quadrant i…
We present semi-analytical similarity solutions for the inside-out, expansion-wave collapse of initially virialized gas clouds with non-isothermal equations of state. Results are given for the family of negative-index polytropes, but we focus especially on the so-called logotrope, P/P_c=1+A ln(rho/rho_c). The formalism and interpretation of the present theory are extensions of those in Shu's (1977) standard model for accretion in self-gravitating isothermal spheres: a collapse front moves outwards into a cloud at rest, and the gas behind it falls back to a collapsed core, or protostar. The inf…
We present the results of the first detailed millimeter and submillimeter molecular line survey of bright rimmed clouds, observed at FCRAO in the CO (J=1-0), C18O (J=1-0), HCO+ (J=1-0), H13CO+ (J=1-0), and N2H+ (J=1-0) transitions, and at the HHT in the CO (J=2-1), HCO+ (J=3-2), HCO+ (J=4-3), H13CO+ (J=3-2), and H13CO+ (J=4-3) molecular line transitions. The source list is composed of a selection of bright rimmed clouds from the catalog of such objects compiled by Sugitani et al. (1991). We also present observations of three Bok globules done for comparison with the bright rimmed clouds. We fi…
