Miju Kang

The Extended Environment of M17: A Star Formation History

M17 is one of the youngest and most massive nearby star-formation regions in the Galaxy. It features a bright H II region erupting as a blister from the side of a giant molecular cloud (GMC). Combining photometry from the Spitzer Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) with complementary infrared (IR) surveys, we identify candidate young stellar objects (YSOs) throughout a 15 ? 1? field that includes the M17 complex. The long sightline through the Galaxy behind M17 creates significant contamination in our YSO sample from unassociated sources with similar IR colors. Removing contaminants, we produce a highly reliable catalog of 96 candidate YSOs with a high probability of association with the M17 complex. We fit model spectral energy distributions to these sources and constrain their physical properties. Extrapolating the mass function of 62 intermediate-mass YSOs (M > 3?M ?), we estimate that >1000 stars are in the process of forming in the extended outer regions of M17. The remaining 34 candidate YSOs are found in a 0.17?deg2 field containing the well-studied M17 H II region and photodissociation region (PDR), where bright diffuse mid-IR emission drastically reduces the sensitivity of the GLIMPSE point-source detections. By inspecting IR survey images from IRAS and GLIMPSE, we find that M17 lies on the rim of a large shell structure ~05 in diameter (~20 pc at 2.1?kpc). We present maps of 12CO and 13CO (J = 2 ? 1) emission observed with the Heinrich Hertz Telescope. The CO emission shows that the shell is a coherent, kinematic structure associated with M17, centered at v = 19?km s?1. The shell is an extended bubble outlining the PDR of a faint, diffuse H II region several Myr old. We identify a group of candidate ionizing stars within the bubble. YSOs in our catalog are concentrated around the bubble rim, providing evidence that massive star formation has been triggered by the expansion of the bubble. The formation of the massive cluster ionizing the M17 H II region itself may have been similarly triggered. We conclude that the star formation history in the extended environment of M17 has been punctuated by successive waves of massive star formation propagating through a GMC complex.

Spitzer and HHT Observations of Bok Globule B335: Isolated Star Formation Efficiency and Cloud Structure*

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 � m shadow. Heinrich Hertz Telescope (HHT) observations of 12 CO 2Y1 confirm the detection of a flattened molecular core with diameter � 10,000 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 � m 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. Subject headingg infrared: ISM — ISM: globules — ISM: individual (Barnard 335) — stars: formation

SPITZER AND HEINRICH HERTZ TELESCOPE OBSERVATIONS OF STARLESS CORES: MASSES AND ENVIRONMENTS

We present Spitzer observations of a sample of 12 starless cores selected to have prominent 24 ?m shadows. The Spitzer images show 8 ?m and 24 ?m shadows and in some cases 70 ?m shadows; these spatially resolved absorption features trace the densest regions of the cores. We have carried out a 12CO (2-1) and 13CO (2-1) mapping survey of these cores with the Heinrich Hertz Telescope (HHT). We use the shadow features to derive optical depth maps. We derive molecular masses for the cores and the surrounding environment; we find that the 24 ?m shadow masses are always greater than or equal to the molecular masses derived in the same region, a discrepancy likely caused by CO freezeout onto dust grains. We combine this sample with two additional cores that we studied previously to bring the total sample to 14 cores. Using a simple Jeans mass criterion, we find that ~2/3 of the cores selected to have prominent 24 ?m shadows are collapsing or near collapse, a result that is supported by millimeter line observations. Of this subset at least half have indications of 70 ?m shadows. All cores observed to produce absorption features at 70 ?m are close to collapse. We conclude that 24 ?m shadows, and even more so the 70 ?m ones, are useful markers of cloud cores that are approaching collapse.

KINEMATICS OF THE AMMONIA DISK AROUND THE PROTOSTAR NGC 1333 IRAS 4A2

The NGC 1333 IRAS 4A protobinary was observed in the ammonia (2, 2) and (3, 3) lines with an angular resolution of 0.3 arcsec. The ammonia emission source of IRAS 4A2 is elongated in the direction perpendicular to the bipolar jet and has a size of 0.55 arcsec or 130 AU. This emission structure was interpreted as a circumstellar disk around the IRAS 4A2 protostar, and the rotation kinematics of the disk was investigated by making a position-velocity diagram along the major axis. Assuming a Keplerian rotation, the disk has a rotation velocity of 1.8 km s{sup -1} at a radius of 20 AU, which implies a central object of about 0.08 solar masses. The collapse age of the protostar is about 50,000 yr. The mass, accretion rate, and age are consistent with what are expected from the standard theory of low-mass star formation. If IRAS 4A2 grows at this rate, it may become a star similar to the Sun.

EMBEDDED YOUNG STELLAR OBJECT CANDIDATES IN THE ACTIVE STAR-FORMING COMPLEX W51: MASS FUNCTION AND SPATIAL DISTRIBUTION

We present 737 candidate young stellar objects (YSOs) near the W51 giant molecular cloud over an area of 125 × 100 selected from Spitzer Space Telescope data. We use spectral energy distribution fits to identify YSOs and distinguish them from main-sequence (MS) or red giant stars, asymptotic giant branch stars, and background galaxies. Based on extinction of each YSO, we separate a total of 437 YSOs associated with the W51 region from the possible foreground sources. We identify 69 highly embedded Stage 0/I candidate YSOs in our field with masses >5 M ☉ (corresponding to mid- to early-B MS spectral types), 46 of which are located in the central active star-forming regions of W51A and W51B. From the YSOs associated with W51, we find evidence for mass segregation showing that the most massive YSOs are concentrated on the W51 H II region complex. We find a variation in the spatial distribution of the mass function (MF) of YSOs in the mass range between 5 M ☉ and 18 M ☉. The derived slopes of the MF are –1.26 and –2.36 in the active star-forming region and the outer region, respectively. The variation of the MF for YSOs embedded in the molecular cloud implies that the distribution of stellar masses in clusters depends on the local conditions in the parent molecular cloud.

TRIGGERED STAR FORMATION IN A DOUBLE SHELL NEAR W51A

We present Heinrich Hertz Telescope CO observations of the shell structure near the active star-forming complex W51A to investigate the process of star formation triggered by the expansion of an H II region. The CO observations confirm that dense molecular material has been collected along the shell detected in Spitzer IRAC images. The CO distribution shows that the shell is blown out toward a lower density region to the northwest. Total hydrogen column density around the shell is high enough to form new stars. We find two CO condensations with the same central velocity of 59 km s–1 to the east and north along the edge of the IRAC shell. We identify two young stellar objects in early evolutionary stages (Stage 0/I) within the densest molecular condensation. From the CO kinematics, we find that the H II region is currently expanding with a velocity of 3.4 km s–1, implying that the shells expansion age is ~1 Myr. This timescale is in good agreement with numerical simulations of the expansion of the H II region (Hosokawa & Inutsuka). We conclude that the star formation on the border of the shell is triggered by the expansion of the H II region.

A CO LINE AND INFRARED CONTINUUM STUDY OF THE ACTIVE STAR-FORMING COMPLEX W51

We present the results of an extensive observational study of the active star-forming complex W51 that was observed in the J = 2 – 1 transition of the 12CO and 13CO molecules over a 125 × 100 region with the University of Arizona Heinrich Hertz Submillimeter Telescope. We use a statistical equilibrium code to estimate physical properties of the molecular gas. We compare the molecular cloud morphology with the distribution of infrared (IR) and radio continuum sources and find associations between molecular clouds and young stellar objects (YSOs) listed in Spitzer IR catalogs. The ratios of CO lines associated with H II regions are different from the ratios outside the active star-forming regions. We present evidence of star formation triggered by the expansion of the H II regions and by cloud-cloud collisions. We estimate that about 1% of the cloud mass is currently in YSOs.

How do stars gain their mass? : A JCMT/SCUBA-2 Transient Survey of Protostars in Nearby Star Forming Regions

Most protostars have luminosities that are fainter than expected from steady accretion over the protostellar lifetime. The solution to this problem may lie in episodic mass accretion—prolonged periods of very low accretion punctuated by short bursts of rapid accretion. However, the timescale and amplitude for variability at the protostellar phase is almost entirely unconstrained. In A James Clerk Maxwell Telescope/SCUBA-2 Transient Survey of Protostars in Nearby Star-forming Regions, we are monitoring monthly with SCUBA-2 the submillimeter emission in eight fields within nearby (<500 pc) star-forming regions to measure the accretion variability of protostars. The total survey area of ~1.6 deg^2 includes ~105 peaks with peaks brighter than 0.5 Jy/beam (43 associated with embedded protostars or disks) and 237 peaks of 0.125–0.5 Jy/beam (50 with embedded protostars or disks). Each field has enough bright peaks for flux calibration relative to other peaks in the same field, which improves upon the nominal flux calibration uncertainties of submillimeter observations to reach a precision of ~2%–3% rms, and also provides quantified confidence in any measured variability. The timescales and amplitudes of any submillimeter variation will then be converted into variations in accretion rate and subsequently used to infer the physical causes of the variability. This survey is the first dedicated survey for submillimeter variability and complements other transient surveys at optical and near-IR wavelengths, which are not sensitive to accretion variability of deeply embedded protostars.

Planck Cold Clumps in the λ Orionis Complex. I : Discovery of an Extremely Young Class 0 Protostellar Object and a Proto-brown Dwarf Candidate in the Bright-rimmed Clump PGCC G192.32-11.88

We are performing a series of observations with ground-based telescopes toward Planck Galactic cold clumps (PGCCs) in the lambda Orionis complex in order to systematically investigate the effects of stellar feedback. In the particular case of PGCC G192.32-11.88, we discovered an extremely young Class 0 protostellar object (G192N) and a proto-brown dwarf candidate (G192S). G192N and G192S are located in a gravitationally bound brightrimmed clump. The velocity and temperature gradients seen in line emission of CO isotopologues indicate that PGCC G192.32-11.88 is externally heated and compressed. G192N probably has the lowest bolometric luminosity (similar to 0.8 L-circle dot) and accretion rate (6.3 x 10(-7) M-circle dot yr(-1)) when compared with other young Class 0 sources (e.g., PACS Bright Red Sources) in the Orion complex. It has slightly larger internal luminosity (0.21 +/- 0.01 L-circle dot) and outflow velocity (similar to 14 km s(-1)) than the predictions of first hydrostatic cores (FHSCs). G192N might be among the youngest Class 0 sources, which are slightly more evolved than an FHSC. Considering its low internal luminosity (0.08 +/- 0.01 L-circle dot) and accretion rate (2.8 x 10(-8) M-circle dot yr(-1)), G192S is an ideal proto-brown dwarf candidate. The star formation efficiency (similar to 0.3%-0.4%) and core formation efficiency (similar to 1%) in PGCC G192.32-11.88 are significantly smaller than in other giant molecular clouds or filaments, indicating that the star formation therein is greatly suppressed owing to stellar feedback.

RADIO VARIABILITY SURVEY OF VERY LOW LUMINOSITY PROTOSTARS

Ten very low luminosity objects were observed multiple times in the 8.5 GHz continuum in search of protostellar magnetic activities. A radio outburst of IRAM 04191+1522 IRS was detected, and the variability timescale was about 20 days or shorter. The results of this survey and archival observations suggest that IRAM 04191+1522 IRS is in active states about half the time. Archival data show that L1014 IRS and L1148 IRS were detectable previously and suggest that at least 20%-30% of very low luminosity protostars are radio variables. Considering the variability timescale and flux level of IRAM 04191+1522 IRS and the previous detection of the circular polarization of L1014 IRS, the radio outbursts of these protostars are probably caused by magnetic flares. However, IRAM 04191+1522 IRS is too young and small to develop an internal convective dynamo. If the detected radio emission is indeed coming from magnetic flares, the discovery implies that the flares may be caused by the fossil magnetic fields of interstellar origin.