Testing the Disk-Locking Paradigm: An Association Between U-V Excess and Rotation in NGC 2264

We present some results from a UVI photometric study of a field in the young open cluster NGC 2264 aimed, in part, at testing whether accretion in pre-main sequence stars is linked to rotation. We confirm that U-V excess is well correlated with H-alpha equivalent width for the stars in our sample. We show that for the more massive stars in the cluster sample (roughly 0.4-1.2 M_sun) there is also a significant association between U-V excess and rotation, in the sense that slow rotators are more likely to show excess U-band emission and variability. This constitutes significant new evidence in support of the disk-locking paradigm.Comment: Accepted by ApJ Letter

Massive Star Formation : the Role of Disks

In this thesis, I study three different evolutionary stages of the massive star formation process looking for supporting evidence for an accretion-based formation scenario of massive stars. The first source studied, the Infrared Dark Cloud IRDC 18223-3, is at one of the earliest observable phases of massive star formation. This source is characterized by a cone-shaped molecular outflow component which is used to establish the outflow orientation. A velocity gradient traced by the molecule N2H+ but more convincingly by CH3OH is indicative of a rotating object oriented orthogonally to the outflow direction. This object is on the order of 28,000 AU in size and does not exhibit Keplerian rotation, but may host a disk within. Modeling this velocity gradient shows that a single rotating and infalling entity is capable of reproducing the observations. Moving to a High Mass Protostellar Object, IRAS 18151-1208, a well-defined outflow orientation is observed as well as an elongation in the 1.3 millimeter dust continuum that is perpendicular to the outflow. This elongation is modeled using a Monte Carlo 3D radiative transfer code. Comparing the modeling results to those of low mass protostars it is deduced that a scaled up version of low-mass star formation provides a plausible description of the observations in this high mass case. In the scaled up version, the density and flaring exponents as well as the relative scale height at one third of the outer radius remain the same as in the low-mass model. The disk mass, outer radius, and central star's mass and luminosity all increase. The third source studied in this thesis, the hot molecular core IRAS 18507+0121, exhibits the rich chemistry characterizing the hot core phase of massive star formation. The outflow orientation is confirmed and each chemical species is looked at for indication of rotation. Somewhat surprisingly, clear signatures of rotation are not detected and several possible explanations for this are discussed such as insufficient spatial resolution. However, along the lines of what has been observed in IRAS 18151-1208, a slight elongation in the dust continuum perpendicular to the outflow orientation is detected. Several approaches are explored as a means of studying whether the observable differences in the massive star formation regions are a result of evolution. Taken individually, no indicator is sufficient to definitively determine an age sequence for the three sources. However, taken collectively, the trends seen in these case studies can be attributed to an evolutionary sequence. The results of this thesis are consistent with an accretion based formation mechanism of massive stars and I conclude that the structural changes of the observed disk-like structures from large-scale to more compact may be the result of evolution

A First Look at the Auriga-California Giant Molecular Cloud With Herschel and the CSO: Census of the Young Stellar Objects and the Dense Gas

We have mapped the Auriga/California molecular cloud with the Herschel PACS and SPIRE cameras and the Bolocam 1.1 mm camera on the Caltech Submillimeter Observatory (CSO) with the eventual goal of quantifying the star formation and cloud structure in this Giant Molecular Cloud (GMC) that is comparable in size and mass to the Orion GMC, but which appears to be forming far fewer stars. We have tabulated 60 compact 70/160um sources that are likely pre-main-sequence objects and correlated those with Spitzer and WISE mid-IR sources. At 1.1 mm we find 18 cold, compact sources and discuss their properties. The most important result from this part of our study is that we find a modest number of additional compact young objects beyond those identified at shorter wavelengths with Spitzer. We also describe the dust column density and temperature structure derived from our photometric maps. The column density peaks at a few x 10^22 cm^-2 (N_H2) and is distributed in a clear filamentary structure along which nearly all the pre-main-sequence objects are found. We compare the YSO surface density to the gas column density and find a strong non-linear correlation between them. The dust temperature in the densest parts of the filaments drops to ~10K from values ~ 14--15K in the low density parts of the cloud. We also derive the cumulative mass fraction and probability density function of material in the cloud which we compare with similar data on other star-forming clouds.Comment: in press Astrophysical Journal, 201

Different Evolutionary Stages in the Massive Star Forming Region W3 Main Complex

We observed three high-mass star-forming regions in the W3 high-mass star formation complex with the Submillimeter Array and IRAM 30 m telescope. These regions, i.e. W3 SMS1 (W3 IRS5), SMS2 (W3 IRS4) and SMS3, are in different evolutionary stages and are located within the same large-scale environment, which allows us to study rotation and outflows as well as chemical properties in an evolutionary sense. While we find multiple mm continuum sources toward all regions, these three sub-regions exhibit different dynamical and chemical properties, which indicates that they are in different evolutionary stages. Even within each subregion, massive cores of different ages are found, e.g. in SMS2, sub-sources from the most evolved UCHII region to potential starless cores exist within 30 000 AU of each other. Outflows and rotational structures are found in SMS1 and SMS2. Evidence for interactions between the molecular cloud and the HII regions is found in the 13CO channel maps, which may indicate triggered star formation.Comment: Accepted for publication in ApJ, 22 pages, 23 figure

Forming an O Star via Disk Accretion?

We present a study of outflow, infall, and rotation in a ~10^5 Lsun (solar luminosity) star-forming region, IRAS 18360-0537, with Submillimeter Array (SMA) and IRAM 30m observations. The 1.3 mm continuum map shows a 0.5 pc dust ridge, of which the central compact part has a mass of ~80 Msun (solar mass) and harbors two condensations, MM1 and MM2. The CO (2--1) and SiO (5--4) maps reveal a biconical outflow centered at MM1, which is a hot molecular core (HMC) with a gas temperature of 320+/-50 K and a mass of ~13 Msun. The outflow has a gas mass of 54 Msun and a dynamical timescale of 8,000 yr. The kinematics of the HMC is probed by high-excitation CH3OH and CH3CN lines, which are detected at sub-arcsecond resolution and unveil a velocity gradient perpendicular to the outflow axis, suggesting a disk-like rotation of the HMC. An infalling envelope around the HMC is evidenced by CN lines exhibiting a profound inverse P-Cygni profile, and the estimated mass infall rate, 1.5x10^{-3} Msun/yr, is well comparable to that inferred from the mass outflow rate. A more detailed investigation of the kinematics of the dense gas around the HMC is obtained from the 13CO and C18O (2--1) lines; the position-velocity diagrams of the two lines are consistent with the model of a free-falling and Keplerian-like rotating envelope. The observations suggest that the protostar of a current mass ~10 Msun embedded within MM1 will develop into an O star via disk accretion and envelope infall.Comment: Accepted for publication in the Ap

Herschel Gould Belt Survey Observations of Dense Cores in the Cepheus Flare Clouds

We present Herschel SPIRE and PACS maps of the Cepheus Flare clouds L1157, L1172, L1228, L1241, and L1251, observed by the Herschel Gould Belt Survey of nearby star-forming molecular clouds. Through modified blackbody fits to the SPIRE and PACS data, we determine typical cloud column densities of (0.5–1.0) × 1021 cm‑2 and typical cloud temperatures of 14–15 K. Using the getsources identification algorithm, we extract 832 dense cores from the SPIRE and PACS data at 160–500 μm. From placement in a mass versus size diagram, we consider 303 to be candidate prestellar cores, and 178 of these to be "robust" prestellar cores. From an independent extraction of sources at 70 μm, we consider 25 of the 832 dense cores to be protostellar. The distribution of background column densities coincident with candidate prestellar cores peaks at (2–4) × 1021 cm‑2. About half of the candidate prestellar cores in Cepheus may have formed as a result of the widespread fragmentation expected to occur within filaments of "transcritical" line mass. The lognormal robust prestellar core mass function (CMF) drawn from all five Cepheus clouds peaks at 0.56 M⊙ and has a width of ∼0.5 dex, similar to that of Aquila's CMF. Indeed, the width of Cepheus's aggregate CMF is similar to the stellar system initial mass function (IMF). The similarity of CMF widths in different clouds and the system IMF suggests a common, possibly turbulent origin for seeding the fluctuations that evolve into prestellar cores and stars

SMA Observations of the Hot Molecular Core IRAS 18566+0408

We present Submillimeter Array (SMA) observations toward the high-mass star-forming region IRAS 18566+0408. Observations at the 1.3 mm continuum and in several molecular line transitions were performed in the compact (2ʺ4 angular resolution) and very-extended (~0ʺ4 angular resolution) configurations. The continuum emission from the compact configuration shows a dust core of 150 M ⊙, while the very-extended configuration reveals a dense (2.6 × 107 cm−3) and compact (~4000 au) condensation of 8 M ⊙. We detect 31 molecular transitions from 14 species including CO isotopologues, SO, CH3OH, OCS, and CH3CN. Using the different k-ladders of the CH3CN line, we derive a rotational temperature at the location of the continuum peak of 240 K. The 12CO(2–1), 13CO(2–1), and SO(65–54) lines reveal a molecular outflow at PA ~ 135° centered at the continuum peak. The extended 12CO(2–1) emission has been recovered with the IRAM 30 m telescope observations. Using the combined data set, we derive an outflow mass of 16.8 M ⊙. The chemically rich spectrum and the high rotational temperature confirm that IRAS 18566+0408 is harboring a hot molecular core. We find no clear velocity gradient that could suggest the presence of a rotational disk-like structure, even at the high-resolution observations obtained with the very-extended configuration

Herschel Gould Belt Survey Observations of Dense Cores in the Cepheus Flare Clouds

Abstract: We present Herschel SPIRE and PACS maps of the Cepheus Flare clouds L1157, L1172, L1228, L1241, and L1251, observed by the Herschel Gould Belt Survey of nearby star-forming molecular clouds. Through modified blackbody fits to the SPIRE and PACS data, we determine typical cloud column densities of (0.5–1.0) × 1021 cm−2 and typical cloud temperatures of 14–15 K. Using the getsources identification algorithm, we extract 832 dense cores from the SPIRE and PACS data at 160–500 μm. From placement in a mass versus size diagram, we consider 303 to be candidate prestellar cores, and 178 of these to be “robust” prestellar cores. From an independent extraction of sources at 70 μm, we consider 25 of the 832 dense cores to be protostellar. The distribution of background column densities coincident with candidate prestellar cores peaks at (2–4) × 1021 cm−2. About half of the candidate prestellar cores in Cepheus may have formed as a result of the widespread fragmentation expected to occur within filaments of “transcritical” line mass. The lognormal robust prestellar core mass function (CMF) drawn from all five Cepheus clouds peaks at 0.56 M⊙ and has a width of ∼0.5 dex, similar to that of Aquila’s CMF. Indeed, the width of Cepheus’s aggregate CMF is similar to the stellar system initial mass function (IMF). The similarity of CMF widths in different clouds and the system IMF suggests a common, possibly turbulent origin for seeding the fluctuations that evolve into prestellar cores and stars

Runaway Stars as Possible Sources of the Elliptical Ring Structures in NGC 7538

Abstract Two large, ∼500 M ☉ elliptical ring structures have been identified in the high-mass star-forming region NGC 7538. The origin of these ring structures is unknown, so we investigate the possibility that a runaway O- or B-type star may have originated in or passed through the region and created either or both of the ring structures via stellar wind. In testing this hypothesis, we identify one candidate star, BD +61 2408, that may have formed the northern ring. It is a B3e star with a mass of ∼8 M ☉ and a surface temperature of ∼20,000 K. Its position, motion, timescale, and spectral type are all consistent with the star being a candidate for having formed one of the ring structures in NGC 7538.</jats:p