I am currently at the Institute for Astronomy where I'm investigating Galactic star formation on the Institute's Submillimeter Postdoctoral Fellowship. My graduate research was completed at UC Berkeley under the guidance of Prof. Wm. J. Welch.

Past & Current Research
While I find all topics of astronomy fascinating, my professional research has focused on Galactic star formation. Following the discovery of a pre-stellar core located in an evolved and isolated molecular cloud (Swift et al. 2005, 2006), I spent the last part of my graduate career designing and completing a comprehensive observational program aimed at understanding the L1551 dark cloud in which that core exists (Swift 2006, Swift & Welch 2008).

Through that effort I have become familiar with virtually all facets of modern star formation research, and I have gained experience observing in wavebands from 1 µm to 1 cm. Currently, my primary topic of interest is the initial conditions of cluster and high-mass star formation (Swift & Williams 2008, Swift 2009 submitted.), although I am also involved in an ongoing, large scale study of circumstellar disks (see Cieza et al. 2008).

Discovery Of A Core:
The impetus behind my PhD thesis work was the discovery of a dense molecular core located in the evolved molecular cloud L1551. Data from BIMA (using the 1 cm receivers), the Kitt Peak 12 m, and the Green Bank Telescope (K-band) reveal ~2 M8 of cold (9 K), gravitationally bound gas showing evidence for gas inflow in the molecular line profiles (Swift et al. 2005). A careful analysis showed that it is very likely that this core will form the next star or stellar system in L1551.

Figure 1: This cold, starless core seen here in N2H+ (gray scale) and CS (spectra) is likely to be the next star or stellar system to form in L1551.

Further study of this core in 13CO, C18O, CS and N2H+ using the Five Colleges 14 m revealed a dynamic and inflowing envelope around this core (Swift et al. 2006). Figure 1 shows the predominantly blue-shifted, self-absorbed CS line profiles overlaid on velocity integrated N2H+ emission. Non-LTE radiative transfer modeling suggests inflow velocities of ~0.1 km/s across the full extent of the core. A statistical treatment of the CS line asymmetries suggest that we are seeing a cold layer of gas moving toward the core that is likely associated with the build up of core mass from the ambient cloud material rather than gravitational collapse of the core.

While the discovery of this core is interesting in its own right, I soon learned that the context in which this core exists has grander implications for our current understanding of low-mass star formation. My focus then shifted toward an understanding of the L1551 cloud as a whole.

A Case Study of L1551 (Swift & Welch 2008):
L1551 is a well-studied cloud in the south of the Taurus complex famous for harboring the first recognized molecular outflow source. It is isolated from other active sites in Taurus and is associated with young stars in all known phases of formation—from Class 0 protostars to Class III weak-line T Tauri stars.

To better understand the stellar component of L1551, literature data were culled and supplemented with the 2MASS point source catalog. Space motions and the projected distribution of stars were used to select out 35 stars that constitute the L1551 association. Prominent pre-main sequence models suggests that ~25% of the association is older than 6 Myr, and the spatial distribution of young stars in relation to their velocity dispersion is consistent with star formation occurring over several million years.

Figure 2: Extinction map of the L1551 cloud overlaid with the positions of the pre-main sequence stars in the field.

Figure 2 shows an extinction map made using Kitt Peak 2.1m FLAMINGOS data that has enabled the most accurate mass determination and radial profile of L1551 to date. The 160 M8 of gas and dust in L1551 is confined within a 0.9 pc radius resulting in ~1045 ergs of gravitational energy.

The three-dimensional velocity dispersion of the gas is clearly of non-thermal origin and contributes 0.68 Egrav of kinetic support against gravity. Thus, like many molecular clouds, L1551 is nearly virial. However, given the age of the cloud implied by the stellar component and the short dissipation time of turbulence, the observed turbulent energy cannot be primordial in nature.

A major source of the observed turbulent motions is revealed in the observational centerpiece of this case study; a 256 pointing interferometric mosaic performed with the BIMA array. Simultaneous detections of the 13CO and C18O (1–0) molecular transitions probe the intermediate density gas while the ~70 m baselines translate to a spatial resolution of ~10”. Single dish maps from the Kitt Peak 12 m provide total flux measurements over the cloud such that all spatial frequencies are well-sampled on scales above 10”.

Figure 3: 13CO(1-0) channel maps of L1551. The dashed contour outlines the high-resolution region obtained with the BIMA mosaic, stars (“x”s), Herbig-Haro objects (triangles), and jet orientations (lines) are denoted.

Figure 3 shows the amazing amount of detail revealed in the 13CO data set. The cloud appears mostly smooth in the central velocity channels (a) and (e) due to the large optical depth of the ambient cloud. However, regions of the cloud that have been excavated by molecular outflow are clearly delineated. The boundaries of the cavities are sharp and highly turbulent.

These boundaries appear as thin structures in the high velocity channels and represent the shocked interfaces between outflow cavities and the ambient cloud material. A two-dimensional spatial power spectrum of these structures reveals a preferential scale of ~0.05 pc. This is the predominant scale for turbulent energy injection from outflow.

The above analyses culminate in a comparison between the rate of energy injection from outflow and the dissipation rate of turbulent energy. They are found to be roughly equal, suggesting that L1551 has been forming stars for several free-fall times with the feedback from embedded stars providing support against global collapse.

With all case studies, there is a risk that the subject is anomalous in unforeseen ways, and the conclusions from this study cannot rule out the possibility that dynamical star formation may be a dominant mode in the Galaxy. However, the overall consistency of these analyses with current theories of outflow regulated star formation both lend credence to those models and emphasizes the need to consider feedback in all theories of star formation.

The Dense Core Mass Function:
The intent in taking my first postdoctoral position at the Institute for Astronomy was to apply the observational techniques of my thesis to higher mass regions using the Submillimeter Array (SMA) and other Mauna Kea telescopes. Once a high-mass star forms, it quickly obliterates its immediate environment, destroying any clues as to how it formed. Thus my research focus returned to the molecular cloud phase.

One of the most fundamental distributions in all of astrophysics is the stellar initial mass function (IMF). Surveys of dense cores in star-forming clouds suggest that the dense core mass function (DCMF) and the IMF have the same general form. This has been broadly interpreted as direct evidence for a one-to-one relationship between cores and the stars to form from them. Moreover, the similarity suggests that the IMF is determined in the molecular cloud phase through thermal or turbulent fragmentation processes.

Given the scant numbers of dense cores and relatively featureless distributions, it is surprising that this idea had not been tested in greater detail. Therefore, despite my observational bent, I designed and wrote computer procedures to perform Monte Carlo simulations that explore the robustness of this interpretation (Swift & Williams 2008).

When comparing a DCMF to an IMF there are fundamentally 2 degrees of freedom that significantly reduce the ability to draw conclusions based solely on the shapes of the distributions. One arises because the total number of stars that will result from an observed core distribution cannot be known and allows the IMF to be vertically shifted without restriction. The other freedom comes about through our ignorance of the star formation efficiency (SFE) in cores and allows the IMF to be shifted horizontally in an unconstrained manner.

Figure 4: Simulated evolution of a distribution of dense cores. Four varying realizations of the IMF (colored lines) are all consistent with the original form of the DCMF over an order of magnitude in mass.

Figure 4 shows the results of a numerical experiment designed to test the ability of state-of-the-art observational studies of dense cores to discern core evolutionary pathways and derive parameters such as the SFE. The histogram is the mass distribution of 300 cores following the expected form, lognormal at low masses with a Salpeter tail toward higher masses. The colored lines represent realizations of the stellar IMF resulting from 4 different core evolution prescriptions.

All models are consistent with the initial DCMF over an order of magnitude in mass, and significant covariance exists between multiplicity, fragmentation and SFE. While the similarity between the DCMF and the IMF may be evidence that the IMF is determined in the molecular cloud phase, at the current level of observational accuracy the comparison between mass functions of dense cores and stars alone is insufficient to discern between different evolutionary models.

This work issues a word of caution when comparing relatively few numbers of cores with the IMF, and accentuates the importance of extending our knowledge of the DCMF to both lower and higher core masses. My current research is aimed at understanding the higher mass end.

Infrared Dark Clouds
The precursors to massive stars are expected to be found in massive, dense clouds. Currently, infrared dark clouds (IRDCs) are the best candidates for pre-cluster environments. They have masses ranging to above 10⁴ M₈, average volume densities up to 10⁵ cm^-3, and some are known to harbor massive, young stars.

Using archival Spitzer and SCUBA data, two clouds have been selected as highly likely to represent pre-cluster environments and chosen for detailed study: MSX G028.53-00.25 and MSX G030.88+00.13. I have obtained wide-field mosaics of these clouds in the 850µm waveband using the SMA that have been combined with SCUBA archival data. JCMT HARPB data cubes, CSO single pointing heterodyne observations, and CFHT WIRCam data in the NIR bands supplement the study.

Figure 5: Spitzer composite image of G030.88+00.13 overlaid with contours from the SMA+JCMT mosaic (orange). MIPS 24µm sources likely to be associated with the cloud are labeled with “X”s.

Figure 5 shows a Spitzer composite image of one of the clouds utilizing the 4 IRAC bands plus the 24 µm MIPS band. Overlaid in orange are contours of the SMA+SCUBA data with the synthesized beam shown in the lower left. A dense core near the projected center of G030.88+00.13 dominates the compact sub-millimeter emission detected with the SMA. The core contains 120 M₈ of mass and has a peak column depth of 2.2 g cm^-2. A non-detection of SMA–1 at 70µm restricts the bolometric temperature and luminosity to below 16 K and 200 L₈, respectively, strong evidence that no massive protostar has yet formed.

Modeling of the emergent spectral energy distributions of the MIPS sources likely associated with the cloud (Robitaille et al. 2007) restricts the current star formation occurring in this cloud to the low-mass regime. However, the mass of the parent cloud, 3800 M₈, and the location of this core in the M_env–L_bol diagram imply that a star or stars with mass ~10 M₈ may form (Larson 1982, Molinari et al. 2008). The high mass and column depth of SMA–1 supports the view that it is a plausible precursor of a massive star (Swift 2009 submitted).

Past Projects
PKS1830-211 - Observations of a gravitationally lensed quasar using the BIMA interferometer in its extended configuration resolved the two main components of a gravitationally lensed quasar. Saturated molecular absorption appears in one of the two images and there is no evidence of absorption in the other image. This fortuitous circumstance has allowed astronomers to track the light curves of the two images separated by ~1" without having to spatially resolve them. The redshift of the lensing galaxy, the redshift of the lensed quasar, and the time delay of the two images have been used to determine the rate of expansion of the universe.

Optical Pointing for Radio Telescopes - Pointing solutions for radio telescopes are typically good to ~ a 10th of a primary beam width. For interferometric observations, pointing errors lead to a loss in sensitivity. However, when mosaicing large fields with an interferometer, pointing errors also lead to a degredation of image fidelity. Tracking or offset pointing on a guide star in the optical could reduce pointing errors on radio dishes to the arcsecond level improving sensitivity, image fidelity and the overall efficiency of radio interferometer observatories. The specifications and perfomance of a cooled CCD video camera mounted on the BIMA pointing optics are outlined in this BIMA Memo. A follow up study characterizing the collimation of the radio and optical pointing on BIMA antenna 1 is written up in this memorandum.