My research interests lie in the field of star and planet formation. As a star forms from the collapse of a gravitationally bound core of interstellar dust and molecular gas, only a small fraction of this material will contribute to the final stellar mass. Some of the dust and gas will orbit the young star in a circumstellar disk, possibly surviving long enough to form a planetary system. Most of the material will be returned to the interstellar medium via jets, outflows, photoevaporation, and the stellar wind. My current research projects include [1] mapping the large-scale interactions of jets with the natal environment, [2] probing the physical structure of the inner outflow cavity walls, [3] monitoring the millimeter variability of young forming stars, [4] carrying out optical spectroscopy observations to derive pre-main-sequence stellar parameters that may affect planet formation processes, [5] studying the evolution of the gaseous material in relation to the dust in the circumstellar disk, and [6] fostering collaborative work between the University of Maryland and the Pontificia Universidad Catolica de Chile in a time of ALMA. In my work, I use a combination of radio observations and radiative transfer modeling of line and continuum emission to analyze the chemical content, physical structure, and dynamical evolution of these young stellar objects throughout the star formation process. I am also interested in the properties of the central star and their connection to specific planet formation processes. My research takes advantage of space-based telescopes, ground-based optical spectrographs, and several international radio interferometers, including the Combined Array for Research in Millimeter-wave Astronomy (CARMA).

ADS Listing for past 5 years

Large-scale Mapping of Star-forming Regions with CARMA
I am involved in a CARMA-23 project to map the NGC 1333 star-forming region in several molecules that trace concentrations of high density gas and outflow activity (P.I. Lee Mundy). The project is a collaboration between science investigators at four CARMA institutes (Maryland, Illinois, Berkeley, and Caltech) and Yale University. The data collected takes advantage of all 23 antennas making up the interferometer, marking the final integration of the eight SZA 3.5-meter antennas from Chicago. The other antennas comprising CARMA are the six 10.4-meter telescopes from OVRO and the nine 6.1-meter dishes from BIMA. Together, the 23-element array provides the best sensitivity and image fidelity of any current interferometer and allows us to resolve the small-scale structure in the region with unprecedented detail. Since we are also interested in the large-scale cloud structure and how this is related to the active regions of star formation within the cloud, we conducted simultaneous single-dish observations, performing the extra empty sky calibration observations to properly calibrate the auto-correlation data. The team is currently busy with the technical challenge of combining the single-dish and interferometry data. Once the map is complete, we can begin to investigate the physical and chemical differentiation throughout this star-forming environment, and particularly the interaction of strong jets with the natal environment.

Figure 1: The star-forming region NGC 1333 as imaged by the Spitzer Space Telescope (Credit: Gutermuth/JPL-Caltech/NASA).

Figure 2: Outflows imaged at millimeter wavelengths toward several embedded protostars (Credit: Jørgensen et al. 2007).

The Physical Structure of Outflow Cavities with Herschel and CARMA
Recent Herschel Space Observatory (HSO) observations show high-J CO emission lines toward several low-mass embedded protostars, highlighting a hot gas component originating in the inner few 1000 AU of the protostellar envelope. Models find that these line intensities can only be reproduced by a combination of UV-heated gas and shocks along the outflow cavity walls. Using millimeter interferometers like CARMA, which are sensitive to the low-J gas lines only, we can image this warm CO gas component and the dust continuum emission with improved resolution, constraining the morphology on 100 AU scales. These data will allow us to constrain the geometry of the outflow, analyze whether the emission is related to the different components and heating mechanisms in ways understood by the current models, and determine how well this emerging outflow picture is able to describe both the warm and hot temperature components toward each source. This research is part of an international collaboration between co-investigators at Copenhagen, Maryland, Austin, Leiden. It is a direct outcome of the Herschel Key Program DIGIT.

Within the DIGIT Key Program, I am responsible for the low-mass embedded sample located in the rho Ophichus dark cloud, sources that I first studied using JCMT and CSO data during my Masters research project in Leiden (van Kempen, van Dishoeck, Salter et al. 2009).

Millimeter Variability of Young Forming Stars
Millimeter continuum observations of young stars presume the flux density levels are dominated by the optically thin thermal emission from the dust in the circumstellar environment. In April 2008, our CARMA observations of the low-mass (T Tauri) spectroscopic binary DQ Tau captured an unusual flare at 3 mm, which peaked at an observed maximum flux of ~0.5 Jy, or about 27 times the quiescent value (Salter, Hogerheijde & Blake 2008). Follow-up millimeter observations using the radio facilities of CARMA, IRAM PdBI, and SMA detected 3 additional flares occurring within 17.5 hours (or 4.6%) of the orbital phase of our first reported flare (Salter, Kóspál, Getman et al. 2010). We explain this non-thermal activity as periodic synchrotron emission initiated by a giant magnetic reconnection event, similar to a solar flare but many times more powerful. In the case of DQ Tau, the two stellar magnetospheres of the highly eccentric (e = 0.556) binary are believed to collide near periastron as the stars approach a minimum separation of 8 stellar radii (~13 solar radii) every 15.8 days. At a separation distance smaller than the theoretical size for a T Tauri stellar magnetosphere, the field lines are forced together and ultimately reconnect. Supporting evidence for a magnetic reconnection event was obtained during simultaneous X-ray observations, which show the relative timing of the X-ray and millimeter flares, the derived flare loop length, and the X-ray luminosity are all consistent with the currently proposed picture (Getman, Broos, Salter et al. 2011).

Our findings offer caution for millimeter flux points in spectral energy distributions that could contain unrecognized flare contributions if measured only once, with the time-consuming nature of observations at these wavelengths leading to a paucity of follow-up measurements. For example, the time-averaged fluxes for the 3 follow-up light curves towards DQ Tau are not obviously out of the ordinary (since we did not catch the peak of another outburst), and might easily have been overlooked were we not specifically looking for variability. Because the geometry of the DQ Tau system seem to be the key parameters in this star-star magnetic interaction scenario, we conducted the first systematic study of millimeter variability toward 12 similar young binaries using the IRAM 30-meter dish (Kóspál, Salter, Hogerheijde et al. 2011). In this sample, UZ Tau E, the binary most similar in orbital geometry to DQ Tau, was the only source to exhibit variability. As a result, we are planning to conduct a long-term monitoring survey of a small sample of nearby sources with the OVRO 40-meter, a collaboration between investigators at Maryland, Leiden, and Caltech. In the near future, ALMA will allow us to perform large surveys of star-forming regions to better characterize millimeter variability toward young stars.

Figure 3: Our CARMA observations of DQ Tau during a quiescent state, left, and during a flare, right (Salter et al. 2008).

Figure 4: Our schematic showing how the stellar magnetospheres in the DQ Tau binary combine near closest approach, top, and separate a short time later, bottom (Salter et al. 2010).

Protoplanetary Disks
As part of my PhD research, I studied the evolution of the gaseous material in relation to the dust in the circumstellar disks around young low-mass (T Tauri) stars. Since the dust in circumstellar disks is much easier to observe, several models exist to describe the dust (temperature and density) structure. Unfortunately, many of these models are degenerative. In addition, the dust represents only 1% of the total disk mass. Our current goal is a better understanding of the gas content in disks, so that we can constrain the disk properties and determine which factors most influence the evolution of each disk and their propensity for planets. For example, as the dust grains grow to larger sizes (the first steps of planet formation), then the UV and X-ray radiation from the central star might penetrate the disk more, altering the chemical content of the gas reservoir. We were unable to detect this effect in a recent single-dish JCMT survey, which led us to conclude for now that the stellar radiation field may play a more important role in the disk-integrated line fluxes (Salter, Hogerheijde, van den Burg et al. 2011).

Figure 5: Three disk models illustrate how the observed molecular line emission profiles change depending on the underlying disk temperature and density structure (Salter et al. 2011).

Figure 6: An artist's rendition of ALMA, the Atacama Large Millimeter Array (Credit: ESO/NAOJ/NRAO).

Fostering Collaborations with Chile in a Time of ALMA
The University of Maryland at College Park and the Pontificia Universidad Catolica de Chile (PUC) have created a joint astronomy PhD program. Students enrolled at one university will be able to take advantage of the telescope facilities and professional expertise available at the other university. As the UMD-PUC post-doc, my job is to help facilitate collaborations between the two institutes and provide millimeter observing expertise as the next generation radio telescope ALMA, located in the Chilean Andes, begins early science in late 2011. Towards this goal, I will spend half my post-doc tenure at Maryland and the other half at PUC.