I am interested in star formation, especially the initial mass function, and obtaining observational constraints for stellar evolution models.

Tidal Radius and Dynamics of the Arches Cluster

The Arches Cluster is a young (2-4 million years old) and massive (~10,000 solar mass) star cluster very close to the center of our galaxy. The Arches provides a unique window into star formation in the extreme environment near the galactic center, which is expected to have conditions more similar to those in early galaxies than the environment near the sun and nearby star clusters. Of particular interest is the Initial Mass Function (IMF) of the Arches Cluster, which describes the characteristic number of stars formed at a given mass during the star formation process.

Before we can understand the IMF of the Arches cluster, we need to know a few of its basic characteristics, namely its size, mass structure, and orbit. However, these are difficult to determine because the cluster is heavily shrouded by thick patches of interstellar gas and dust, and worse, different amounts of dust cover different regions of the cluster. To combat this, I am working with Jessica Lu to identify cluster members via astrometry, or the precise measurement of stellar positions over time. Since the cluster is orbiting the galactic center at a different velocity than the surrounding field stars, we can use astrometry to determine which stars are part of the cluster and which are not. Once we identify cluster members, we can measure the cluster's tidal radius, or the radius at which stars are no longer gravitationally bound to the cluster, as well as get a better handle on the cluster's mass structure and orbit. This will bring us an important step closer to accurately measuring the cluster's IMF.

The Arches Cluster, a young massive star cluster near the galactic center. Credit: HST NICMOS, D. Figer (StScI) and NASA

Extragalactic Spectroscopy of Blue Supergiants

Blue supergiants (BSGs) are the advanced evolutionary stage of stars 12-40 times the mass of the sun. They are so bright that we can obtain resolved low-resolution spectra of these objects in nearby galaxies out to 10 Mpc (~32 million light years) away, far beyond the distance where resolved spectroscopy of other stars is possible. Blue supergiants are thus a useful tool for studying these galaxies.

Blue supergiants in dwarf galaxy NGC 3109, identified by blue and red circles. These stars are bright enough that we can obtain resolved low-resolution spectra of them despite the fact that the galaxy is 1.27 Mpc (~4 million light years) away. The other bright objects in the image are not associated with the galaxy.

By comparing the spectra of blue supergiants to stellar atmosphere models, we can derive two important pieces of information: how many metals (elements heavier than helium, in astronomer-speak) are present in the star and its distance from Earth. The metal content, or metallicity, of a star is important because it reflects the metallicity of its surrounding environment, which in turn yields information about the evolution of the galaxy at large. From spectroscopy we can also determine the surface temperature and gravity of the star, which can be converted into a luminosity (or intrinsic brightness) through the Flux-Weighted Gravity-Luminosity Relation (FGLR). By comparing how bright the star appears on Earth to how bright it actually is, we can determine the star's (and thus the galaxy's) distance. For more information about the FGLR and its advantages as a distance indicator, see Kudritzki et al (2008) and Kudritzki et al (2012).

Working with Rolf Kudrtizki, Fabio Bresolin and collaborators, I did quantitative spectroscopy on 12 late-B and early-A type blue supergiants in dwarf galaxy NGC 3109, providing an independent measure of the galaxy's metallicity and distance. This work is part of an ongoing effort to study Local-Group and nearby galaxies using blue supergiants, important because galaxy metallicities are difficult to measure and because it provides a crucial check of distances determined using other methods such as Cepheid Variables. For details see Hosek et al. (2014).

The observed Flux-Weighted Gravity-Luminosity Relation (FGLR) of blue supergiant stars. If we can determine a BSG's surface temperature T and gravity g (combined in the variable g_f = g / (T * 10^-4)^4, in cgs units), this relation allows us to determine the star's absolute magnitude, which we can use to calculate a distance.

Undergraduate Research

Outburst Dust Production of Comets

The intrinsic brightness of a comet is an indicator of how active it is, or how much material it is ejecting into space. In general, comets become brighter as they get closer to the sun and warm up, which causes water and other volatiles to sublimate from its surface. Every so often, a comet will dramatically brighten and then fade back to its original brightness over the span of a few days. These events are called outbursts, and are thought to be triggered by a sudden massive release of dust into space. Because they are so unpredictable, outbursts are difficult to study in their entirety.

Comet 29P/Schwassmann-Wachmann 1 (SW1) is an example of a comet that is known to outburst frequently. Working with NASA Marshall Space Flight Center Meteoroid Environment Office, I monitored he brightness of the comet during its 2011 and 2012 apparitions. We observed two outbursts over this time, and because of the frequency of our observations, we were able to record the events from nearly beginning to end. This allowed us to roughly calculate the total mass of dust produced in each event and estimate that a significant fraction (greater than 50%) of the yearly dust production of SW1 comes from outburst activity. This study demonstrates that outbursts can be a significant source of cometary dust, and so these events must be monitored for in Earth-crossing comets to improve meteor shower forcasts. See Hosek et al. (2013) for details.

The May 2011 outburst of SW1. Left: Image of the comet before (left) and after (right) the observed outburst. Right: Afρ, a proxy for dust production, versus time during the May 2011 outburst. Note the rapid increase in dust production followed by the prolonged decay. This outburst was observed over a span of 9 days.