I am interested in star formation in extreme environments and obtaining observational constraints for stellar evolution models.

The Arches and Quintuplet Star Clusters: Star Formation Near the Galactic Center

The Arches and Quintuplet clusters are young (2-5 million years old) and massive (~10,000 solar mass) star clusters very close to the center of our galaxy. They provide a unique window into star formation in the extreme environment near the galactic center, which is expected be more similar to star forming environments in early galaxies than the environment near our sun. Of particular interest is the Initial Mass Function (IMF) of the Arches and Quintuplet clusters, which describes the characteristic number of stars formed at a given mass during the star formation process.

Before we can understand the IMF of these clusters, we first need to identify which stars are actually part of the cluster as opposed to the foreground and background stellar populations. This is challenging because the Arches and Quintuplet are heavily shrouded by thick patches of interstellar gas and dust which vary in intensity across the clusters. As a result, standard methods of identifying cluster members from their observed brightnesses and colors are not effective. In an alternative approach, I am working with Jessica Lu (my advisor) and collaborators 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. We demonstrate the ability to do this for the Arches cluster in Hosek et al. 2015.

The Arches Cluster, a young massive star cluster near the Galactic Center. In this infrared Hubble image, F153M is red, F139M is green, and F127M is blue. Credit: HST WFC3IR, 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.