Hubble image of the merging galaxy cluster Abell 520

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Stephen Holland's Research Interests

My primary scientific interest is the explosive deaths of massive stars, and using these explosions to probe the local environments about these stars. I also work on understanding the stellar populations of Galactic and extragalactic globular clusters.

Gamma-Ray Bursts

Gamma-Ray Burst

Much of my research over the past decade has concentrated on studying the nature of gamma-ray bursts, their afterglows, and the physical environments where they occur. Understanding the physical environments that gamma-ray bursts occur in is fundamental for understanding the physical cause of burst afterglows.

An unsolved problem with gamma-ray bursts is the detailed nature of their local environments. This is a critical problem because afterglows are the result of the interaction between outflow from the gamma-ray burst's central engine and the local environment. The progenitors of long-soft gamma-ray bursts (ie, those with observed durations of more than two seconds and soft spectra) are thought to supernovae from some types of massive stars that have experienced significant mass loss, such as Wolf-Rayet stars or luminous blue variables, In this scenario a gamma-ray burst's local environment (and thus the details of its afterglow) should be dominated by the stellar wind blown off the star near the end of its life. The resulting density profile, ρ, around the star will be ρ(R) ∝ R-2 where R is the distance from the star. The density profile can be studied by combining data from the X-ray to the infrared bands to provide a powerful window into the environments that gamma-ray bursts explode into. Different local environments imprint different signatures on the light curves, and the time evolution of the spectral energy distributions, of GRB afterglows, so detailed observations of GRB afterglows over a wide range of wavelengths can be used to probe the burst's environment and constrain the behaviour of the progenitor prior to the explosion. A surprising result of these studies has been that most of the gamma-ray bursts with well-studied local environments have a homogeneous local density structure (ρ(R) = constant), not a wind-dominated one. This is difficult to explain if the progenitors are massive stars at the end of their lives.

The standard relativistic fireball model for GRB afterglows predicts that the afterglow will have a synchrotron spectrum, and that the flux and spectrum of an afterglow will evolve in a well-understood way with time. These predictions depend on the nature of the environment that the burst is expanding into in several ways. For example, the evolution of the synchrotron cooling frequency with time (whether it moves to higher or lower frequencies) depends on the environment's density profile. This provides a powerful tool for probing the density structure near the progenitor. I have used used X-ray and ultraviolet/optical/infrared observations to probe the local environments of many gamma-ray bursts. Increasingly, observations are suggesting that a simple ρ ∝ R-2 or ρ = constant profile is an oversimplification. GRB 990123 occurred in a homogeneous insterstellar medium (Holland et al. 2000). GRB 021004 appeared to have occurred in an environemnt that was homogeneous on scales of approximately one parsec (= 3.24 light years) but clumpy on smaller scales (Lazzati et al. 2002; Holland et al. 2003). GRB 011211 expanded into a wind-dominated environment (Holland et al. 2002; Price et al. 2002; Garnavich et al. 2003). GRB 081029 showed evidence for a complex environment (Holland et al. 2012), and the temporal and spectral evolution of the dark burst GRB 090417B were well described by assuming that there was a sheet of dust along the line of sight (Holland et al. 2010; see Figure 1). Taken together these results, and other multi-wavelength data, suggest that gamma-ray burst environments can be much more complex than a simple, one-parameter density profile. I am continuing these studies ground-based telescopes as well as data from the Swift, and HST observations to obtain a wide range of ultraviolet, optical, and infrared data on the afterglows and host galaxies of GRBs.

GRB 090417b light curve and spectral hardness evolution

Figure 1. A dust scattering model gives a very good fit to the GRB 090417B X-ray data. Top panel: the fit to the Swift X-ray light curve. Bottom panel: the fit to the spectral index evolution. The model used is a sheet of dust approximately 30 -- 80 pc (about 100 -- 250 light years) from the progenitor of GRB 090417B. The dust sheet imparts an extinction of AV ∼ 11 mag, which explains the dark nature of this burst. The model predictions for the sheet of are shown as solid lines and data as crosses. The data agree well with the model.

The spectral energy distributions of the host galaxies of gamma-ray bursts can be used to estimate their ages and metallicities. In cases where ultra-high resolution images are available this can be done for the local star-forming regions where the bursts occurred. This data can further constrain the circumburst environment and provides information about the so-called dark gamma-ray bursts. These are gamma-ray bursts where optical afterglows are expected based on their X-ray properties, but are not seen. There is growing evidence that many dark gamma-ray bursts are due to high levels of dust in their host galaxies (Holland et al. 2010). This has implications for using gamma-ray bursts to probe the star-formation rate at high redshifts.

Supernovae

Supernova

Stars that have more than about eight times more mass than the Sun end their lives as supernovae. When the star has burned all of the hydrogen in its core the core will contract until the pressure and temperature are high enough that helium fusion burning starts. This continues until the helium is exhausted, and the cycle repeats for heavier elements such as carbon, neon, oxygen, and sillicon. When the sillicon has been exhausted the core is made of nickel and iron, which do not undergo nuclear fusion. At this point the star is a supergiant and there is nothing to prevent gravity from causing the star to collapse in on itself. This collapse triggers a masive explosion in the core and the start becomes a supernova. In some cases a neutron star or a black hole is left behind. These are called core-collapse supernovae, and come in many different varieties depending on the mass of the original star and the details of how much of the star's atmosphere remains when the supernova occurs.

A second type of supernova, called a Type Ia supernova, occurs when large amounts of material fall onto a white dwarf. When enough material has been accreted that the mass of the white dwarf exceeds 1.4 times the mass of the Sun the white dwarf collapses under its own gravity and explodes. Type Ia supernovae are very useful because their intrinsic luminosity is tightly related to the rate that they fade. This means that Type Ia supernova can be used as high-precision standard candles to measure distances in the Universe. This is what led to the discovery in 1998 that the expansion of the Universe is accelerating.

Since 2005 I have been part of a the Swift Supernova Team. The ultraviolet imaging and spectroscopic capabilities of UltraViolet Optical Telescope (UVOT) on Swift make it an ideal instrument for observing supernovae. Ultraviolet observations of Type Ia supernova have shown that they are not as good standard candles in the ultraviolet as they are in the optical. Our work has also shown that Type Ia supernova are likely to be the result of the merger of two white dwarfs.

Stellar Populations in the Halo of M31

An Ultraviolet Image of M31

M31, also known as the Andromeda Galaxy, is the nearest large galaxy to our own. The galaxy is visible to the naked eye from a dark site from early October to early March, and it is the only external galaxy that can be seen with the naked eye from the norther hemisphere.

The Andromeda galaxy is an ideal place to study stellar populations because all the stars are at approximately the same distance from us, and M31 is far enough that a single observation can obtain data on a large number of stars. This makes it possible to build up a statistically significant data set using a reasonable amount of resources. For many years it was assumed that M31 is a twin of our own galaxy. However, studies in the late 1990s (eg, Holland et al. 1996) showed that there is a wide range in the chemical composition of stars in M31's halo. This is unlike the halo of the Milky Way, which has stars with a fairly uniform, and low, metallicities. Furhter work has confirmed this result and strongly suggests that M31 was built up by cannibalizing its small neighbouring galaxies over several billion years.

Globular Clusters

The globular cluster M13

Globular clusters are spherical collections of between about 10,000 and 1,000,000 stars that are usually found in the halos of galaxies. In most cases all of the stars have the same age and chemical composition, although there are a few globular clusters that have small internal variations. The fact that all the stars were born at about the same time, and have about the same chemical compositon, makes globular clusters ideal laboratories for studying stellar evolution. Our Galaxy has a few hundred known globular clusters, but M31 has at least 1000, and we do not know why one galaxy has so many more than the other. Studies such as Holland et al. (1997) and Holland et al. (1998) have shown that the globular clusters in the two galaxies have similar ages, a similar metallicity distribution, and a similar luminosity function. This suggests that the globular clusters formed in much the same way in the two galaxies, even though the formation histories of the Milky Way and M31 may have been very different.

In 1997 early results from the Hipparcus mission suggested that M31 was much farther away then previously thought. I used HST observations of 14 globular clusters in M31 to derive a new distance to M31 (Holland (1998) of 783 kpc (or 2.55 million light-years), in agreement with previous values. This remains the accepted distance to M31.


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Stephen Holland,
Most recently updated: June 13, 2012

sholland@stsci.edu

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