ABSTRACT
Research Interests:
Research Interests:
Research Interests:
ABSTRACT
The globular cluster NGC 1851 was observed with STIS/FUV-MAMA and the F25QTZ filter in 1999. We have examined this data and obtained additional data using the ACS/SBC in 2006. The optical identification of the ultaviolet sources has... more
The globular cluster NGC 1851 was observed with STIS/FUV-MAMA and the F25QTZ filter in 1999. We have examined this data and obtained additional data using the ACS/SBC in 2006. The optical identification of the ultaviolet sources has allowed us to identify the various poplulations in this cluster and the significance of those populations. The data allows us to examine the various ultraviolet sources for variability and we have identified a number of unique sources.
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Wolf-Rayet (WR) stars are very massive, luminous stars, likely descended from O stars with initial masses greater than 30 solar masses. The byproducts of nuclear burning have been brought to the surface of these He-rich stars and a high... more
Wolf-Rayet (WR) stars are very massive, luminous stars, likely descended from O stars with initial masses greater than 30 solar masses. The byproducts of nuclear burning have been brought to the surface of these He-rich stars and a high rate of mass loss is evident through strong emission lines of He, C, N and O. The unique emission line spectra of WR stars make them readily identifiable. Despite the importance of WR stars both in the evolutionary paths of massive stars and as tracers of active star formation, only 250 of these stars have been identified in the Galaxy due to absorption. To detect Galactic WR stars, a narrowband infrared imaging survey has been completed in seven filters. The survey covers the Galactic plane +/-1 degree and was taken in J, narrowband filters centered on HeI, HeII, CIV, Bracket Gamma and in continuum filters both redward and blueward of the narrowband filters. WR candidates are being selected by their excess emission in the narrowband filters; we pres...
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We present an analysis of optical HST/STIS and HST/FOS spectroscopy of blue stragglers found in the globular clusters 47Tuc, M3, NGC6752 and NGC6397. Spectroscopic masses, and rotation rates are presented which impose constraints on their... more
We present an analysis of optical HST/STIS and HST/FOS spectroscopy of blue stragglers found in the globular clusters 47Tuc, M3, NGC6752 and NGC6397. Spectroscopic masses, and rotation rates are presented which impose constraints on their evolutionary history. 6 blue stragglers in our sample cannot be fit with state of the art non-LTE stellar atmosphere models. The 6 misfits possess Balmer jumps which are too large for the effective temperatures implied by their Paschen continua. We find that our data for these stars are consistent with models only if we account for extra absorption of stellar Balmer photons by an ionized circumstellar disk. Column densities of HI and CaII are derived as are the the disks' thicknesses. This is the first time that a circumstellar disk is detected around blue stragglers. The presence of magnetically-locked disks attached to the stars has been suggested as a mechanism to lose the large angular momentum imparted by the collision event at the birth o...
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Globular clusters are gravitationally bound stellar systems containing on the order of 105 stars. Due to the high stellar densities in the cores of these clusters, close encounters and even physical collisions between stars are... more
Globular clusters are gravitationally bound stellar systems containing on the order of 105 stars. Due to the high stellar densities in the cores of these clusters, close encounters and even physical collisions between stars are inevitable. These dynamical interactions can produce exotic types of single and binary stars that are extremely rare in the galactic field, but which may be important to the dynamical evolution of their host clusters. A common feature of these dynamically-formed stellar populations is that many of their members are relatively hot, and thus bright in the far-ultraviolet (FUV) waveband. In this short review, we describe how space-based FUV observations are being used to find and study these populations.
Seitenzahl et al. (2009) have predicted that $\sim 3$ years after its explosion, the light we receive from a Type Ia supernova will come mostly from reprocessing of electrons and X-rays emitted by the radioactive decay chain $^{57}{\rm... more
Seitenzahl et al. (2009) have predicted that $\sim 3$ years after its explosion, the light we receive from a Type Ia supernova will come mostly from reprocessing of electrons and X-rays emitted by the radioactive decay chain $^{57}{\rm Co}~\to~^{57}{\rm Fe}$, instead of positrons from the decay chain $^{56}{\rm Co}~\to~^{56}{\rm Fe}$ that dominates the supernova light at earlier times. Using the Hubble Space Telescope, we followed the light curve of the Type Ia supernova SN2012cg out to $1055$ days after maximum light. Our measurements are consistent with the light curves predicted by the contribution of energy from the reprocessing of electrons and X-rays emitted by the decay of $^{57}$Co. This provides conclusive evidence that $^{57}$Co is produced in Type Ia supernova explosions. The ratio of luminosities produced by the decays of $^{57}$Co and $^{56}$Co, a strong constraint on any Type Ia supernova explosion model, is in the range $(0.4$ - $8.5)\times10^{-3}$.
Tony Moffat has been inspiring the hunt for new Wolf-Rayet stars for over 40 years. These extraordinary objects offer critical tests of stellar evolution theory, and are predicted to be progenitors of type Ib and Ic supernovae. We're... more
Tony Moffat has been inspiring the hunt for new Wolf-Rayet stars for over 40 years. These extraordinary objects offer critical tests of stellar evolution theory, and are predicted to be progenitors of type Ib and Ic supernovae. We're only going to know if that prediction is correct (in our lifetimes) by locating and spectrographically confirming of order 10 000 WR stars - a daunting but increasingly doable task. Our 2009 prediction that roughly 6 500 Wolf-Rayet stars live in our Galaxy has been followed by demonstrations in the past few years that, via narrowband infrared imaging and spectroscopy, we can find and confirm almost all Galactic WR stars. The rest of the Local Group is unlikely to contain more than 1 000 WR stars, so the Milky Way is THE place to search exhaustively for them. I briefly describe how we hunt and gather WR stars and give a current (mid-2011) Local Group census of them.
Wolf-Rayet (WR) stars mark the final evolutionary stages of massive stellar evolution (M > 25 Msun) and the short lives of their precursors (< 10 Myr) make them excellent tracers of recent star formation. The WR phase (the final few... more
Wolf-Rayet (WR) stars mark the final evolutionary stages of massive stellar evolution (M > 25 Msun) and the short lives of their precursors (< 10 Myr) make them excellent tracers of recent star formation. The WR phase (the final few x 10^5 yrs of a massive star's life) is largely categorized by ferocious winds and high mass-loss rates that chemically enrich the interstellar medium. The observational signatures of these dense, fast winds are optical and near-IR spectra dominated by strong, velocity-broadened emission lines. At even longer mid infrared wavelengths, WR stars show excess due to free-free emission. Utilizing both 2MASS and WISE photometry, we present refined color-color selection criterion for identifying optically hidden Galactic Wolf-Rayet (WR) stars. Furthermore, using W4 (22 micron) images of the known population, we have identified a plethora of incredible structures. These ``bubbles" surrounding Wolf-Rayets trace an environment which is fundamental f...
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We present N-body models to complement ACS imaging of the metal-poor core-collapsed cluster NGC 6397 during HST Cycle 13 (see poster by Richer et al. at this meeting). We focus on the results of a simulation that began with 100,000 stars,... more
We present N-body models to complement ACS imaging of the metal-poor core-collapsed cluster NGC 6397 during HST Cycle 13 (see poster by Richer et al. at this meeting). We focus on the results of a simulation that began with 100,000 stars, 5% primordial binaries and Population II metallicity. After 15 Gyr of evolution the model cluster has about 20% of the stars remaining and is core-collapsed. We find that in the region of the model corresponding to the observed field of NGC 6397 (about two half-mass radii from the cluster center) the white dwarf population has sufferred little modification from dynamical processes -- contamination of the luminosity function by binaries and white dwarfs with non-standard evolution histories is minimal and does not affect measurement of the cluster age. For the luminosity function of the main sequence stars we find that although this has been altered significantly by dynamics over the cluster lifetime, especially in the central and outer regions, tha...
We are systematically surveying all known and suspected Z Cam-type dwarf novae for classical nova shells. This survey is motivated by the discovery of the largest known classical nova shell, which surrounds the archetypal dwarf nova Z... more
We are systematically surveying all known and suspected Z Cam-type dwarf novae for classical nova shells. This survey is motivated by the discovery of the largest known classical nova shell, which surrounds the archetypal dwarf nova Z Camelopardalis. The Z Cam shell demonstrates that at least some dwarf novae must have undergone classical nova eruptions in the past, and that at least some classical novae become dwarf novae long after their nova thermonuclear outbursts, in accord with the hibernation scenario of cataclysmic binaries. Here we report the detection of a fragmented "shell", 3 arcmin in diameter, surrounding the dwarf nova AT Cancri. This second discovery demonstrates that nova shells surrounding Z Cam-type dwarf novae cannot be very rare. The shell geometry is suggestive of bipolar, conical ejection seen nearly pole-on. A spectrum of the brightest AT Cnc shell knot is similar to that of the ejecta of the classical nova GK Per, and of Z Cam, dominated by [NII] e...
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ABSTRACT We undertake a survey of the Wolf-Rayet population of NGC 6744, a spiral galaxy located at an estimated distance of 11.6 Mpc, in order to determine whether the distribution of this population is consistent with the distributions... more
ABSTRACT We undertake a survey of the Wolf-Rayet population of NGC 6744, a spiral galaxy located at an estimated distance of 11.6 Mpc, in order to determine whether the distribution of this population is consistent with the distributions of various ccSNe subtypes. 242 Wolf-Rayet candidate sources are identified, 40% of which are only detected in narrow-band helium II imaging, not in broad-band imaging. The spatial distribution of WR candidates is compared to the distributions of ccSNe subtype populations in the broad-band B filter and the narrow-band Hα filter. WR stars appear to follow the Type Ic distribution in the faintest 30% of the galaxy in the B image, but follow the Ib or II distribution in the brightest regions, possibly due to the difficulty of detecting WR stars in the brightest regions. WR candidates are found to be closely associated with HII regions; however, the treatment of the residual background strongly affects the distribution, and this result must be investigated further.
Cataclysmic variables (classical novae and dwarf novae) are binary star systems in which a red dwarf transfers hydrogen-rich matter, via an accretion disk, to its white dwarf companion. In dwarf novae, an instability is believed to... more
Cataclysmic variables (classical novae and dwarf novae) are binary star systems in which a red dwarf transfers hydrogen-rich matter, via an accretion disk, to its white dwarf companion. In dwarf novae, an instability is believed to episodically dump much of the accretion disk onto the white dwarf, brightening these systems up to a hundred-fold. Thermonuclear-powered eruptions a thousand times more
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We have looked for dwarf novae eruptions in the cluster cores of both 47 Tuc and NGC 6397. The cluster cores were imaged in parallel when the deep white dwarf field was imaged. We have also included archival images of the cluster cores... more
We have looked for dwarf novae eruptions in the cluster cores of both 47 Tuc and NGC 6397. The cluster cores were imaged in parallel when the deep white dwarf field was imaged. We have also included archival images of the cluster cores taken with WFPC2, ACS and WFC3. We describe here all the data, our methodology and the total
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The globular cluster NGC 6681 (M70), a post-core-collapse galactic cluster, has been imaged extensively in the nearand far-ultraviolet with HST as a calibration target for both STIS and ACS. An aggregate dataset of 605 UV exposures taken... more
The globular cluster NGC 6681 (M70), a post-core-collapse galactic cluster, has been imaged extensively in the nearand far-ultraviolet with HST as a calibration target for both STIS and ACS. An aggregate dataset of 605 UV exposures taken on 53 occasions yields the deepest nearand far-UV survey of a globular cluster to date. Reduction and analysis of this massive, multi-epoch dataset