The Search for White Dwarf Binaries in Astronomical Survey Data

Extremely low mass (ELM) white dwarfs are excellent laboratories for studying a variety of astrophysical phenomena, including common-envelope evolution, gravitational wave emission, and merger physics. A significant amount of work has been done to identify extremely low mass white dwarfs in the northern hemisphere using various photometric surveys, such as SDSS and Pan-STARRS. In this dissertation, I present my follow-up observations of a low-mass, eclipsing, white dwarf binary and use the data to begin the process of directly measuring the binary's rate of orbital decay due to gravitational wave emission. These measurements can be used to obtain an independent measurement of the mass of the binary as well as constrain the effects of tides through a measured offset in orbital decay rate from a purely gravitational wave model (see Piro, 2011; Benacquista et al., 2011). I then begin the search for ELM white dwarfs in the southern hemisphere (the ELM Survey South) through a large, targeted, spectroscopic survey using the SOAR 4.1-meter telescope. This search makes use of the southern-sky photometric surveys ATLAS and SkyMapper to identify a population of white dwarf binaries in color-color space which yields a small number of low-mass white dwarfs in a large pool of candidates. The target selection criteria are expanded to include the European Space Agency's Gaia Data Release 2 astrometry and photometry as soon as it became available. By using reliable parallax measurements and precise magnitudes, one is able to selectively target ELM white dwarfs with significantly higher accuracy than when using a pure photometric approach. In preparation for the next phase of the ELM Survey South, I make use of the Zwicky Transient Facility (ZTF) public photometric survey to demonstrate the effectiveness of identifying exotic variables, including double white dwarf binaries, in large survey data sets. I make use of the Box Least Squares (BLS) period finding algorithm to identify eclipsing binaries within the public ZTF data archive and present photometric and spectroscopic analyses to a select few systems, including white dwarfs and hot subdwarf stars with main sequence companions. The methods developed here will be used as a starting point towards expanding the search for ELM white dwarfs in the southern hemisphere with the upcoming large-scale photometric surveys BlackGEM and the Vera Rubin Observatory's LSST program

Periodic optical variability and debris accretion in white dwarfs: a test for a causal connection

Recent Kepler photometry has revealed that about half of white dwarfs (WDs) have periodic, low-level (~ 1e-4 - 1e-3), optical variations. Hubble Space Telescope (HST) ultraviolet spectroscopy has shown that up to about one half of WDs are actively accreting rocky planetary debris, as evidenced by the presence of photospheric metal absorption lines. We have obtained HST ultraviolet spectra of seven WDs that have been monitored for periodic variations, to test the hypothesis that these two phenomena are causally connected, i.e. that the optical periodic modulation is caused by WD rotation coupled with an inhomogeneous surface distribution of accreted metals. We detect photospheric metals in four out of the seven WDs. However, we find no significant correspondence between the existence of optical periodic variability and the detection of photospheric ultraviolet absorption lines. Thus the null hypothesis stands, that the two phenomena are not directly related. Some other source of WD surface inhomogeneity, perhaps related to magnetic field strength, combined with the WD rotation, or alternatively effects due to close binary companions, may be behind the observed optical modulation. We report the marginal detection of molecular hydrogen in WD J1949+4734, only the fourth known WD with detected H_2 lines. We also re-classify J1926+4219 as a carbon-rich He-sdO subdwarf.Comment: MNRAS, in pres

A refined search for pulsations in white dwarf companions to millisecond pulsars

We present optical high-speed photometry of three millisecond pulsars with low-mass (< 0.3M⊙) white dwarf companions, bringing the total number of such systemswith follow-up time-series photometry to five. We confirm the detection of pulsations in one system, the white dwarf companion to PSR J1738+0333, and show that the pulsation frequencies and amplitudes are variable over many months. A full asteroseismic analysis for this star is underconstrained, but the mode periods we observe are consistent with expectations for an M⁎ = 0.16-0.19 M⊙ white dwarf, as suggested from spectroscopy. We also present the empirical boundaries of the instability strip for low-mass white dwarfs based on the full sample of white dwarfs, and discuss the distinction between pulsating low-mass white dwarfs and subdwarf A/F stars.Instituto de Astrofísica de La PlataFacultad de Ciencias Astronómicas y Geofísica

A refined search for pulsations in white dwarf companions to millisecond pulsars

We present optical high-speed photometry of three millisecond pulsars with low-mass (< 0.3M⊙) white dwarf companions, bringing the total number of such systemswith follow-up time-series photometry to five. We confirm the detection of pulsations in one system, the white dwarf companion to PSR J1738+0333, and show that the pulsation frequencies and amplitudes are variable over many months. A full asteroseismic analysis for this star is underconstrained, but the mode periods we observe are consistent with expectations for an M⁎ = 0.16-0.19 M⊙ white dwarf, as suggested from spectroscopy. We also present the empirical boundaries of the instability strip for low-mass white dwarfs based on the full sample of white dwarfs, and discuss the distinction between pulsating low-mass white dwarfs and subdwarf A/F stars.Instituto de Astrofísica de La PlataFacultad de Ciencias Astronómicas y Geofísica

Chandra and Very Large Array Observations of the Nearby Sd Galaxy NGC 45

We present an analysis of high angular resolution observations made in the X-ray and the radio with the Chandra X-ray Observatory and the Karl Jansky Very Large Array (VLA), respectively, of the nearby spiral galaxy NGC 45. This galaxy is the third that we have considered in a study of the supernova remnant (SNR) populations of nearby spiral galaxies and the present work represents the first detailed analysis of the discrete X-ray and radio source populations of this galaxy. We analyzed data sets from the three pointed observations made of this galaxy with Chandra along with a merged data set obtained from combining these data sets: the total effective exposure time of the merged data set is 63515 s. A total of 25 discrete X-ray sources are found in the entire field of view of the ACIS-S3 chip, with 16 sources found within the visual extent of the galaxy. We estimate that as many as half of the sources detected in the entire field of view of the ACIS-S3 chip and seven of the sources detected in the optical extent of NGC 45 may be background sources. We analyzed the spectral properties of the discrete X-ray sources within the galaxy and conclude that the majority of these sources are X-ray binaries. We have searched for counterparts at different wavelengths to the discrete X-ray sources and we find two associations: one with a star cluster and the other with a background galaxy. We have found one source that is clearly variable within one observation and seven that are seen to vary from one observation to another. We also conduct a photometric analysis to determine the near-infrared fluxes of the discrete X-ray sources in Spitzer Infrared Array Camera channels. We constructed a cumulative luminosity function of the discrete X-ray sources seen toward NGC 45: taking into account simultaneously the luminosity function of background sources, the fitted slope of the cumulative luminosity function Γ = –1.3_(-1.6)^(+0.7) (all error bounds correspond to 90% confidence intervals). The VLA observations reveal seven discrete radio sources: we find no overlaps between these sources and the X-ray detected sources. Based on their measured spectral indices and their locations with respect to the visible extent of NGC 45, we classify one source as a candidate radio SNR associated with the galaxy and the others as likely background galaxies seen in projection toward NGC 45. Finally, we discuss the properties of a background cluster of galaxies (denoted as CXOU J001354.2–231254.7) seen in projection toward NGC 45 and detected by the Chandra observations. The fit parameters to the extracted Chandra spectra of this cluster are a column density N_H = 0.07(<0.14) × 10^(22) cm^(−2), a temperature kT = 4.22_(-1.42)^(+2.08) keV, an abundance Z = 0.30(<0.75) relative to solar and a redshift z = 0.28 ± 0.14. From the fit parameters we derive an electron number density n_e = 4(±1) × 10^(−3) cm^(−3), an unabsorbed X-ray luminosity L_(0.5-7.0keV) ~ 8.77(±0.96) × 10^(43) erg s^(−1) for the cluster and an X-ray emitting mass M = 2.32(±1.75) × 10^(12)M_☉