[Show abstract][Hide abstract] ABSTRACT: We present optical and X-ray observations of two tidally distorted, extremely low-mass white dwarfs (WDs) with massive companions.
There is no evidence of neutron stars in our Chandra and XMM observations of these objects. SDSS J075141.18−014120.9 (J0751) is an eclipsing double WD binary containing a 0.19 M⊙ WD with a 0.97 M⊙ companion in a 1.9 h orbit. J0751 becomes the fifth eclipsing double WD system currently known. SDSS J174140.49+652638.7
(J1741) is another binary containing a 0.17 M⊙ WD with an unseen M ≥ 1.11 M⊙ WD companion in a 1.5-h orbit. With a mass ratio of ≈0.1, J1741 will have stable mass transfer through an accretion disc
and turn into an interacting AM Canum Venaticorum (AM CVn) system in the next ≈160 Myr. With a mass ratio of 0.2, J0751 is
likely to follow a similar evolutionary path. These are the first known AM CVn progenitor binary systems and they provide
important constraints on the initial conditions for AM CVn. Theoretical studies suggest that both J0751 and J1741 may create
thermonuclear supernovae in ∼108 yr, either .Ia or Ia. Such explosions can account for ∼1 per cent of the Type Ia supernova rate.
Full-text · Article · Oct 2013 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] ABSTRACT: We present optical spectroscopy, astrometry, radio and X-ray observations of the runaway binary LP 400−22. We refine the orbital
parameters of the system based on our new radial velocity observations. Our parallax data indicate that LP 400−22 is significantly
more distant (3σ lower limit of 840 pc) than initially predicted. LP 400−22 has a tangential velocity in excess of 830 km s−1; it is unbound to the Galaxy. Our radio and X-ray observations fail to detect a recycled millisecond pulsar companion, indicating
that LP 400−22 is a double white dwarf system. This essentially rules out a supernova runaway ejection mechanism. Based on
its orbit, a Galactic Centre origin is also unlikely. However, its orbit intersects the locations of several globular clusters;
dynamical interactions between LP 400−22 and other binary stars or a central black hole in a dense cluster could explain the
origin of this unusual binary.
Full-text · Article · Jul 2013 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] ABSTRACT: X-ray emission traces the strength of a star's magnetic dynamo and is
tightly linked to age and stellar rotation. Early studies found that
X-ray luminosity decreases steadily as stars age. However, recent
studies suggest that the picture of X-ray luminosity decaying from 10
Myr to 5 Gyr as a single power-law, and independently of mass, is too
simplistic. Open clusters, with their homogeneous, coeval populations,
provide an ideal environment for calibrating empirically how stellar
activity depends on age and rotation. M37, a Hyades analog, has been
carefully surveyed in the optical, and we have obtained complementary
deep (450 ksec) Chandra ACIS-I imaging of this open cluster. M37's rich
membership, good match to Chandra's field-of-view, and large number of
measured periods 700) make it ideal for examining the evolution of
coronal X-ray emission. Here we report the first results from our
analysis of the combined X-ray and optical data. These data will enable
determinations of X-ray luminosity as a function of rotation period,
tests of whether magnetic effects produce anomalous optical
colors/luminosities for high X-ray luminosity sources, and theta
characterization of M37's interacting binaries.
[Show abstract][Hide abstract] ABSTRACT: We construct a photometrically calibrated catalog of non-variable sources
from the Palomar Transient Factory (PTF) observations. The first version of
this catalog presented here, the PTF photometric catalog 1.0, contains
calibrated R_PTF-filter magnitudes for about 21 million sources brighter than
magnitude 19, over an area of about 11233 deg^2. The magnitudes are provided in
the PTF photometric system, and the color of a source is required in order to
convert these magnitudes into other magnitude systems. We estimate that the
magnitudes in this catalog have typical accuracy of about 0.02 mag with respect
to magnitudes from the Sloan Digital Sky Survey. The median repeatability of
our catalog's magnitudes for stars between 15 and 16 mag, is about 0.01 mag,
and it is better than 0.03 mag for 95% of the sources in this magnitude range.
The main goal of this catalog is to provide reference magnitudes for
photometric calibration of visible light observations. Subsequent versions of
this catalog, which will be published incrementally online, will be extended to
a larger sky area and will also include g_PTF-filter magnitudes, as well as
variability and proper motion information.
Full-text · Article · Jun 2012 · Publications of the Astronomical Society of the Pacific
[Show abstract][Hide abstract] ABSTRACT: The Palomar Transient Factory (PTF) is a synoptic survey designed to
explore the transient sky and to study stellar variability. The project
utilizes the 48-inch Samuel Oschin Schmidt Telescope at Palomar
Observatory. The telescope has a digital camera equipped with 11 active
CCDs, each 2Kx4K pixels, and has been surveying the northern sky since
March 2009. Each PTF image covers 7.26 deg2 with a pixel
scale of 1.01arcsec/pix. The median point-spread function FWHM is about
2arcsec and it is uniform over the camera field of view.
The PTF main survey is currently performed in the g-band during dark
time and in the Mould R-band (close to r-band) toduring bright time, but
most of the data taken prior to January 2011 were obtained using the
R-band filter. In addition, a few nights around times of full Moon are
used for surveying the sky with narrow-band Hα filters.
(1 data file).
[Show abstract][Hide abstract] ABSTRACT: We present new radial velocity and X-ray observations of extremely low-mass
(ELM, 0.2 Msol) white dwarf candidates in the Sloan Digital Sky Survey (SDSS)
Data Release 7 area. We identify seven new binary systems with 1-18 h orbital
periods. Five of the systems will merge due to gravitational wave radiation
within 10 Gyr, bringing the total number of merger systems found in the ELM
Survey to 24. The ELM Survey has now quintupled the known merger white dwarf
population. It has also discovered the eight shortest period detached binary
white dwarf systems currently known. We discuss the characteristics of the
merger and non-merger systems observed in the ELM Survey, including their
future evolution. About half of the systems have extreme mass ratios. These are
the progenitors of the AM Canum Venaticorum systems and supernovae .Ia. The
remaining targets will lead to the formation of extreme helium stars,
subdwarfs, or massive white dwarfs. We identify three targets that are
excellent gravitational wave sources. These should be detected by the Laser
Interferometer Space Antenna (LISA)-like missions within the first year of
operation. The remaining targets are important indicators of what the Galactic
foreground may look like for gravitational wave observatories.
Preview · Article · Mar 2012 · The Astrophysical Journal
[Show abstract][Hide abstract] ABSTRACT: A large fraction of field mid-to-late M dwarfs are found to be
magnetically active, yet the exact mechanisms governing this activity
are not well-understood. Simulations and observations indicate that
rotation may play a role in magnetic field generation for these
fully-convective stars, but the interpretation has been hampered by a
lack of measured rotation rates for slowly-rotating stars as these are
inaccessible to the vsini method. We present results from a study of the
magnetic activity-rotation relation for a large sample of field M dwarfs
for which we have measured rotation periods from MEarth and PTF programs
(including many slow rotators; 50-150 days) and low-resolution
spectroscopic observations from FLWO/FAST and SDSS. We quantify the
magnetic activity of the stars using the chromospheric H-alpha emission
line and find that there is a clear trend of decreasing activity with
increased rotation periods for all M dwarf spectral types. In addition,
a kinematic analysis of the three-dimensional space motions of the stars
confirms that those with higher levels of magnetic activity are
consistent with being younger than their less active (or inactive)
We acknowledge MEarth funding from the Packard Fellowship for Science
and Engineering and the NSF (AST-0807690).
[Show abstract][Hide abstract] ABSTRACT: This paper completes the series of cataclysmic variables (CVs) identified
from the Sloan Digital Sky Survey I/II. The coordinates, magnitudes and spectra
of 33 CVs are presented. Among the 33 are eight systems known previous to SDSS
(CT Ser, DO Leo, HK Leo, IR Com, V849 Her, V405 Peg, PG1230+226 and
HS0943+1404), as well as nine objects recently found through various
photometric surveys. Among the systems identified since the SDSS are two polar
candidates, two intermediate polar candidates and one candidate for containing
a pulsating white dwarf. Our followup data have confirmed a polar candidate
from Paper VII and determined tentative periods for three of the newly
identified CVs. A complete summary table of the 285 CVs with spectra from SDSS
I/II is presented as well as a link to an online table of all known CVs from
both photometry and spectroscopy that will continue to be updated as future
[Show abstract][Hide abstract] ABSTRACT: The Columbia-Cornell-Caltech Palomar Transient Factory (CCCP) survey is designed to obtain stellar rotations in a number of nearby open clusters. Among the interesting objects detected in CCCP fields observed to date are several candidate eclipsing binaries with M star primaries. These systems, whether in the field or in our target clusters, are ideal laboratories for testing models for the masses and temperatures of late-type stars. The challenge is in obtaining and analyzing the high quality follow-up spectroscopy that is required for these tests. Here we report on our use of the near-infrared TripleSpec spectrometer on the 200-inch telescope at Palomar Observatory to follow up on these interesting systems. While these observations are of moderate resolution (R 3000), they do provide sufficient traction to constrain the radial velocity separation of the two components of the binary, allowing us to place constraints on the orbits of these systems.
[Show abstract][Hide abstract] ABSTRACT: Large uncertainties remain in our picture of the evolution of stellar rotation and activity in Sun-like stars after they reach the age of the Hyades, roughly 0.5 Gyr. Rotation periods are scarce for stars at these ages, seriously complicating the calibration of an age-rotation-activity relation that could be applied to field stars. The Columbia/Cornell/Caltech Palomar Transient Factory (CCCP) survey of open clusters is an effort to systematically map stellar rotation in nearby open clusters. We present preliminary results from our on-going CCCP campaign on NGC 752, a 1 Gyr open cluster for which we have several hundred individual R-band observations. At a distance of only 450 parsecs, and with deep archival X-ray observations, it represents the best opportunity we have to study the age-rotation-activity relation in main sequence stars at an age about a quarter that of the Sun.
[Show abstract][Hide abstract] ABSTRACT: For several decades, observational surveys have demonstrated a closerelationship between a star's rotation period, magnetic activity, and age. The age-activity-rotation relation is well documented in clusters younger than 500 Myrs, but its subsequent evolution is less clear. Using wide-field, multi-epoch photometry from the Palomar Transient Factory, we have monitored 530 members of the Praesepe open cluster, a 600 Myr nearby Hyades analog. With light curves containing >150 measurements over more than three months, we have measured rotation periods for 50 K & M-type cluster members. These rotation periods span the gap between the periods measured for solar-type Hyads and the lowest-mass Praesepe members, and indicate that the orderly mass-rotation relation seen for higher mass Praesepe members begins to break down at 0.6 Msun. Below this critical mass, a range of rotation periods spanning an order of magnitude are observed for stars with similar masses. Combining these rotation measurements with archival X-ray and Halpha observations completes the portrait of the 600 Myr age-activity-rotation relation, and enables a detailed comparison with Praesepe's sister cluster, the Hyades.
[Show abstract][Hide abstract] ABSTRACT: Over the past 40 years, observational surveys have established the existence of a tight relationship between a star's age, rotation period, and magnetic activity. This age-rotation-activity relation documents the interplay between a star's magnetic dynamo and angular momentum evolution, and provides a valuable age estimator for isolated field stars. While the age-rotation-activity relation has been studied extensively in clusters younger than 500 Myr, empirically measured rotation periods are scarce for older ages. Using the Palomar Transient Factory (PTF), we have begun a survey of stellar rotation to map out the late-stage evolution of the age-rotation-activity relation: the Columbia/Cornell/Caltech PTF (CCCP) survey of open clusters. The first CCCP target is the nearby ~600 Myr Hyades-analog Praesepe, where PTF has produced light curves spanning more than 3 months and containing >150 measurements for ~650 cluster members. Analyzing these light curves, we have measured rotation periods for 40 K & M cluster members, filling the gap between the periods previously reported for solar-type Hyads (Radick et al. 1987, Prosser et al. 1995) and for a handful of low-mass Praesepe members (Scholz et al. 2007). Our measurements indicate that Praesepe's period-color relation undergoes at transition at a characteristic spectral type of ~M1 --- from a well-defined singular relation at higher mass, to a more scattered distribution of both fast and slow-rotators at lower masses. The location of this transition is broadly consistent with expectations based on observations of younger clusters and the assumption that stellar-spin down is the dominant mechanism influencing angular momentum evolution at ~600 Myr. In addition to presenting the results of our photometric monitoring of Praesepe, we summarize the status and future of the CCCP survey. Comment: Contribution to Proceedings of Cool Stars 16 Workshop; 8 pages in ASP format; 3 figures
[Show abstract][Hide abstract] ABSTRACT: We describe new radial velocity and X-ray observations of extremely low-mass white dwarfs (ELM WDs, ~0.2 M
☉) in the Sloan Digital Sky Survey Data Release 4 and the MMT Hypervelocity Star survey. We identify four new short period binaries, including two merger systems. These observations bring the total number of short period binary systems identified in our survey to 20. No main-sequence or neutron star companions are visible in the available optical photometry, radio, and X-ray data. Thus, the companions are most likely WDs. Twelve of these systems will merge within a Hubble time due to gravitational wave radiation. We have now tripled the number of known merging WD systems. We discuss the characteristics of this merger sample and potential links to underluminous supernovae, extreme helium stars, AM CVn systems, and other merger products. We provide new observational tests of the WD mass-period distribution and cooling models for ELM WDs. We also find evidence for a new formation channel for single low-mass WDs through binary mergers of two lower mass objects.
Full-text · Article · Nov 2010 · The Astrophysical Journal
[Show abstract][Hide abstract] ABSTRACT: During a synoptic survey of the North American Nebula region, the Palomar Transient Factory (PTF) detected an optical outburst (dubbed PTF10nvg) associated with the previously unstudied flat or rising spectrum infrared source IRAS 20496+4354. The PTF R-band light curve reveals that PTF10nvg brightened by more than 5 mag during the current outburst, rising to a peak magnitude of R
PTF ≈ 13.5 in 2010 September. Follow-up observations indicate that PTF10nvg has undergone a similar ~5 mag brightening in the K band and possesses a rich emission-line spectrum, including numerous lines commonly assumed to trace mass accretion and outflows. Many of these lines are blueshifted by ~175 km s–1 from the North American Nebula's rest velocity, suggesting that PTF10nvg is driving an outflow. Optical spectra of PTF10nvg show several TiO/VO band heads fully in emission, indicating the presence of an unusual amount of dense (>1010 cm–3), warm (1500-4000 K) circumstellar material. Near-infrared spectra of PTF10nvg appear quite similar to a spectrum of McNeil's Nebula/V1647 Ori, a young star which has undergone several brightenings in recent decades, and 06297+1021W, a Class I protostar with a similarly reached near-infrared emission line spectrum. While further monitoring is required to fully understand this event, we conclude that the brightening of PTF10nvg is indicative of enhanced accretion and outflow in this Class-I-type protostellar object, similar to the behavior of V1647 Ori in 2004-2005.
Full-text · Article · Nov 2010 · The Astronomical Journal
[Show abstract][Hide abstract] ABSTRACT: We measure the radial velocity curve of the eclipsing detached white dwarf
binary NLTT 11748. The primary exhibits velocity variations with a
semi-amplitude of 273 km/s and an orbital period of 5.641 hr. We do not detect
any spectral features from the secondary star, or any spectral changes during
the secondary eclipse. We use our composite spectrum to constrain the
temperature and surface gravity of the primary to be Teff = 8690 (140) K and
log g = 6.54 (0.05), which correspond to a mass of 0.18 Msol. For an
inclination angle of 89.9 deg. derived from the eclipse modeling, the mass
function requires a 0.76 Msol companion. The merger time for the system is 7.2
Gyr. However, due to the extreme mass ratio of 0.24, the binary will most
likely create an AM CVn system instead of a merger.
Full-text · Article · Aug 2010 · The Astrophysical Journal Letters
[Show abstract][Hide abstract] ABSTRACT: The ROSAT All-Sky Survey (RASS) was the first imaging X-ray survey of the entire sky. Combining the RASS Bright and Faint Source Catalogs (Cat. , 1RXS and ) yields an average of about three X-ray sources per square degree. However, while X-ray source counterparts are known to range from distant quasars to nearby M dwarfs, the RASS data alone are often insufficient to determine the nature of an X-ray source. As a result, large-scale follow-up programs are required to construct samples of known X-ray emitters. We use optical data produced by the Sloan Digital Sky Survey (SDSS) to identify 709 stellar X-ray emitters cataloged in the RASS and falling within the SDSS Data Release 1 footprint. Most of these are bright stars with coronal X-ray emission unsuitable for SDSS spectroscopy, which is designed for fainter objects (g>15[mag]). Instead, we use SDSS photometry, correlations with the Two Micron All Sky Survey and other catalogs, and spectroscopy from the Apache Point Observatory 3.5m telescope to identify these stellar X-ray counterparts. Our sample of 707 X-ray-emitting F, G, K, and M stars is one of the largest X-ray-selected samples of such stars. We identify 17 new X-ray-emitting DA (hydrogen) WDs, of which three are newly identified WDs. We report on follow-up observations of three candidate cool X-ray-emitting WDs (one DA and two DB (helium) WDs); we have not confirmed X-ray emission from these WDs. (7 data files).
[Show abstract][Hide abstract] ABSTRACT: We present 348 X-ray-emitting stars identified from correlating the Extended Chandra Multiwavelength Project (ChaMP), a wide-area serendipitous survey based on archival X-ray images, with the Sloan Digital Sky Survey (SDSS-DR6). We use morphological star/galaxy separation, matching to an SDSS quasar catalog, an optical color-magnitude cut, and X-ray data-quality tests to create our catalog, the ChaMP Extended Stellar Survey (ChESS), from a sample of 2121 matched ChaMP/SDSS sources. Our cuts retain 92% of the spectroscopically confirmed stars in the original sample while excluding 99.6% of the 684 spectroscopically confirmed extragalactic sources. Fewer than 3% of the sources in our final catalog are previously identified stellar X-ray emitters. For 42 catalog members, spectroscopic classifications are available in the literature. We present new spectral classifications and Halpha measurements for an additional 79 stars. The catalog is dominated by main-sequence stars; we estimate the fraction of giants in ChESS is ~10%. We identify seven giant stars (including a possible Cepheid and an RR Lyrae star) as ChaMP sources, as well as three cataclysmic variables. Future papers will present analyses of source variability and comparisons of this catalog to models of stellar activity in the Galactic disk. (4 data files).
[Show abstract][Hide abstract] ABSTRACT: We report on follow-up observations of candidate isolated neutron stars (INSs) identified from correlations of the ROSAT All-Sky Survey (RASS) and the Sloan Digital Sky Survey (SDSS) in Agueros et al. (2006). We obtained Chandra X-ray Telescope exposures for 13 candidates in order to pinpoint the source of X-ray emission in optically blank RASS error circles. These observations eliminated 12 targets as good INS candidates. We discuss subsequent XMM-Newton, Gemini, and Apache Point Observatory observations of the remaining candidate.
[Show abstract][Hide abstract] ABSTRACT: We present 348 X-ray emitting stars identified from correlating the Extended Chandra Multiwavelength Project (ChaMP), a serendipitous wide-area X-ray survey, with the Sloan Digital Sky Survey (SDSS). We use morphological star/galaxy separation, an SDSS quasar catalog, an optical color-magnitude cut, and X-ray data quality tests to create our catalog, the ChaMP Extended Stellar Survey (ChESS), from a sample of 2121 matched ChaMP/SDSS sources. Our cuts retain 92% of the spectroscopically confirmed stars while excluding 99.6% of the 684 spectoscopically confirmed extragalactic sources. Fewer than 3% of the sources in our final catalog are previously identified stellar X-ray emitters; we expect ~10% of the catalog is composed by giants, and identify seven giant stars and three cataclysmic variables. We derive distances, X-ray and bolometric luminosities for these stars, revealing that this catalog fills the gap between the nearby stars identified by the ROSAT All-Sky Survey and the more distant stars detected in deep Chandra and XMM-Newton surveys. For 36 newly identified X-ray emitting M stars we calculate L_(Halpha)/L_(bol). L_(H\alpha)/L_(bol) and L_(X)/L_(bol) are linearly related below L_(X)/L_(bol) ~ 3 x 10^(-4), while L_(Halpha)/L_(bol) appears to turn over at larger L_(X)/L_(bol) values. Stars with reliable SDSS photometry have an ~0.1 mag blue excess in (u-g), likely due to increased chromospheric continuum emission. Photometric metallicity estimates suggest the sample is split between the young and old disk populations of the Galaxy; the lowest activity sources belong to the old disk population, a clear signature of the decay of magnetic activity with age. Future papers will present analyses of source variability and comparisons of this catalog to models of stellar activity in the Galactic disk.
Full-text · Article · Jun 2008 · The Astrophysical Journal Supplement Series