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John R. Thorstensen

Publications and source records attributed to John R. Thorstensen.

At least 37 records · Page 2Linked to original sources
arXiv2019

Two Long-Period Cataclysmic Variable Stars: ASASSN-14ho and V1062 Cyg

We report spectroscopy and photometry of the cataclysmic variable stars ASASSN-14ho and V1062 Cyg. Both are dwarf novae with spectra dominated by their secondary stars, which we classify as approxomately K4 and M0.5, respectively. Their orbital periods, determined mostly from the secondary stars' radial velociites, proved to be nearly identical, respectively 350.14 +- 0.15 and 348.25 +- 0.60 min. The H-alpha emission line in V1062 Cyg displays a relatively sharp emission component that tracks the secondary's motion, which may arise on the irradiated face of the secondary; tihs is not often seen and may indicate an unusually strong flux of ionizing radiation. Both systems exhibit double-peaked orbital modulation consistent with ellipsoidal variation from the changing aspect of the secondary. We model these variations to constrain the orbital inclination i, and estimate approximate component masses based oni and the secondary velocity amplitude K2.

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arXiv2018

An Optical Study of Two VY Sculptoris-Type Cataclysmic Binary Stars: V704 And and RX J2338+431

We report observations of the known cataclysmic variable star (CV) V704 And, and also confirm that the optical counterpart of the ROSAT Galactic Plane Survey source RX J2338+431 is a heretofore-neglected CV. Photometric and spectroscopic observations from MDM Observatory show both systems to be novalike variables that exhibit dips of 4-5 magnitudes from their mean brightnesses, establishing them as members of the VY~Scl subclass. From high-state emission-line radial velocities, we determine orbital periods of 0.151424(3) d (3.63 hr) for V704 And and 0.130400(1) d (3.13 hr) for RX J2338+431. In V704 And, we find that the H-alpha emission-line measures cluster into distinct regions on a plot of equivalent width versus full width at half-maximum, which evidently correspond to high, intermediate, and low photometric states. This allows us to assign spectra to photometric states when contemporaneous photometry is not available, an apparently novel method that may be useful in studies of other novalikes. Our low-state spectra of RX J2338+431 show features of an M-type secondary star, from which we estimate a distance of 890 +- 200 pc, in good agreement with the Gaia DR2 parallax.

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arXiv2017

Activity and Kinematics of White Dwarf-M Dwarf Binaries from the SUPERBLINK Proper Motion Survey

We present an activity and kinematic analysis of high proper motion white dwarf-M dwarf binaries (WD+dMs) found in the SUPERBLINK survey, 178 of which are new identifications. To identify WD+dMs, we developed a UV-optical-IR color criterion and conducted a spectroscopic survey to confirm each candidate binary. For the newly identified systems, we fit the two components using model white dwarf spectra and M dwarf template spectra to determine physical parameters. We use H$α$ chromospheric emission to examine the magnetic activity of the M dwarf in each system, and investigate how its activity is affected by the presence of a white dwarf companion. We find that the fraction of WD+dM binaries with active M dwarfs is significantly higher than their single M dwarf counterparts at early and mid spectral types. We corroborate previous studies that find high activity fractions at both close and intermediate separations. At more distant separations the binary fraction appears to approach the activity fraction for single M dwarfs. Using derived radial velocities and the proper motions, we calculate 3D space velocities for the WD+dMs in SUPERBLINK. For the entire SUPERBLINK WD+dMs, we find a large vertical velocity dispersion, indicating a dynamically hotter population compared to high proper motion samples of single M dwarfs. We compare the kinematics for systems with active M dwarfs and those with inactive M dwarfs, and find signatures of asymmetric drift in the inactive sample, indicating that they are drawn from an older population.

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arXiv2017

X-ray and Optical Study of the Gamma-ray Source 3FGL J0838.8$-$2829: Identification of a Candidate Millisecond Pulsar Binary and an Asynchronous Polar

We observed the field of the Fermi source 3FGL J0838.8-2829 in optical and X-rays, initially motivated by the cataclysmic variable (CV) 1RXS J083842.1-282723 that lies within its error circle. Several X-ray sources first classified as CVs have turned out to be gamma-ray emitting millisecond pulsars (MSPs). We find that 1RXS J083842.1-282723 is in fact an unusual CV, a stream-fed asynchronous polar in which accretion switches between magnetic poles (that are $\approx$120$^{\circ}$ apart) when the accretion rate is at minimum. High-amplitude X-ray modulation at periods of 94.8$\pm$0.4 minutes and 14.7$\pm$1.2 hr are seen. The former appears to be the spin period, while latter is inferred to be one-third of the beat period between the spin and the orbit, implying an orbital period of 98.3$\pm$0.5 minutes. We also measure an optical emission-line spectroscopic period of 98.413$\pm$0.004 minutes which is consistent with the orbital period inferred from the X-rays. In any case, this system is unlikely to be the gamma-ray source. Instead, we find a fainter variable X-ray and optical source, XMMU J083850.38-282756.8, that is modulated on a time scale of hours in addition to exhibiting occasional sharp flares. It resembles the black widow or redback pulsars that have been discovered as counterparts of Fermi sources, with the optical modulation due to heating of the photosphere of a low-mass companion star by, in this case, an as-yet undetected MSP. We propose XMMU J083850.38-282756.8 as the MSP counterpart of 3FGL J0838.8-2829.

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arXiv2016

A Trip to the Cataclysmic Binary Zoo: Detailed Follow-Up of 35 Recently-Discovered Systems

We report follow-up studies of 35 recently-discovered cataclysmic variables (CVs), 32 of which were found in large, automated synoptic sky surveys. The objects were selected for observational tractability. For 34 of the objects we present mean spectra and spectroscopic orbital periods, and for one more we give an eclipse-based period. Thirty-two of the period determinations are new, and three of these refine published estimates based on superhump periods. The remaining three of our determinations confirm previously published periods. Twenty of the stars are confirmed or suspected dwarf novae with periods shorter than 3 hours, but we also find three apparent polars (AM Her stars), and six systems with P > 5 h, five of which have secondary stars visible in their spectra, from which we estimate distances when possible. The orbital period distribution of this sample is very similar to that of previously discovered CVs.

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arXiv2016

Population Properties of Brown Dwarf Analogs to Exoplanets

We present a kinematic analysis of 152 low surface gravity M7-L8 dwarfs by adding 8 parallaxes, 38 radial velocities, and 19 proper motions. We find 39 objects to be high-likelihood or bona fide members of nearby moving groups, 92 objects to be ambiguous members and 21 objects that are non-members. We find that gravity classification and photometric color separate 5-150 Myr sources from > 3 Gyr field objects, but they do not correlate one-to-one with the narrower 5 -150 Myr age range. The absolute magnitudes of low-gravity sources from J band through W3 show a flux redistribution when compared to equivalent field sources that is correlated with spectral subtype. Clouds, which are a far more dominant opacity source for L dwarfs, are the likely cause. On CMDs, the latest-type low-gravity L dwarfs drive the elbow of the L/T transition up to 1 mag redder and 1 mag fainter than field dwarfs at M_J but are consistent with or brighter than the elbow at M_W1 and M_W2. Furthermore, there is an indication on CMD's (such as M_J versus (J-W2) of increasingly redder sequences separated by gravity classification. Examining bolometric luminosities for planets and low-gravity objects, we confirm that young M dwarfs are overluminous while young L dwarfs are normal compared to the field. This translates into warmer M dwarf temperatures compared to the field sequence while lower temperatures for L dwarfs.

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arXiv2015

Ultracool white dwarfs and the age of the Galactic disc

We present parallax observations and a detailed model atmosphere analysis of 54 cool and ultracool ($T_{\rm eff}$ < 4000 K) white dwarfs (WDs) in the solar neighbourhood. For the first time, a large number of cool and ultracool WDs have distance and tangential velocities measurements available. Our targets have distances ranging from 21 pc to >100 pc, and include five stars within 30 pc. Contrary to expectations, all but two of them have tangential velocities smaller than 150 km s$^{-1}$ thus suggesting Galactic disc membership. The oldest WDs in this sample have WD cooling ages of 10 Gyr, providing a firm lower limit to the age of the thick disc population. Many of our targets have uncharacteristically large radii, indicating that they are low mass WDs. It appears that we have detected the brighter population of cool and ultracool WDs near the Sun. The fainter population of ultracool CO-core WDs remain to be discovered in large numbers. The Large Synoptic Survey Telescope should find these elusive, more massive ultracool WDs in the solar neighbourhood.

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arXiv2015

ASAS-SN 13cl : A Newly-Discovered Cataclysmic Binary with an Anomalously Warm Secondary

The spectrum of the recently discovered cataclysmic variable star (CV) ASAS-SN 13cl shows that a secondary star with spectral type K4 (+- 2 subclasses) contributes roughly half the optical light. The radial velocities of the secondary are modulated on an orbital period P_orb = 4.86 hr with a velocity semiamplitude K = 246 +- 9 km/s, and the light curve shows ellipsoidal variations and an apparent grazing eclipse. At this orbital period, the secondary stars in most CVs are substantially cooler, with spectral types near M3. ASN-13cl therefore joins the small group of CVs with anomalously warm secondary stars, which apparently form when the onset of mass transfer occurs after the secondary has undergone significant nuclear evolution.

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arXiv2015

Spectroscopic Orbital Periods for 29 Cataclysmic Variables from the Sloan Digital Sky Survey

We report follow-up spectroscopy of 29 cataclysmic variables from the Sloan Digital Sky Survey (SDSS), 22 of which were discovered by SDSS and seven other previously known systems that were recovered in SDSS. The periods for 16 of these objects were included in the tabulation by Gaensicke et al. (2009). While most of the systems have periods less than 2 hours, only one has a period in the 80-86 minute 'spike' found by Gaensicke et al. (2009), and 11 have periods longer than 3 hours, indicating that the present sample is skewed toward longer-period, higher-luminosity objects. Seven of the objects have spectra resembling dwarf novae, but have apparently never been observed in outburst, suggesting that many cataclysmics with relatively low variability amplitude remain to be discovered. Some of the objects are notable. SDSS J07568+0858 and SDSS J08129+1911 were previously known to have deep eclipses; in addition to spectroscopy, we use archival data from the CRTTS to refine their periods. We give a parallax-based distance of 195 (+54, -39) pc for LV Cnc (SDSS J09197+0857), which at Porb = 81 m has the shortest orbital period in our sample. SDSS J08091+3814 shows both the spectroscopic phase offset and phase-dependent absorption found in SW Sextantis stars. The average spectra of SDSS J08055+0720 and SDSS J16191+1351 show contributions from K-type secondaries, and SDSS J080440+0239 shows a contribution from an early M star. We use these to constrain the distances. SDSS J09459+2922 has characteristics typical of a magnetic system. SDSS11324+6249 may be a novalike variable, and if so, its orbital period (99 min) is unusually short for that subclass.

astro-ph.SR↗
arXiv2014

Optical Counterparts of Two Fermi Millisecond Pulsars: PSR J1301+0833 and PSR J1628-3205

Using the 1.3m and 2.4m telescopes of the MDM Observatory, we identified the close companions of two eclipsing millisecond radio pulsars discovered by the Green Bank Telescope in searches of Fermi Gamma-ray Space Telescope sources, and measured their light curves. PSR J1301+0833 is a black widow pulsar in a 6.5 hr orbit whose companion star is strongly heated on the side facing the pulsar. It varies from R = 21.8 to R > 24 around the orbit. PSR J1628-3205 is a "redback," a nearly Roche-lobe filling system in a 5.0 hr orbit whose optical modulation in the range 19.0 < R < 19.4 is dominated by strong ellipsoidal variations, indicating a large orbital inclination angle. PSR J1628-3205 also shows evidence for a long-term variation of about 0.2 mag, and an asymmetric temperature distribution possibly due to either off-center heating by the pulsar wind, or large starspots. Modelling of its light curve restricts the inclination angle to i > 55 degrees, the mass of the companion to 0.16 < M_c < 0.30 M_sun, and the effective temperature to 3560 < T_eff < 4670 K. As is the case for several redbacks, the companion of PSR J1628-3205 is less dense and hotter than a main-sequence star of the same mass.

astro-ph.HE↗
arXiv2014

Cataclysmic Variables in the SUPERBLINK Proper Motion Survey

We have discovered a new high proper motion cataclysmic variable (CV) in the SUPERBLINK proper motion survey, which is sensitive to stars with proper motions greater than 40 mas/yr. This CV was selected for follow-up observations as part of a larger search for CVs selected based on proper motions and their NUV-V and V-K$_{s}$ colors. We present spectroscopic observations from the 2.4m Hiltner Telescope at MDM Observatory. The new CV's orbital period is near 96 minutes, its spectrum shows the double-peaked Balmer emission lines characteristic of quiescent dwarf novae, and its V magnitude is near 18.2. Additionally, we present a full list of known CVs in the SUPERBLINK catalog.

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arXiv2013

Optical and X-ray Studies of Ten X-ray Selected Cataclysmic Binaries

We report on ground-based optical observations of ten cataclysmic binaries that were discovered through their X-ray emission. Time-resolved radial velocity spectroscopy yields unambiguous orbital periods for eight objects and ambiguous results for the remaining two. The orbital periods range from 87 min to 9.38 hr. We also obtained time-series optical photometry for six targets, four of which have coherent pulsations. These periods are 1218 s for 1RXS J045707.4+452751, 628 s for AX J1740.2-2903, 477 s for AX J1853.3-0128, and 935 s for IGR J19267+1325. A total of seven of the sources have coherent oscillations in X-rays or optical, indicating that they are intermediate polars (DQ Herculis stars). Time-resolved spectroscopy of one object, Swift J2218.4+1925, shows that it is an AM Herculis star, or polar, and IGR J19552+0044 may also be in that class. For another object, Swift J0476.2-1611, we find an orbital period of 9.384 hr and detect the spectrum of the secondary star. The secondary's spectral contribution implies a distance of 900 (+190, -150) pc, where the error bars are estimated using a Monte Carlo technique to account for correlated uncertainties.

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arXiv2012

Spectroscopy and Photometry of Cataclysmic Variable Candidates from the Catalina Real Time Survey

The Catalina Real Time Survey (CRTS) has found over 500 cataclysmic variable (CV) candidates, most of which were previously unknown. We report here on followup spectroscopy of 36 of the brighter objects. Nearly all the spectra are typical of CVs at minimum light. One object appears to be a flare star, while another has a spectrum consistent with a CV but lies, intriguingly, at the center of a small nebulosity. We measured orbital periods for eight of the CVs, and estimated distances for two based on the spectra of their secondary stars. In addition to the spectra, we obtained direct imaging for an overlapping sample of 37 objects, for which we give magnitudes and colors. Most of our new orbital periods are shortward of the so-called period gap from roughly 2 to 3 hours. By considering the cross-identifications between the Catalina objects and other catalogs such as the Sloan Digital Sky Survey, we argue that a large number of cataclysmic variables remain uncatalogued. By comparing the CRTS sample to lists of previously-known CVs that CRTS does not recover, we find that the CRTS is biased toward large outburst amplitudes (and hence shorter orbital periods). We speculate that this is a consequence of the survey cadence.

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arXiv2012

11-12 Gyr old White Dwarfs 30 parsecs Away

We present a detailed model atmosphere analysis of two of the oldest stars known in the solar neighborhood, the high proper motion white dwarfs SDSS J110217.48+411315.4 (hereafter J1102) and WD 0346+246 (hereafter WD0346). We present trigonometric parallax observations of J1102, which places it at a distance of only 33.7 +- 2.0 pc. Based on the state of the art model atmospheres, optical, near-, mid-infrared photometry, and distances, we constrain the temperatures, atmospheric compositions, masses, and ages for both stars. J1102 is an 11 Gyr old (white dwarf plus main-sequence age), 0.62 Msol white dwarf with a pure H atmosphere and Teff = 3830 K. WD0346 is an 11.5 Gyr old, 0.77 Msol white dwarf with a mixed H/He atmosphere and Teff = 3650 K. Both stars display halo kinematics and their ages agree remarkably well with the ages of the nearest globular clusters, M4 and NGC 6397. J1102 and WD0346 are the closest examples of the oldest halo stars that we know of.

astro-ph.GA↗
arXiv2011

The New Eclipsing Cataclysmic Variable SDSS 154453+2553

The cataclysmic variable SDSS154453+2553 was recently identified in the Sloan Digital Sky Survey. We obtained spectra and photometry at the MDM Observatory, which revealed an eclipse with a 6.03 hour period. The Hα emission line exhibits a strong rotational disturbance during eclipse, indicating that it arises in an accretion disk. A contribution from an M-type companion is also observed. Time-series photometry during eclipse gives an ephemeris of 2454878.0062(15) + 0.251282(2)E. We present spectroscopy through the orbit and eclipse photometry. Our analysis of the secondary star indicates a distance of 800 {\pm} 180 pc.

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arXiv2010

Optical Studies of Twenty Longer-Period Cataclysmic Binaries

We obtained time-series radial velocity spectroscopy of twenty cataclysmic variable stars, with the aim of determining orbital periods P_orb. All of the stars reported here prove to have P_orb > 3.5 h. For sixteen of the stars, these are the first available period determinations, and for the remaining four (V709 Cas, AF Cam, V1062 Tau, and RX J2133+51) we use new observations to improve the accuracy of previously-published periods. Most of the targets are dwarf novae, without notable idiosyncracies. Of the remainder, three (V709 Cas, V1062 Tau, and RX J2133+51) are intermediate polars (DQ Her stars); one (IPHAS 0345) is a secondary-dominated system without known outbursts, similar to LY UMa; one (V1059 Sgr) is an old nova; and two others (V478 Her and V1082 Sgr) are long-period novalike variables. The stars with new periods are IPHAS 0345 (0.314 d); V344 Ori (0.234 d); VZ Sex (0.149 d); NSVS 1057+09 (0.376 d); V478 Her (0.629 d); V1059 Sgr (0.286 d); V1082 Sgr (0.868 d); FO Aql (0.217 d); V587 Lyr (0.275 d); V792 Cyg (0.297 d); V795 Cyg (0.181 d); V811 Cyg (0.157 d); V542 Cyg (0.182 d); PQ Aql (0.247 d); V516 Cyg (0.171 d); and VZ Aqr(0.161 d). Noteworthy results on individual stars are as follows. We see no indication of the underlying white dwarf star in V709 Cas, as has been previously claimed; based on the non-detection of the secondary star, we argue that the system is farther away that had been thought and the white dwarf contribution is probably negligible. V478 Her had been classified as an SU UMa-type dwarf nova, but this is incompatible with the long orbital period we find. We report the first secondary-star velocity curve for V1062 Tau. In V542 Cyg, we find a late-type contribution that remains stationary in radial velocity, yet the system is unresolved in a direct image, suggesting that it is a hierarchical triple system.

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arXiv2009

SDSS J102347.6+003841: A Millisecond Radio Pulsar Binary That Had A Hot Disk During 2000-2001

The Sloan Digital Sky Survey (SDSS) source J102347.6+003841 was recently revealed to be a binary 1.69 millisecond radio pulsar with a 4.75 hr orbital period and a ~0.2 M_sun companion. Here we analyze the SDSS spectrum of the source in detail. The spectrum was taken on 2001 February 1, when the source was in a bright state and showed broad, double-peaked hydrogen and helium lines -- dramatically different from the G-type absorption spectrum seen from 2003 onward. The lines are consistent with emission from a disk around the compact primary. We derive properties of the disk by fitting the SDSS continuum with a simple disk model, and find a temperature range of 2000--34000 K from the outer to inner edge of the disk. The disk inner and outer radii were approximately 10^9 and 5.7x10^10 cm, respectively. These results further emphasize the unique feature of the source: it is likely a system at the end of its transition from an X-ray binary to a recycled radio pulsar. The disk mass is estimated to have been ~10^23 g, most of which would have been lost due to pulsar wind ablation (or due to the propeller effect if the disk had extended inside the light cylinder of the pulsar) before the final disk disruption event. The system could undergo repeated episodes of disk formation. Close monitoring of the source is needed to catch the system in its bright state again, so that this unusual example of a pulsar-disk interaction can be studied in much finer detail

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