arXiv subjects

T. R. Marsh

Publications and source records attributed to T. R. Marsh.

At least 145 records · Page 8Linked to original sources

arXiv2014

KIC 2856960: the impossible triple star

KIC 2856960 is a star in the Kepler field which was observed by Kepler for 4 years. It shows the primary and secondary eclipses of a close binary of 0.258d as well as complex dipping events that last for about 1.5d at a time and recur on a 204d period. The dips are thought to result when the close binary passes across the face of a third star. In this paper we present an attempt to model the dips. Despite the apparent simplicity of the system and strenuous efforts to find a solution, we find that we cannot match the dips with a triple star while satisfying Kepler's laws. The problem is that to match the dips the separation of the close binary has to be larger than possible relative to the outer orbit given the orbital periods. Quadruple star models can get round this problem but require the addition of a so-far undetected intermediate period of order 5 -- 20d that has be a near-perfect integer divisor of the outer 204d period. Although we have no good explanation for KIC 2856960, using the full set of Kepler data we are able to update several of its parameters. We also present a spectrum showing that KIC 2856960 is dominated by light from a K3- or K4-type star.

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arXiv2014

ULTRASPEC: a high-speed imaging photometer on the 2.4-m Thai National Telescope

ULTRASPEC is a high-speed imaging photometer mounted permanently at one of the Nasmyth focii of the 2.4-m Thai National Telescope (TNT) on Doi Inthanon, Thailand's highest mountain. ULTRASPEC employs a 1024x1024 pixel frame-transfer, electron-multiplying CCD (EMCCD) in conjunction with re-imaging optics to image a field of 7.7'x7.7' at (windowed) frame rates of up to ~200 Hz. The EMCCD has two outputs - a normal output that provides a readout noise of 2.3 e- and an avalanche output that can provide essentially zero readout noise. A six-position filter wheel enables narrow-band and broad-band imaging over the wavelength range 330-1000 nm. The instrument saw first light on the TNT in November 2013 and will be used to study rapid variability in the Universe. In this paper we describe the scientific motivation behind ULTRASPEC, present an outline of its design and report on its measured performance on the TNT.

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arXiv2014

First Lunar Occultation Results from the 2.4 m Thai National Telescope equipped with ULTRASPEC

The recently inaugurated 2.4\,m Thai National Telescope (TNT) is equipped, among other instruments, with the ULTRASPEC low-noise, frame-transfer EMCCD camera. At the end of its first official observing season, we report on the use of this facility to record high time resolution imaging using small detector subarrays with sampling as fast as several $10^2$\,Hz. In particular, we have recorded lunar occultations of several stars which represent the first contribution to this area of research made from South-East Asia with a telescope of this class. Among the results, we discuss an accurate measurement of $α$~Cnc, which has been reported previously as a suspected close binary. Attempts to resolve this star by several authors have so far met with a lack of unambiguous confirmation. With our observation we are able to place stringent limits on the projected angular separation ($<0\farcs003$) and brightness ($Δ{\rm m} > 5$) of a putative companion. We also present a measurement of the binary {HR~7072}, which extends considerably the time coverage available for its yet undetermined orbit. We discuss our precise determination of the flux ratio and projected separation in the context of other available data. We conclude by providing an estimate of the performance of ULTRASPEC at TNT for lunar occultation work. This facility can help to extend the lunar occultation technique in a geographical area where no comparable resources were available until now.

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arXiv2014

Heavy metals in a light white dwarf: Abundances of the metal-rich, extremely low-mass GALEX J1717+6757

Using the Hubble Space Telescope, we detail the first abundance analysis enabled by far-ultraviolet spectroscopy of a low-mass (~0.19 Msun) white dwarf (WD), GALEX J1717+6757, which is in a 5.9-hr binary with a fainter, more-massive companion. We see absorption from nine metals, including roughly solar abundances of Ca, Fe, Ti, and P. We detect a significantly sub-solar abundance of C, and put upper limits on N and O that are also markedly sub-solar. Updated diffusion calculations indicate that all metals should settle out of the atmosphere of this 14,900 K, log(g) = 5.67 WD in the absence of radiative forces in less than 20 yr, orders of magnitude faster than the cooling age of hundreds of Myr. We demonstrate that ongoing accretion of rocky material that is often the cause of atmospheric metals in isolated, more massive WDs is unlikely to explain the observed abundances in GALEX J1717+6757. Using new radiative levitation calculations, we determine that radiative forces can counteract diffusion and support many but not all of the elements present in the atmosphere of this WD; radiative levitation cannot, on its own, explain all of the observed abundance patterns, and additional mechanisms such as rotational mixing may be required. Finally, we detect both primary and secondary eclipses using ULTRACAM high-speed photometry, which we use to constrain the low-mass WD radius and rotation rate as well as update the ephemeris from the discovery observations of this WD+WD binary.

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arXiv2014

WASP 1628+10 - an EL CVn-type binary with a very-low-mass stripped-red-giant star and multi-periodic pulsations

The star 1SWASP J162842.31+101416.7 (WASP 1628+10) is one of several EL CVn-type stars recently identified using the WASP database, i.e., an eclipsing binary star in which an A-type dwarf star (WASP 1628+10A) eclipses the remnant of a disrupted red giant star (WASP1628+10B). We have measured the masses, radii and luminosities of the stars in WASP 1628+10 using photometry obtained in three bands (u', g', r') with the Ultracam instrument and medium-resolution spectroscopy. The properties of the remnant are well-matched by models for stars in a rarely-observed state evolving to higher effective temperatures at nearly constant luminosity prior to becoming a very low-mass white dwarf composed almost entirely of helium, i.e., we confirm that WASP 1628+10B is a pre-He-WD. WASP 1628+10A appears to be a normal A2V star with a mass of $1.36 \pm 0.05 M_{\odot}$. By fitting models to the spectrum of this star around the H$γ$ line we find that it has an effective temperature T$_{\rm eff,A} = 7500 \pm 200$K and a metallicity [Fe/H]$ = -0.3 \pm 0.3$. The mass of WASP 1628+10B is only $0.135 \pm 0.02M_{\odot}$. The effective temperature of this pre-He-WD is approximately 9200K. The Ultracam photometry of WASP 1628+10 shows variability at several frequencies around 40 cycles per day, which is typical for $δ$ Sct-type pulsations often observed in early A-type stars like WASP 1628+10A. We also observe frequencies near 114 cycles/day and 129 cycles/day, much higher than the frequencies normally seen in $δ$ Sct stars. Additional photometry through the primary eclipse will be required to confirm that these higher frequencies are due to pulsations in WASP 1628+10B. If confirmed, this would be only the second known example of a pre-He-WD showing high-frequency pulsations.

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arXiv2014

One thousand cataclysmic variables from the Catalina Real-time Transient Survey

Over six years of operation, the Catalina Real-time Transient Survey (CRTS) has identified 1043 cataclysmic variable (CV) candidates --- the largest sample of CVs from a single survey to date. Here we provide spectroscopic identification of 85 systems fainter than g<19, including three AMCVn binaries, one helium-enriched CV, one polar and one new eclipsing CV. We analyse the outburst properties of the full sample and show that it contains a large fraction of low accretion rate CVs with long outburst recurrence times. We argue that most of the high accretion rate dwarf novae in the survey footprint have already been found and that future CRTS discoveries will be mostly low accretion rate systems. We find that CVs with white dwarf dominated spectra have significantly fewer outbursts in their CRTS light curves compared to disc-dominated CVs, reflecting the difference in their accretion rates. Comparing the CRTS sample to other samples of CVs, we estimate the overall external completeness to be 23.6 per cent, but show that as much as 56 per cent of CVs have variability amplitudes that are too small to be selected using the transient selection criteria employed by current ground-based surveys. The full table of CRTS CVs, including their outburst and spectroscopic properties examined in this paper, is provided in the online materials.

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arXiv2014

A parameter study of the eclipsing CV in the Kepler field, KIS J192748.53+444724.5

We present high-speed, three-colour photometry of the eclipsing dwarf nova KIS J192748.53+444724.5 which is located in the Kepler field. Our data reveal sharp features corresponding to the eclipses of the accreting white dwarf followed by the bright spot where the gas stream joins the accretion disc. We determine the system parameters via a parameterized model of the eclipse fitted to the observed lightcurve. We obtain a mass ratio of q = 0.570 +/- 0.011 and an orbital inclination of 84.6 +/- 0.3 degrees. The primary mass is M_w = 0.69 +/- 0.07 Msun. The donor star's mass and radius are found to be M_d = 0.39 +/- 0.04 Msun and R_d = 0.43 +/- 0.01 Rsun, respectively. From the fluxes of the white dwarf eclipse we find a white dwarf temperature of T_w = 23000 +/- 3000 K, and a photometric distance to the system of 1600 +/- 200 pc, neglecting the effects of interstellar reddening. The white dwarf temperature in KISJ1927 implies the white dwarf is accreting at an average rate of Mdot = (1.4 +/- 0.8) x10e-9 Msun/yr, in agreement with estimates of the secular mass loss rate from the donor.

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arXiv2014

SDSSJ001153.08-064739.2, a cataclysmic variable with an evolved donor in the period gap

Secondary stars in cataclysmic variables (CVs) follow a well defined period-density relation. Thus, canonical donor stars in CVs are generally low-mass stars of spectral type M. However, several CVs have been observed containing secondary stars which are too hot for their inferred masses. This particular configuration can be explained if the donor stars in these systems underwent significant nuclear evolution before they reached contact. In this paper we present SDSSJ001153.08-064739.2 as an additional example belonging to this peculiar type of CV and discuss in detail its evolutionary history. We perform spectroscopic and photometric observations and make use of available Catalina Real-Time Transient Survey photometry to measure the orbital period of SDSSJ001153.08-064739.2 as 2.4 hours and estimate the white dwarf (Mwd>0.65Msun) and donor star (0.21Msun<Mdon<0.45Msun) masses, the mass ratio (q = 0.32 +- 0.08), the orbital inclination (47 degrees < i < 70 degrees), derive an accurate orbital ephemeris (T0 = 2453383.578 + E x 0.10028081), and report the detection of an outburst. We show that SDSSJ001153.08-064739.2 is one of the most extreme cases in which the donor star is clearly too hot for its mass. SDSSJ001153.08-064739.2 is therefore not only a peculiar CV containing an evolved donor star but also an accreting CV within the period gap. Intriguingly, approximately half of the total currently-observed sample of these peculiar CVs are located in the period gap with nearly the same orbital period.

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arXiv2014

Two new AM Canum Venaticorum binaries from the Sloan Digital Sky Survey III

The AM Canum Venaticorum (AM CVn) binaries are a rare group of ultra-short period, mass-transferring white dwarf binaries, some of which may be Type Ia supernova progenitors. More than a third of the total known population of AM CVn binaries have been discovered via the Sloan Digital Sky Survey (SDSS). Here, we discuss our search for new AM CVns in the SDSS spectroscopic data base, and present two new AM CVns discovered in SDSS-III spectroscopy, SDSS J113732.32+405458.3 and SDSS J150551.58+065948.7. The AM CVn binaries exhibit a connection between their spectral appearance and their orbital period, the spectra of these two new AM CVns suggest that they may be long period systems. Using the radial velocity variations of the emission lines, we measure a possible orbital period of 59.6 +/- 2.7 minutes for SDSS J113732.32+405458.3. Since our search of SDSS spectroscopy has revealed only these two new systems, it is unlikely that a large population of AM CVn binaries have been missed, and their discovery should have little effect on previous calculations of the AM CVn space density.

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arXiv2014

Precise parameters for both white dwarfs in the eclipsing binary CSS 41177

We present ULTRACAM photometry and X-Shooter spectroscopy of the eclipsing double white dwarf binary CSS 41177, the only such system that is also a double-lined spectroscopic binary. Combined modelling of the light curves and radial velocities yield masses and radii for both white dwarfs without the need to assume mass-radius relations. We find that the primary white dwarf has a mass of M1 = 0.38(2) Msun and a radius of R1 = 0.0222(4) Rsun. The secondary white dwarf's mass and radius are M2 = 0.32(1) Msun and R2 = 0.0207(4) Rsun, and its temperature and surface gravity (T2 = 11678(313) K, log(g2) = 7.32(2)) put it close to the white dwarf instability strip. However, we find no evidence for pulsations to roughly 0.5% relative amplitude. Both masses and radii are consistent with helium white dwarf models with thin hydrogen envelopes of 0.0001 Mstar. The two stars will merge in 1.14 Gyr due to angular momentum loss via gravitational wave emission.

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arXiv2013

Timing variations in the secondary eclipse of NN Ser

The eclipsing white dwarf plus main-sequence binary NN Serpentis provides one of the most convincing cases for the existence of circumbinary planets around evolved binaries. The exquisite timing precision provided by the deep eclipse of the white dwarf has revealed complex variations in the eclipse arrival times over the last few decades. These variations have been interpreted as the influence of two planets in orbit around the binary. Recent studies have proved that such a system is dynamically stable over the current lifetime of the binary. However, the existence of such planets is by no means proven and several alternative mechanisms have been proposed that could drive similar variations. One of these is apsidal precession, which causes the eclipse times of eccentric binaries to vary sinusoidally on many year timescales. In this paper we present timing data for the secondary eclipse of NN Ser and show that they follow the same trend seen in the primary eclipse times, ruling out apsidal precession as a possible cause for the variations. This result leaves no alternatives to the planetary interpretation for the observed period variations, although we still do not consider their existence as proven. Our data limits the eccentricity of NN Ser to e<0.001. We also detect a 3.3+/-1.0 second delay in the arrival times of the secondary eclipses relative to the best planetary model. This delay is consistent with the expected 2.84+/-0.04 second Romer delay of the binary, and is the first time this effect has been detected in a white dwarf plus M dwarf system.

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arXiv2013

Optical transmission photometry of the highly inflated exoplanet WASP-17b

We present ground-based high-precision observations of the transit of WASP-17b using the multi-band photometer ULTRACAM on ESO's NTT in the context of performing transmission spectrophotometry of this highly inflated exoplanet. Our choice of filters (SDSS u', g' and r' bands) is designed to probe for the presence of opacity sources in the upper atmosphere. We find evidence for a wavelength dependence in the planet radius in the form of enhanced absorption in the SDSS r' band, consistent with a previously detected broad sodium feature. We present a new independent measurement of the planetary radius at Rpl = 1.97 +/- 0.06 Rjup, which confirms this planet as the most inflated exoplanet known to date. Our measurements are most consistent with an atmospheric profile devoid of enhanced TiO opacity, previously predicted to be present for this planet.

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arXiv2013

The AM CVn binary SDSS J173047.59+554518.5

The AM Canum Venaticorum (AM CVn) binaries are a rare group of hydrogen-deficient, ultra-short period, mass-transferring white dwarf binaries, and are possible progenitors of type Ia supernovae. We present time-resolved spectroscopy of the recently-discovered AM CVn binary SDSS J173047.59+554518.5. The average spectrum shows strong double-peaked helium emission lines, as well as a variety of metal lines, including neon; this is the second detection of neon in an AM CVn binary, after the much brighter system GP Com. We detect no calcium in the accretion disc, a puzzling feature that has been noted in many of the longer-period AM CVn binaries. We measure an orbital period, from the radial velocities of the emission lines, of 35.2 +/- 0.2 minutes, confirming the ultra-compact binary nature of the system. The emission lines seen in SDSS J1730 are very narrow, although double-peaked, implying a low inclination, face-on accretion disc; using the measured velocities of the line peaks, we estimate i < 11 degrees. This low inclination makes SDSS J1730 an excellent system for the identification of emission lines.

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arXiv2013

EL CVn-type binaries - Discovery of 17 helium white dwarf precursors in bright eclipsing binary star systems

The star 1SWASP J024743.37-251549.2 was recently discovered to be a binary star in which an A-type dwarf star eclipses the remnant of a disrupted red giant star (WASP0247-25B). The remnant is in a rarely-observed state evolving to higher effective temperatures at nearly constant luminosity prior to becoming a very low-mass white dwarf composed almost entirely of helium, i.e., it is a pre-He-WD. We have used the WASP photometric database to find 17 eclipsing binary stars with orbital periods P=0.7 to 2.2 days with similar lightcurves to 1SWASP J024743.37-251549.2. The only star in this group previously identified as a variable star is the brightest one, EL CVn, which we adopt as the prototype for this class of eclipsing binary star. The characteristic lightcurves of EL CVn-type stars show a total eclipse by an A-type dwarf star of a smaller, hotter star and a secondary eclipse of comparable depth to the primary eclipse. We have used new spectroscopic observations for 6 of these systems to confirm that the companions to the A-type stars in these binaries have very low masses (approximately 0.2 solar masses). This includes the companion to EL CVn which was not previously known to be a pre-He-WD. EL CVn-type binary star systems will enable us to study the formation of very low-mass white dwarfs in great detail, particularly in those cases where the pre-He-WD star shows non-radial pulsations similar to those recently discovered in WASP0247-25B.

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arXiv2013

Binaries discovered by the MUCHFUSS project; FBS 0117+396: An sdB+dM binary with a pulsating primary

The project Massive Unseen Companions to Hot Faint Underluminous Stars from SDSS (muchfuss) aims to discover subdwarf-B stars with massive compact companions such as overmassive white dwarfs (M > 1.0 M_sun), neutron stars or black holes. From the 127 subdwarfs with substantial radial-velocity variations discovered in the initial survey, a number of interesting objects have been selected for extensive follow-up. After an initial photometry run with BUSCA revealed that FBS 0117+396 is photometrically variable both on long and short timescales, we chose it as an auxiliary target during a 6-night multi-color photometry run with Ultracam. Spectroscopy was obtained at a number of observatories in order to determine the binary period and obtain a radial-velocity amplitude. After establishing an orbital period of P = 0.252 d, and removing the signal associated with the irradiated hemisphere of the M-dwarf companion, we were able to detect ten pulsation periods in the Fourier spectrum of the light curve. Two pulsation modes are found to have short periods of 337 and 379 s, and at least eight modes are found with periods between 45 minutes and 2.5 hours. This establishes that FBS 0117+396 is an sdB+dM reflection binary, in which the primary is a hybrid pulsator, and the first one found with this particular melange of flavours.

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arXiv2013

The planets around NN Ser: still there

We present 25 new eclipse times of the white dwarf binary NN Ser taken with the high-speed camera ULTRACAM on the WHT and NTT, the RISE camera on the Liverpool Telescope, and HAWK-I on the VLT to test the two-planet model proposed to explain variations in its eclipse times measured over the last 25 years. The planetary model survives the test with flying colours, correctly predicting a progressive lag in eclipse times of 36 seconds that has set in since 2010 compared to the previous 8 years of precise times. Allowing both orbits to be eccentric, we find orbital periods of 7.9 +/- 0.5 yr and 15.3 +/- 0.3 yr, and masses of 2.3 +/- 0.5 Mjup and 7.3 +/- 0.3 Mjup. We also find dynamically long-lived orbits consistent with the data, associated with 2:1 and 5:2 period ratios. The data scatter by 0.07 seconds relative to the best-fit model, by some margin the most precise of any of the proposed eclipsing compact object planet hosts. Despite the high precision, degeneracy in the orbit fits prevents a significant measurement of a period change of the binary and of N-body effects. Finally, we point out a major flaw with a previous dynamical stability analysis of NN Ser, and by extension, with a number of analyses of similar systems.

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arXiv2013

A magnetic white dwarf in a detached eclipsing binary

SDSS J030308.35+005444.1 is a close, detached, eclipsing white dwarf plus M dwarf binary which shows a large infrared excess which has been interpreted in terms of a circumbinary dust disk. In this paper we present optical and near-infrared photometric and spectroscopic data for this system. At optical wavelengths we observe heated pole caps from the white dwarf caused by accretion of wind material from the main-sequence star on to the white dwarf. At near-infrared wavelengths we see the eclipse of two poles on the surface of the white dwarf by the main-sequence star, indicating that the white dwarf is magnetic. Our spectroscopic observations reveal Zeeman split emission lines in the hydrogen Balmer series, which we use to measure the magnetic field strength as 8MG. This measurement indicates that the cyclotron lines are located in the infrared, naturally explaining the infrared excess without the need for a circumbinary dust disk. We also detect magnetically-confined material located roughly midway between the two stars. Using measurements of the radial velocity amplitude and rotational broadening of the M star we constrain the physical parameters of the system, a first for a magnetic white dwarf, and the location of the poles on the surface of the white dwarf. SDSS J030308.35+005444.1 is a pre-cataclysmic variable that will likely evolve into an intermediate polar in ~1Gyr.

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arXiv2013

ULTRACAM z'-band Detection of the Secondary Eclipse of WASP-12b

We present z'-band secondary eclipse photometry of the highly irradiated hot Jupiter WASP-12b using ULTRACAM on the 4.2m William Herschel Telescope. We measure a decrease in flux of δ= 0.130+/-0.013% during the passage of the planet behind the star, which is significantly deeper than the previous measurement at this wavelength (0.082+/-0.015%, López-Morales et al. 2010). Our secondary eclipse is best fit with a mid-eclipse phase, ϕ, that is compatible with a circular orbit ϕ= 0.501+/-0.002, in agreement with previous results (Croll et al. 2011). In combination with existing data, our eclipse depth measurement allows us to constrain the characteristics of the planet's atmosphere, which is consistent with a carbon-rich model, with no evidence for a strong thermal inversion. If the difference in eclipse depth reported here compared to that of López-Morales et al. (2010) is of physical origin, as opposed to due to systematics, it may be caused by temporal variability in the flux, due to atmospheric dynamics.

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