arXiv subjects

C. M. Copperwheat

Publications and source records attributed to C. M. Copperwheat.

70 records · Page 4Linked to original sources

arXiv2011

Radial velocity measurements of Subdwarf B stars

Subdwarf B (sdB) stars are hot, sub-luminous stars which are thought to be core-helium burning with thin hydrogen envelopes. The mechanism by which these stars lose their envelopes has been controversial but it has been argued that binary star interaction is the main cause. Over the past decade we have conducted a radial velocity study of a large sample of sdB stars, and have shown that a significant fraction of the field sdB population exists in binary systems. In 2002 and 2003 we published 23 new binary sdB stars and the definitions of their orbits. Here we present the continuation of this project. We give the binary parameters for 28 systems, 18 of which are new. We present also our radial velocity measurements of a further 108 sdBs. Of these, 88 show no significant evidence of orbital motion. The remaining 20 do show radial velocity variations, and so are good candidates for further study. Based on these results, our best estimate for the binary fraction in the sdB population is 46 - 56 per cent. This is a lower bound since the radial velocity variations of very long period systems would be difficult to detect over the baseline of our programme, and for some sources we have only a small number of measurements.

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arXiv2011

Cataclysmic Variables below the Period Gap: Mass Determinations of 14 Eclipsing Systems

We present high-speed, three-colour photometry of the eclipsing cataclysmic variables CTCV 1300, CTCV 2354 and SDSS 1152. All three systems are below the observed "period gap" for cataclysmic variables. For each system we determine the system parameters by fitting a parameterised model to the observed eclipse light curve by chi-squared minimisation. We also present an updated analysis of all other eclipsing systems previously analysed by our group. New donor masses are generally between 1 and 2 sigma of those originally published, with the exception of SDSS 1502 and DV UMa. We note that the donor mass of SDSS 1501 has been revised upwards by 0.024Msun. This system was previously identified as having evolved passed the minimum orbital period for cataclysmic variables, but the new mass determination suggests otherwise. Our new analysis confirms that SDSS 1035 and SDSS 1433 have evolved past the period minimum for cataclysmic variables, corroborating our earlier studies. We find that the radii of donor stars are oversized when compared to theoretical models, by approximately 10 percent. We show that this can be explained by invoking either enhanced angular momentum loss, or by taking into account the effects of star spots. We are unable to favour one cause over the other, as we lack enough precise mass determinations for systems with orbital periods between 100 and 130 minutes, where evolutionary tracks begin to diverge significantly. We also find a strong tendency towards high white dwarf masses within our sample, and no evidence for any He-core white dwarfs. The dominance of high mass white dwarfs implies that erosion of the white dwarf during the nova outburst must be negligible, or that not all of the mass accreted is ejected during nova cycles, resulting in the white dwarf growing in mass. (Abridged)

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arXiv2011

A search for optical bursts from RRAT J1819-1458: II. Simultaneous ULTRACAM-Lovell Telescope observations

The Rotating RAdio Transient (RRAT) J1819-1458 exhibits ~3 ms bursts in the radio every ~3 min, implying that it is visible for only ~1 per day. Assuming that the optical light behaves in a similar manner, long exposures of the field would be relatively insensitive due to the accumulation of sky photons. A much better way of detecting optical emission from J1819-1458 would then be to observe with a high-speed optical camera simultaneously with radio observations, and co-add only those optical frames coincident with the dispersion-corrected radio bursts. We present the results of such a search, using simultaneous ULTRACAM and Lovell Telescope observations. We find no evidence for optical bursts in J1819-1458 at magnitudes brighter than i'=19.3 (5-sigma limit). This is nearly 3 magnitudes fainter than the previous burst limit, which had no simultaneous radio observations.

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arXiv2011

The photometric period in ES Ceti

We present ULTRACAM photometry of ES Cet, an ultracompact binary with a 620s orbital period. The mass transfer in systems such as this one is thought to be driven by gravitational radiation, which causes the binary to evolve to longer periods since the semi-degenerate donor star expands in size as it loses mass. We supplement these ULTRACAM+WHT data with observations made with smaller telescopes around the world over a nine year baseline. All of the observations show variation on the orbital period, and by timing this variation we track the period evolution of this system. We do not detect any significant departure from a linear ephemeris, implying a donor star that is of small mass and close to a fully degenerate state. This finding favours the double white dwarf formation channel for this AM CVn star. An alternative explanation is that the system is in the relatively short-lived phase in which the mass transfer rate climbs towards its long-term value.

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arXiv2011

The Planets around the Post-Common Envelope Binary NN Serpentis

We have detected 2 circumbinary planets around the close binary system NN Serpentis using the orbital time delay effect measured via the sharp eclipses of the white dwarf primary. The present pre-cataclysmic binary was formed when the original - 2 M primary expanded into a red giant, causing the secondary star to drop from its original orbit at a separation of about 1.4 A.U. down to its current separation at 0.0043 A.U. A quasi-adiabatic evolution of the circumbinary planets' orbits during the common-envelope phase would have placed them in unstable configurations, suggesting that they may have suffered significant orbital drag effects and were originally in much larger orbits. Alternatively, they may have been created as 2nd-generation planets during the last million years from the substantial amount of material lost during the creation of the binary, making them the youngest planets known. Either solution shows how little we actually understand about planetary formation.

astro-ph.EP↗

arXiv2010

Two planets orbiting the recently formed post-common envelope binary NN Serpentis

Planets orbiting post-common envelope binaries provide fundamental information on planet formation and evolution. We searched for such planets in NN Ser ab, an eclipsing short-period binary that shows long-term eclipse time variations. Using published, reanalysed, and new mid-eclipse times of NN Ser ab obtained between 1988 and 2010, we find excellent agreement with the light-travel-time effect by two additional bodies superposed on the linear ephemeris of the binary. Our multi-parameter fits accompanied by N-body simulations yield a best fit for the objects NN Ser (ab)c and d locked in a 2:1 mean motion resonance, with orbital periods P_c=15.5 yrs and P_d=7.7 yrs, masses M_c sin i_c = 6.9 M_Jup and M_d sin i_d = 2.2 M_Jup, and eccentricities e_c=0 and e_d=0.20. A secondary chi**2 minimum corresponds to an alternative solution with a period ratio of 5:2. We estimate that the progenitor binary consisted of an A star with ~2 M_Sun and the present M dwarf secondary at an orbital separation of ~1.5 AU. The survival of two planets through the common-envelope phase that created the present white dwarf requires fine tuning between the gravitational force and the drag force experienced by them in the expanding envelope. The alternative is a second-generation origin in a circumbinary disk created at the end of this phase. In that case, the planets would be extremely young with ages not exceeding the cooling age of the white dwarf of 10**6 yrs.

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arXiv2010

Orbital periods of cataclysmic variables identified by the SDSS. VIII. A slingshot prominence in SDSS J003941.06+005427.5?

We present VLT spectroscopy and NTT photometry of the faint cataclysmic binary SDSS J003941.06+005427.5. This object shows triple-peaked H-alpha emission with all three peaks variable in both strength and velocity. We measure an orbital period of 91.395 +/- 0.093 min from the velocity variations of the wings of the H-alpha emission line. Using the GALEX and SDSS photometry of this object, we determine a white dwarf temperature of 15000 K and a very late (>=L2) spectral type for the companion star. These measurements, plus the relatively long orbital period, suggest that SDSS J003941.06+005427.5 may be a post-bounce cataclysmic variable. Doppler maps of the H-alpha and He I 6678A emission features show an accretion disc with a non-uniform brightness and departures from Keplerian flow. The third emission peak is detected only in H-alpha and at a relatively low velocity amplitude of 202 +/- 3 km/s. We are unable to explain this emission as arising from either the white dwarf, the secondary star, or the accretion disc. We tentatively attribute this mysterious central peak to a coronal loop anchored at the secondary star. If confirmed, this would be the first example of a slingshot prominence in a CV with a low mass-transfer rate and/or a fully convective secondary star.

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arXiv2010

SDSS J0926+3624: The shortest period eclipsing binary star

With orbital periods of the order of tens of minutes or less, the AM Canum Venaticorum stars are ultracompact, hydrogen deficient binaries with the shortest periods of any binary subclass, and are expected to be among the strongest gravitational wave sources in the sky. To date, the only known eclipsing source of this type is the P = 28 min binary SDSS J0926+3624. We present multiband, high time resolution light curves of this system, collected with WHT/ULTRACAM in 2006 and 2009. We supplement these data with additional observations made with LT/RISE, XMM_Newton and the Catalina Real-Time Transient Survey. From light curve models we determine the mass ratio to be q = M2 / M1 = 0.041 +/- 0.002 and the inclination to be 82.6 +/- 0.3 deg. We calculate the mass of the primary white dwarf to be 0.85 +/- 0.04 solar masses and the donor to be 0.035 +/- 0.003 solar masses, implying a partially degenerate state for this component. We observe superhump variations that are characteristic of an elliptical, precessing accretion disc. Our determination of the superhump period excess is in agreement with the established relationship between this parameter and the mass ratio, and is the most precise calibration of this relationship at low q. We also observe a quasi-periodic oscillation in the 2006 data, and we examine the outbursting behaviour of the system over a 4.5 year period.

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arXiv2010

Orbital Period Variations in Eclipsing Post Common Envelope Binaries

We present high speed ULTRACAM photometry of the eclipsing post common envelope binaries DE CVn, GK Vir, NN Ser, QS Vir, RR Cae, RX J2130.6+4710, SDSS 0110+1326 and SDSS 0303+0054 and use these data to measure precise mid-eclipse times in order to detect any period variations. We detect a large (~ 250 sec) departure from linearity in the eclipse times of QS Vir which Applegate's mechanism fails to reproduce by an order of magnitude. The only mechanism able to drive this period change is a third body in a highly elliptical orbit. However, the planetary/sub-stellar companion previously suggested to exist in this system is ruled out by our data. Our eclipse times show that the period decrease detected in NN Ser is continuing, with magnetic braking or a third body the only mechanisms able to explain this change. The planetary/sub-stellar companion previously suggested to exist in NN Ser is also ruled out by our data. Our precise eclipse times also lead to improved ephemerides for DE CVn and GK Vir. The width of a primary eclipse is directly related to the size of the secondary star and variations in the size of this star could be an indication of Applegate's mechanism or Wilson (starspot) depressions which can cause jitter in the O-C curves. We measure the width of primary eclipses for the systems NN Ser and GK Vir over several years but find no definitive variations in the radii of the secondary stars. However, our data are precise enough (dRsec / Rsec < 10^-5) to show the effects of Applegate's mechanism in the future. We find no evidence of Wilson depressions in either system. We also find tentative indications that flaring rates of the secondary stars depend on their mass rather than rotation rates.

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arXiv2009

Orbital periods of cataclysmic variables identified by the SDSS. VII. Four new eclipsing systems

We present photometry of nine cataclysmic variable stars identified by the Sloan Digital Sky Survey, aimed at measuring the orbital periods of these systems. Four of these objects show deep eclipses, from which we measure their orbital periods. The light curves of three of the eclipsing systems are also analysed using the LCURVE code, and their mass ratios and orbital inclinations determined. SDSS J075059.97+141150.1 has an orbital period of 134.1564 +/- 0.0008 min, making it a useful object with which to investigate the evolutionary processes of cataclysmic variables. SDSS J092444.48+080150.9 has a period of 131.2432 +/- 0.0014 min and is probably magnetic. The white dwarf ingress and egress phases are very deep and short, and there is no clear evidence that this object has an accretion disc. SDSS J115207.00+404947.8 and SDSS J152419.33+220920.1 are nearly identical twins, with periods of 97.5 +/- 0.4 and 93.6 +/- 0.5 min and mass ratios of 0.14 +/- 0.03 and 0.17 +/- 0.03, respectively. Their eclipses have well-defined white dwarf and bright spot ingress and egress features, making them excellent candidates for detailed study. All four of the orbital periods presented here are shorter than the 2-3 hour period gap observed in the known population of cataclysmic variables.

astro-ph.SR↗

arXiv2009

Precise mass and radius values for the white dwarf and low mass M dwarf in the pre-cataclysmic binary NN Serpentis

We derive precise system parameters for the pre-cataclysmic binary, NN Ser. From light curve fitting we find an orbital inclination of i = 89.6 +/- 0.2 deg. From the HeII absorption line we find K_{WD}= 62.3 +/- 1.9 km/s. The irradiation-induced emission lines from the surface of the secondary star give a range of observed radial velocities. The corrected values give a radial velocity of K_{sec}= 301 +/- 3 km/s, with an error dominated by the systematic effects of the model. This leads to a binary separation of a = 0.934 +/- 0.009 R_{sun}, radii of R_{WD} = 0.0211 +/- 0.0002 R_{sun} and R_{sec} = 0.149 +/- 0.002 R_{sun} and masses of M_{WD} = 0.535 +/- 0.012 M_{sun} and M_{sec} = 0.111 +/- 0.004 M_{sun}. The masses and radii of both components of NN Ser were measured independently of any mass-radius relation. For the white dwarf, the measured mass, radius and temperature show excellent agreement with a `thick' hydrogen layer of fractional mass M_{H}/{M}_{WD} = 10^{-4}. The measured radius of the secondary star is 10% larger than predicted by models, however, correcting for irradiation accounts for most of this inconsistency, hence the secondary star in NN Ser is one of the first precisely measured very low mass objects to show good agreement with models. ULTRACAM r', i' and z' photometry taken during the primary eclipse determines the colours of the secondary star as (r'-i')_{sec}= 1.4 +/- 0.1 and (i'-z')_{sec} = 0.8 +/- 0.1 which corresponds to a spectral type of M4 +/- 0.5. This is consistent with the derived mass, demonstrating that there is no detectable heating of the unirradiated face, despite intercepting radiative energy from the white dwarf which exceeds its own luminosity by over a factor of 20.

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arXiv2009

Physical properties of IP Pegasi: an eclipsing dwarf nova with an unusually cool white dwarf

We present high speed photometric observations of the eclipsing dwarf nova IP Peg taken with the triple-beam camera ULTRACAM mounted on the William Herschel Telescope. The primary eclipse in this system was observed twice in 2004, and then a further sixteen times over a three week period in 2005. Our observations were simultaneous in the Sloan u', g' and r' bands. By phase-folding and averaging our data we make the first significant detection of the white dwarf ingress in this system and find the phase width of the white dwarf eclipse to be 0.0935 +/- 0.0003, significantly higher than the previous best value of between 0.0863 and 0.0918. The mass ratio is found to be q = M2 /M1 = 0.48 +/- 0.01, consistent with previous measurements, but we find the inclination to be 83.8 +/- 0.5 deg, significantly higher than previously reported. We find the radius of the white dwarf to be 0.0063 +/- 0.0003 solar radii, implying a white dwarf mass of 1.16 +/- 0.02 solar masses. The donor mass is 0.55 +/- 0.02 solar masses. The white dwarf temperature is more difficult to determine, since the white dwarf is seen to vary significantly in flux, even between consecutive eclipses. This is seen particularly in the u'-band, and is probably the result of absorption by disc material. Our best estimate of the temperature is 10,000 - 15,000K, which is much lower than would be expected for a CV with this period, and implies a mean accretion rate of less than 5 times 10^-11 solar masses per year, more than 40 times lower than the expected rate.

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arXiv2009

Orbital periods of cataclysmic variables identified by the SDSS. VI. The 4.5-hr period eclipsing system SDSS J100658.40+233724.4

We present time-resolved spectroscopy and photometry of SDSS J100658.40+233724.4, which we have discovered to be an eclipsing cataclysmic variable with an orbital period of 0.18591324 days (267.71507 min). The observed velocity amplitude of the secondary star is 276 +/- 7 km/s, which an irradiation correction reduces to 258 +/- 12 km/s. Doppler tomography of emission lines from the infrared calcium triplet supports this measurement. We have modelled the light curve using the LCURVE code and Markov Chain Monte Carlo simulations, finding a mass ratio of 0.51 +/- 0.08. From the velocity amplitude and the light curve analysis we find the mass of the white dwarf to be 0.78 +/- 0.12 Msun and the masses and radii of the secondary star to be 0.40 +/- 0.10 Msun and 0.466 +/- 0.036 Rsun, respectively. The secondary component is less dense than a normal main sequence star but its properties are in good agreement with the expected values for a CV of this orbital period. By modelling the spectral energy distribution of the system we find a distance of 676 +/- 40 pc and estimate a white dwarf effective temperature of 16500 +/- 2000 K.

astro-ph.SR↗

arXiv2009

Optical pulsations from the anomalous X-ray pulsar 1E 1048.1-5937

We present high-speed optical photometry of the anomalous X-ray pulsar 1E 1048.1-5937 obtained with ULTRACAM on the 8.2-m Very Large Telescope in June 2007. We detect 1E 1048.1-5937 at a magnitude of i'=25.3+/-0.2, consistent with the values found by Wang et al. (2008) and hence confirming their conclusion that the source was approximately 1 mag brighter than in 2003-2006 due to an on-going X-ray flare that started in March 2007. The increased source brightness enabled us to detect optical pulsations with an identical period (6.458 s) to the X-ray pulsations. The rms pulsed fraction in our data is 21+/-7%, approximately the same as the 2-10 keV X-ray rms pulsed fraction. The optical and X-ray pulse profiles show similar morphologies and appear to be approximately in phase with each other, the latter lagging the former by only 0.06+/-0.02 cycles. The optical pulsations in 1E 1048.1-5937 are very similar in nature to those observed in 4U 0142+61. The implications of our observations for models of anomalous X-ray pulsars are discussed.

astro-ph.HE↗

arXiv2008

ULTRACAM observations of two accreting white dwarf pulsators

In this paper we present high time-resolution observations of GW Librae and SDSS J161033.64-010223.3 -- two cataclysmic variables which have shown periodic variations attributed to non-radial pulsations of the white dwarf. We observed both these systems in their quiescent states and detect the strong pulsations modes reported by previous authors. The identification of further periodicities in GW Lib is limited by the accretion-driven flickering of the source, but in the case of SDSS 1610 we identify several additional low-amplitude periodicities. In the case of SDSS 1610, there is evidence to suggest that the two primary signals have a different colour dependence, suggesting that they may be different spherical harmonic modes. We additionally observed GW Lib during several epochs following its 2007 dwarf nova outburst: the first time a dwarf nova containing a pulsating white dwarf has been observed in such a state. We do not observe any periodicities, suggesting that the heating of the white dwarf had either switched-off the pulsations entirely, or reduced their relative amplitude in flux to the point where they are undetectable. Further observations eleven months after the outburst still do not show the pulsation modes previously observed, but do show the emergence of two new periodic signals. In addition to the WD pulsations, our observations of GW Lib in quiescence show a larger-amplitude modulation in luminosity with a period of approximately 2.1 hours. This has been previously observed, and its origin is unclear: it is unrelated to the orbital period. We find this modulation to vary over the course of our observations in phase and/or period. Our data support the conclusion that this is an accretion-related phenomenon which originates in the accretion disc.

astro-ph↗

arXiv2007

Irradiation models for ULXs and fits to optical data

We have constructed a model which describes the optical emission from ultraluminous X-ray sources (ULXs), and have used it to constrain the parameters of seven ULX systems. Our model assumes a binary nature for ULXs, and accounts for optical emission from an X-ray irradiated companion star and accretion disk. We apply our model to six different ULX optical counterparts observed with HST, and one observed with the ESO VLT, and determine the mass, radius and age of the donor stars in these systems. In addition, we obtained constraints for the black hole (BH) mass in some cases. We use the mass accretion rate implied by the X-ray luminosity of these sources as an additional constraint on the donor star, by assuming the mass transfer is driven by the stellar nuclear evolution. We find that in general the donors are older and less massive than previously thought, and are consistent with being of spectral type B. We discuss how these results affect our understanding of the evolution and history of ULXs. Where we can constrain the BH masses, we find them to be consistent with stellar mass BHs or intermediate mass BHs of order 100 solar masses. We make predictions for future observations of optical/infrared ULX counterparts, calculating binary periods for different BH masses in each of the seven sources.

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