arXiv subjects

P. D. Dobbie

Publications and source records attributed to P. D. Dobbie.

48 records · Page 3Linked to original sources

arXiv2006

New Praesepe white dwarfs and the initial mass-final mass relation

We report the spectroscopic confirmation of four further white dwarf members of Praesepe. This brings the total number of confirmed white dwarf members to eleven making this the second largest collection of these objects in an open cluster identified to date. This number is consistent with the high mass end of the initial mass function of Praesepe being Salpeter in form. Furthermore, it suggests that the bulk of Praesepe white dwarfs did not gain a substantial recoil kick velocity from possible asymmetries in their loss of mass during the asymptotic giant branch phase of evolution. By comparing our estimates of the effective temperatures and the surface gravities of WD0833+194, WD0840+190, WD0840+205 and WD0843+184 to modern theoretical evolutionary tracks we have derived their masses to be in the range 0.72-0.76Msun and their cooling ages \~300Myrs. For an assumed cluster age of 625+/-50Myrs the infered progenitor masses are between 3.3-3.5Msun. Examining these new data in the context of the initial mass-final mass relation we find that it can be adequately represented by a linear function (a0=0.289+/-0.051, a1=0.133+/-0.015) over the initial mass range 2.7Msun to 6Msun. Assuming an extrapolation of this relation to larger initial masses is valid and adopting a maximum white dwarf mass of 1.3Msun, our results support a minimum mass for core-collapse supernovae progenitors in the range ~6.8-8.6Msun.

astro-ph↗

arXiv2005

A search for binarity using FUSE observations of DAO white dwarfs

We report on a search for evidence of binarity in Far-Ultraviolet Spectroscopic Explorer (FUSE) observations of DAO white dwarfs. Spectra recorded by FUSE are built up from a number of separate exposures. Observation of changes in the position of photospheric heavy element absorption lines between exposures, with respect to the stationary interstellar medium lines, would reveal radial velocity changes - evidence of the presence of a binary system. This technique is successful in picking out all the white dwarfs already known to be binaries, which comprise 5 out of the sample of 16, but significant radial velocity shifts were found for only one additional star, Ton 320. This object is also known to have an infrared excess. DAOs can be separated broadly into low or normal mass objects. Low mass white dwarfs can be formed as a result of binary evolution, but it has been suggested that the lower mass DAOs evolve as single stars from the extended horizontal branch, and we find no evidence of binarity for 8 out of the 12 white dwarfs with relatively low mass. The existence of higher mass DAOs can also be explained if they are within binary systems, but of the four higher mass stars in the sample studied, PG 1210+533 and LB 2 do not exhibit significant radial velocity shifts, although there were only two exposures for the former object and the latter has an infrared excess.

astro-ph↗

arXiv2005

Heavy element abundances in DAO white dwarfs measured from FUSE data

We present heavy element abundance measurements for 16 DAO white dwarfs, determined from Far-Ultraviolet Spectroscopic Explorer {FUSE} spectra. Evidence of absorption by heavy elements was found in the spectra of all the objects. Measurements were made using models that adopted the temperatures, gravities and helium abundances determined from both optical and FUSE data by Good et al. (2004). Despite the different evolutionary paths that the types of DAO white dwarfs are thought to evolve through, their abundances were not found to vary significantly, apart from for the silicon abundances. Abundances measured when the FUSE derived values of temperature, gravity and helium abundance were adopted were, in general, a factor 1-10 higher than those determined when the optical measure of those parameters was used. Satisfactory fits to the absorption lines were achieved in approximately equal number. The models that used the FUSE determined parameters seemed better at reproducing the strength of the nitrogen and iron lines, while for oxygen, the optical parameters were better.

astro-ph↗

arXiv2005

Crossing into the substellar regime in Praesepe

We present the results of a deep optical 2.6 square degree survey with near infrared follow-up measurements of the intermediate-aged Praesepe open cluster. The survey is complete to I=21.3, Z=20.5, corresponding to ~0.06 Solar Masses assuming a cluster age of 0.5 Gyrs. Using 3-5 pass-bands to constrain cluster membership, we identify 32 new low mass cluster members, at least 4 of which are likely to be substellar. We use the low mass census to trace the region where the sequence moves away from the NEXTGEN towards the Dusty regime at Teff = 2200K. In doing so, we identify four unresolved binaries, yielding a substellar binary fraction of ~30 percent. The binary fractions appear to decrease below 0.1 Solar Masses, in contrast to the rising fractions found in the Pleiades. We also identify a paucity of late M dwarfs, thought to be due to a steepening in the mass-luminosity relation at these spectral types, and compare the properties of this gap in the sequence to those observed in younger clusters. We note an overdensity of faint sources in the region of the so-called subcluster (possibly an older smaller cluster within Praesepe), and subsequently derive the luminosity and mass functions for the main Praesepe cluster, revealing a turn-over near the substellar boundary. We conclude by presenting astrometric measurements for low mass Praesepe candidates from the literature, and rule out as a likely foreground dwarf RPr1, hitherto thought to be a substellar member.

astro-ph↗

arXiv2004

A near-IR spectroscopic search for very-low-mass cool companions to notable DA white dwarfs

We have undertaken a detailed near-IR spectroscopic analysis of eight notable white dwarfs, predominantly of southern declination. In each case the spectrum failed to reveal compelling evidence for the presence of a spatially unresolved, cool, late-type companion. Therefore, we have placed an approximate limit on the spectral-type of a putative companion to each degenerate. From these limits we conclude that if GD659, GD50, GD71 or WD2359-434 possesses an unresolved companion then most probably it is substellar in nature (M<0.072Msun). Furthermore, any spatially unresolved late-type companion to RE J0457-280, RE J0623-374, RE J0723-274 or RE J2214-491 most likely has M<0.082Msun. These results imply that if weak accretion from a nearby late-type companion is the cause of the unusual photospheric composition observed in a number of these degenerates then the companions are of very low mass, beyond the detection thresholds of this study. Furthermore, these results do not contradict a previously noted deficit of very-low-mass stellar and brown dwarf companions to main sequence F,G,K and early-M type primaries (a<1000AU).

astro-ph↗

arXiv2004

Praesepe and the seven white dwarfs

We report the discovery, from our preliminary survey of the Praesepe open cluster, of two new spectroscopically confirmed white dwarf candidate members. We derive the effective temperatures and surface gravities of WD0837+218 and WD0837+185 (LB5959) to be 17845 +555 -565K and log g = 8.48 +0.07 -0.08 and 14170 +1380 -1590K and log g =8.46 +0.15 -0.16 respectively. Using theoretical evolutionary tracks we estimate the masses and cooling ages of these white dwarfs to be 0.92+/-0.05Msun and 280 +40 -30Myrs and 0.90+/-0.10Msun and 500 +170 -100Myrs respectively. Adopting reasonable values for the cluster age we infer the progenitors of WD0837+218 and WD0837+185 had masses of 2.6 < M < Mcrit Msun and 2.4 < M < 3.5Msun respectively, where Mcrit is the maximum mass of a white dwarf progenitor. We briefly discuss these findings in the context of the observed deficit of white dwarfs in open clusters and the initial mass-final-mass relationship.

astro-ph↗

arXiv2004

RX J2130.6+4710 - an eclipsing white dwarf - M-dwarf binary star

(Abridged) We report the detection of eclipses in the close white-dwarf - M-dwarf binary star RXJ2130.6+4710. We estimate that the spectral type of the M-dwarf is M3.5Ve or M4Ve. We estimate that the effective temperature of the white dwarf is Teff = 18000K +- 1000K. We have used the width of the primary eclipse and duration of totality measured precisely from Ultracam u' data combined with the amplitude of the ellipsoidal effect in the I band and the semi-amplitudes of the spectroscopic orbits to derive masses and radii for the M-dwarf and white dwarf. The M-dwarf has a mass of 0.555 +- 0.023 solar masses and a radius of 0.534 +- 0.053 solar radii, which is a typical radius for stars of this mass. The mass of the white dwarf is 0.554 +- 0.017 solar masses and its radius is 0.0137 +- 0.0014 solar radii, which is the radius expected for a carbon-oxygen white dwarf of this mass and effective temperature RXJ2130.6+4710 is a rare example of a pre-cataclysmic variable star which will start mass transfer at a period above the period gap for cataclysmic variables.

astro-ph↗

arXiv2003

An Optical and Near Infrared Search for Brown Dwarfs in the Pleiades Cluster

We have carried out a brown dwarf search over an area of 14' x 23' near the central portion of the Pleiades open cluster in five optical and near infrared bands (i', Z, J, H, Ks) with 10 sigma detection limits of i' = 22.0, J = 20.0 and Ks = 18.5 mag. The surveyed area has large extinction in excess of Av = 3 in the Pleiades region. We detected four new brown dwarf candidates from the colour-colour (J-K, i'-J) and the colour-magnitude (J, i'-K) diagrams. We estimated their masses as 0.046 Msun down to 0.028 Msun. The least massive one is estimated to have a mass smaller than Roque 25 or int-pl-IZ-69, and maybe the lowest mass object found so far in the Pleiades cluster.

astro-ph↗

arXiv2003

Brown dwarfs and low-mass stars in the Pleiades and Praesepe:- Membership and binarity

We present near infrared J-, H- and K-band photometry and optical spectroscopy of low-mass star and brown dwarf (BD) candidates in the Pleiades and Praesepe open clusters. We find that Pleiades stars with K=10.5--13 are rather redder than the NextGen isochrones. We also identify this effect amongst alphaPer sources from the literature, but find no evidence of it for field stars from the literature. It is possible that the I-K colour of youthful stars is affected by the presence of magnetic activity. In the 2-colour diagram we find no colour change across the "M dwarf gap" (which would be indicative of dust formation). For Teff=2800--3150K, the colours of Praesepe and Pleiades sources differ, which may result from gravity sensitive molecular opacities. Praesepe sources cooler than 2500K agree with the dusty models, but Pleiades sources agree with the NextGen models down to 2000K. It is possible that dust formation does not begin until lower Teffs in sources with lower surface gravities. We also identify unresolved binaries in both clusters, and estimate mass ratios (q) for Pleiades BDs. Most of these have q>0.7, however, 3/18 appear to have lower q values. We determine the binary fraction (BF) for numerous mass ranges in each cluster, and find that it is generally rising towards lower masses. We find a BD BF of 50(+11/-10)%. We also find some evidence suggesting that the BF-q distribution is flat for 0.5-0.35Mo, in contrast to solar type stars.

astro-ph↗

arXiv2002

The alpha-element abundances in the most oxygen-poor planetary nebula PN G 135.9+55.9

We present extensive new spectroscopy and imaging of PN G 135.9+55.9. We use these data as constraints to photoionization models to derive limits on the oxygen abundance. We find that PN G 135.9+55.9 has an oxygen abundance less than 1/50 of the solar value. Our models favour a value of 12 + \log O/H between 5.8 and 6.5 dex, confirming that PN G 135.9+55.9 is the most oxygen-poor planetary nebula known (Tovmassian et al. 2001). We also derive Ne/O = 0.5 +/- 0.3, S/O < 0.094, and Ar/O < 0.23. Although the value of Ne/O is nominally high, it need not imply that the progenitor of PN G 135.9+55.9 converted any of its initial oxygen abundance to neon. The helium abundance appears to be very low, He/H ~ 0.08, but a precise determination will require a much more detailed study. We find that Ha/Hb is lower than expected and perhaps variable, a finding for which we have no clear explanation.

astro-ph↗

arXiv2001

The low mass white dwarf companion to Beta Crateris

We present FUSE H Lyman series spectroscopy of the hot white dwarf companion to the 4th magnitude A1III star Beta Crateris, which shows that is has an unusually low mass, 0.43Msun, and has almost certainly evolved through binary interaction. This system could be a long-sought remnant of Algol-type evolution, although radial velocity measurements appear to show that the pair are not close. Instead, micro-variations in Beta Crt's proper motion as measured by Hipparcos suggests that the period could be as high as 10 years. However, a low mass white dwarf in a system with a period >3 years is difficult to explain by conventional models for binary evolution. We speculate on alternative models for the evolution of this system which involve an eccentric binary or multiple components.

astro-ph↗

arXiv1999

Opacities along the line of sight to and in the atmosphere of the white dwarf in the close detached DAO+dM binary RE J0720-318 (V* IN CMa)

We present the results from a multi-wavelength study of the mixed H+He composition DAO white dwarf RE J0720-318. A detailed analysis of UV and EUV spectroscopic data with state-of-the-art non-LTE photospheric models demonstrates that the observed opacity to EUV radiation probably results from a more complex structure than a simple H+He, chemically layered atmosphere. Instead, EUV photometry and phase resolved EUV spectroscopy indicate a likely spatial non-uniformity in the surface distribution of helium, which is consistent with a model in which material is accreted from the wind of the dM secondary. The rotational modulation of the spatially inhomogeneous EUV opacity allows us to estimate the rotation period of the white dwarf (0.463 +/- 0.004 days). We have also reviewed two plausible origins proposed by Burleigh et al. (1997) and Dupuis et al. (1997) to account for the unusual N(HI)/N(HeI)~1 along this line of sight. We conclude that it is probably due to the presence of a cloud of ionized gas along this line of sight, rather than a circumbinary disk. The cloud, residing between 123-170pc distant in the direction of the CMa ISM tunnel, may be >40pc in length.

astro-ph↗