arXiv subjects

Howard E. Bond

Publications and source records attributed to Howard E. Bond.

At least 145 records · Page 8Linked to original sources

arXiv2005

The uBVI Photometric System. I. Motivation, Implementation, and Calibration

This paper describes the design principles for a CCD-based photometric system that is highly optimized for ground-based measurement of the size of the Balmer jump in stellar energy distributions. It is shown that, among ultraviolet filters in common use, the Thuan-Gunn u filter is the most efficient for this purpose. This filter is combined with the standard Johnson-Kron-Cousins B, V, and I bandpasses to constitute the uBVI photometric system. Model stellar atmospheres are used to calibrate color-color diagrams for the uBVI system in terms of the fundamental stellar parameters of effective temperature, surface gravity, and metallicity. The u-B index is very sensitive to log g, but also to [Fe/H]. It is shown that an analog of the Strömgren c_1 index, defined as (u-B)-(B-V), is much less metallicity dependent, but still sensitive to log g. The effect of interstellar reddening on u-B is determined through synthetic photometric calculations, and practical advice is given on dealing with flat fields, atmospheric extinction, the red leak in the u filter, and photometric reductions. The uBVI system offers a wide range of applicability in detecting stars of high luminosity in both young (yellow supergiants) and old (post-AGB stars) populations, using stars of both types as standard candles to measure extragalactic distances with high efficiency, and in exploring the horizontal branch in globular clusters. In many stellar applications, it can profitably replace the classical uBVI system. Paper II in this series will present a network of well-calibrated standard stars for the uBVI system.

astro-ph↗

arXiv2005

The uBVI Photometric System. II. Standard Stars

Paper I of this series described the design of a CCD-based photometric system that is optimized for ground-based measurements of the size of the Balmer discontinuity in stellar spectra. This "uBVI" system combines the Thuan-Gunn u filter with the standard Johnson-Kron-Cousins BVI filters, and it can be used to discover luminous yellow supergiants in extragalactic systems and post-asymptotic-giant-branch stars in globular clusters and galactic halos. In the present paper we use uBVI observations obtained on 54 nights with 0.9-m telescopes at Kitt Peak and Cerro Tololo to construct a catalog of standardized u magnitudes for standard stars taken from the 1992 catalog of Landolt. We describe the selection of our 14 Landolt fields, and give details of the photometric reductions, including red-leak and extinction corrections, transformation of all of the observations onto a common magnitude system, and establishment of the photometric zero point. We present a catalog of u magnitudes of 103 stars suitable for use as standards. We show that data obtained with other telescopes can be transformed to our standard system with better than 1% accuracy.

astro-ph↗

arXiv2004

A Period and a Prediction for the Of?p Spectrum Alternator HD 191612

The observational picture of the enigmatic O-type spectrum variable HD191612 has been sharpened substantially. A symmetrical, low-amplitude light curve with a period near 540 d has recently been reported from Hipparcos photometry. This period satisfies all of the spectroscopy since at least 1982, including extensive new observations during 2003 and 2004, and it has predicted the next transition during September--October 2004. Measurements of the H alpha equivalent width reveal a sharp emission peak in the phase diagram, in contrast to the apparently sinusoidal light curve. The He II absorption-line strength is essentially constant, while He I varies strongly, possibly filled in by emission in the O6 state, thus producing the apparent spectral-type variations. The O8 state appears to be the "normal" one. Two intermediate O7 observations have been obtained, which fall at the expected phases, but these are the only modern observations of the transitions so far. The period is too long for rotation or pulsation; although there is no direct evidence as yet for a companion, a model in which tidally induced oscillations drive an enhanced wind near periastron of an eccentric orbit appears promising. Further observations during the now predictable transitions may provide a critical test. Ultraviolet and X-ray observations during both states will likely also prove illuminating.

astro-ph↗

arXiv2003

Indications of a Large Fraction of Spectroscopic Binaries Among Nuclei of Planetary Nebulae

Previous work indicates that about 10% of planetary-nebula nuclei (PNNi) are photometrically variable short-period binaries with periods of hours to a few days. These systems have most likely descended from common-envelope (CE) interactions in initially much wider binaries. Population-synthesis studies suggest that these very close pairs could be the short-period tail of a much larger post-CE binary population with periods of up to a few months. We have initiated a radial-velocity (RV) survey of PNNi with the WIYN 3.5-m telescope and Hydra spectrograph, which is aimed at discovering these intermediate-period binaries. We present initial results showing that 10 out of 11 well-observed PNNi have variable RVs, suggesting that a significant binary population may be present. However, further observations are required because we have as yet been unable to fit our sparse measurements with definite orbital periods, and because some of the RV variability might be due to variations in the stellar winds of some of our PNNi.

astro-ph↗

arXiv2003

An energetic stellar outburst accompanied by circumstellar light echoes

Some classes of stars, including supernovae and novae, undergo explosive outbursts that eject stellar material into space. In 2002, the previously unknown variable star V838 Monocerotis brightened suddenly by a factor of about 10^4. Unlike a supernova or nova, V838 Mon did not explosively eject its outer layers; rather, it simply expanded to become a cool supergiant with a moderate-velocity stellar wind. Superluminal light echoes were discovered as light from the outburst propagated into surrounding, pre-existing circumstellar dust. Here we report high-resolution imaging and polarimetry of the light echoes, which allow us to set direct geometric distance limits to the object. At a distance of >6 kpc, V838 Mon at its maximum brightness was temporarily the brightest star in the Milky Way. The presence of the circumstellar dust implies that previous eruptions have occurred, and spectra show it to be a binary system. When combined with the high luminosity and unusual outburst behavior, these characteristics indicate that V838 Mon represents a hitherto unknown type of stellar outburst, for which we have no completely satisfactory physical explanation.

astro-ph↗

arXiv2002

WeBo 1: A Young Barium Star Surrounded by a Ring-Like Planetary Nebula

WeBo 1 (PN G135.6+01.0), a previously unrecognized planetary nebula with a remarkable thin-ring morphology, was discovered serendipitously on Digitized Sky Survey images. The central star is found to be a late-type giant with overabundances of carbon and s-process elements. The giant is chromospherically active and photometrically variable, with a probable period of 4.7 days; this suggests that the star is spotted, and that 4.7 days is its rotation period. We propose a scenario in which one component of a binary system became an AGB star with a dense stellar wind enriched in C and s-process elements; a portion of the wind was accreted by the companion, contaminating its atmosphere and spinning up its rotation. The AGB star has now become a hot subdwarf, leaving the optical companion as a freshly contaminated barium star inside an ionized planetary nebula.

astro-ph↗

arXiv2002

Confirmation of SBS 1150+599A As An Extremely Metal-Poor Planetary Nebula

SBS 1150+599A is a blue stellar object at high galactic latitude discovered in the Second Byurakan Survey. New high-resolution images of SBS 1150+599A are presented, demonstrating that it is very likely to be an old planetary nebula in the galactic halo, as suggested by Tovmassian et al (2001). An H-alpha image taken with the WIYN 3.5-m telescope and its "tip/tilt" module reveals the diameter of the nebula to be 9.2", comparable to that estimated from spectra by Tovmassian et al. Lower limits to the central star temperature were derived using the Zanstra hydrogen and helium methods to determine that the star's effective temperature must be > 68,000K and that the nebula is optically thin. New spectra from the MMT and FLWO telescopes are presented, revealing the presence of strong [Ne V] lambda 3425, indicating that the central star temperature must be > 100,000K. With the revised diameter, new central star temperature, and an improved central star luminosity, we can constrain photoionization models for the nebula significantly better than before. Because the emission-line data set is sparse, the models are still not conclusive. Nevertheless, we confirm that this nebula is an extremely metal-poor planetary nebula, having a value for O/H that is less than 1/100 solar, and possibly as low as 1/500 solar.

astro-ph↗

arXiv2002

FIRST J102347.6+003841: The First Radio-Selected Cataclysmic Variable

We have identified the 1.4 GHz radio source FIRST J102347.6+003841 (hereafter FIRST J1023+0038) with a previously unknown 17th-mag Galactic cataclysmic variable (CV). The optical spectrum resembles that of a magnetic (AM Herculis- or DQ Herculis-type) CV. Five nights of optical CCD photometry showed variations on timescales of minutes to hours, along with rapid flickering. A re-examination of the FIRST radio survey data reveals that the radio detection was based on a single 6.6 mJy flare; on two other occasions the source was below the ~1 mJy survey limit. Several other magnetic CVs are known to be variable radio sources, suggesting that FIRST J1023+0038 is a new member of this class (and the first CV to be discovered on the basis of radio emission). However, FIRST J1023+0038 is several optical magnitudes fainter than the other radio-detected magnetic CVs. It remains unclear whether the source simply had a very rare and extraordinarily intense radio flare at the time of the FIRST observation, or is really an unusually radio-luminous CV; thus further observations are urged.

astro-ph↗

arXiv2001

Hubble Space Telescope Spectroscopy of V471 Tauri: Oversized K Star, Paradoxical White Dwarf

We have used the GHRS onboard the HST to obtain Lyman-alpha spectra of the hot white-dwarf (WD) component of the short-period eclipsing DA+dK2 pre-cataclysmic binary V471 Tauri, a member of the Hyades star cluster. Radial velocities of the WD, combined with ground-based measurements of the dK velocities, eclipse timings, and a determination of the dK star's rotational velocity, yield dynamical masses for the components of M(WD)=0.84 and M(dK)=0.93 Msun. Model-atmosphere fitting of the Ly-alpha profile provides the effective temperature (34,500 K) and surface gravity (log g=8.3) of the WD. The radius of the dK component is 18% larger than that of a normal Hyades dwarf of the same mass. This expansion is attributed to the extensive coverage of the surface by starspots, causing the star to expand in response. The WD radius, determined from a radiometric analysis and from eclipse ingress timings, is 0.0107 Rsun. The position of the star in the M-R plane is in full accord with theory for a degenerate CO WD. The high temperature and mass of the WD present an evolutionary paradox: the WD is the most massive known in the Hyades, but also the hottest and youngest. We suggest that the explanation is that the WD is indeed very young, and is descended from a triple consisting of a blue straggler and a more-distant dK companion. We estimate that the common-envelope efficiency parameter, alpha_CE, was of order 0.3-1.0, in good agreement with recent hydrodynamical simulations.

astro-ph↗

arXiv2001

Detection of Coronal Mass Ejections in V471 Tauri with the Hubble Space Telescope

V471 Tauri, an eclipsing system consisting of a hot DA white dwarf (WD) and a dK2 companion in a 12.5-hour orbit, is the prototype of the pre-cataclysmic binaries. The late-type component is magnetically active, due to its being constrained to rotate synchronously with the short orbital period. During a program of UV spectroscopy of V471 Tau, carried out with the Goddard High Resolution Spectrograph (GHRS) onboard the Hubble Space Telescope, we serendipitously detected two episodes in which transient absorptions in the Si III 1206 A resonance line appeared suddenly, on a timescale of <2 min. The observations were taken in a narrow spectral region around Ly-alpha, and were all obtained near the two quadratures of the binary orbit, i.e., at maximum projected separation (~3.3 Rsun) of the WD and K star. We suggest that these transient features arise when coronal mass ejections (CME's) from the K2 dwarf pass across the line of sight to the WD. Estimates of the velocities, densities, and masses of the events in V471 Tau are generally consistent with the properties of solar CME's. Given our detection of 2 events during 6.8 hr of GHRS observing, along with a consideration of the restricted range of latitudes and longitudes on the K star's surface that can give rise to trajectories passing in front of the WD as seen from Earth, we estimate that the active V471 Tau dK star emits some 100-500 CME's per day, as compared to 1-3 per day for the Sun. The K dwarf's mass-loss rate associated with CME's is at least (5-25) x 10^{-14} Msun/yr, but it may well be orders of magnitude higher if most of the silicon is in ionization states other than Si III.

astro-ph↗

arXiv2001

Post-AGB Stars in Globular Clusters and Galactic Halos

We discuss three aspects of post-AGB (PAGB) stars in old populations. (1) HST photometry of the nucleus of the planetary nebula (PN) K 648 in the globular cluster (GC) M15 implies a mass of 0.60 Msun, in contrast to the mean masses of white dwarfs in GCs of ~0.5 Msun. This suggests that K 648 is descended from a merged binary, and we infer that single Pop II stars do not produce visible PNe. (2) Yellow PAGB stars are the visually brightest stars in old populations (Mv ~ -3.3) and are easily recognizable because of their large Balmer jumps; thus they show great promise as a Pop II standard candle. Two yellow PAGB stars in the GC NGC 5986 have the same V magnitudes to within +/-0.05 mag, supporting an expected narrow luminosity function. (3) Using CCD photometry and a u filter lying below the Balmer jump, we have detected yellow PAGB stars in the halo of M31 and in its dwarf elliptical companion NGC 205. With the Milky Way zero point, we reproduce the Cepheid distance to M31, and find that NGC 205 is ~100 kpc further away than M31. The star counts imply a yellow PAGB lifetime of about 25,000 yr, and their luminosities imply masses near 0.53 Msun.

astro-ph↗

arXiv2000

Resolving Sirius-like binaries with the Hubble Space Telescope

We present initial results from a Hubble Space Telescope ultraviolet imaging survey of stars known to have hot white-dwarf companions which are unresolved from the ground. The hot companions, discovered through their EUV or UV emission, are hidden by the overwhelming brightnesses of the primary stars at visible wavelengths. Out of 17 targets observed, we have resolved eight of them with the Wide Field Planetary Camera~2, using various ultraviolet filters. Most of the implied orbital periods for the resolved systems are hundreds to thousands of years, but in at least three cases (56 Persei, zeta Cygni, and REJ1925-566) it shouldbe possible to detect the orbital motions within the next few years, and they may eventually yield new dynamically determined masses for the white-dwarf components. The 56 Persei and 14 Aurigae systems are found to be quadruple and quintuple, respectively, including the known optical components as well as the newly resolved white-dwarf companions. The mild barium star zeta Cygni, known to have an 18 year spectroscopic period, is marginally resolved. All of these new resolved Sirius-type binaries will be useful in determining gravitational redshifts and masses of the white-dwarf components.

astro-ph↗

arXiv2000

Resolving Sirius-like binaries with the Hubble Space Telescope

We have imaged seventeen recently discovered Sirius-like binary systems with HST/WFPC2 and resolved the white dwarf secondary in eight cases. Most of the implied orbital periods are of order several hundred years, but in three cases (56 Per, Zeta Cygni and REJ1925-566) the periods are short enough that it may be possible to detect orbital motion within a few years. It will then be possible to derive dynamically determined masses for the white dwarfs, and potentially these stars could be used as stringent tests of the mass-radius relation and initial-final mass relation.

astro-ph↗

arXiv2000

Hubble Space Telescope Observations of the Planetary Nebula K648 in the Globular Cluster M15

We have obtained observations of the planetary nebula K648 in the Galactic globular cluster M15 with the Hubble Space Telescope's WFPC2 camera, covering an interval of 7 days. The frames provide both time-sampled broad-band photometry of the central star and high-resolution images of the nebula in the light of H-alpha, [OIII], and [NII]. In the deep narrow-band images, K648 is a fairly typical double-shelled elliptical, but with a bright arc at one end of the major axis that is especially prominent in [NII]; this feature is probably a collection of FLIERs. The nebula is surrounded by a faint, smooth elliptical halo, which appears undisturbed by any interaction with the interstellar medium. Adopting Teff = 40,000 +- 3,000 K based upon published spectral-line analyses, and employing our new broad-band optical flux data along with the known cluster distance, we find log L/L_sun = 3.78 +- 0.08 for the K648 central star. Theoretical post-asymptotic-giant-branch evolutionary tracks imply a mass of 0.60 +- 0.02 M_sun for this luminosity, which is significantly higher than the mean mass of white dwarfs in globular clusters and the halo field (0.50 +- 0.02). The K648 central star exhibits no significant photometric variability in our data; thus we find no direct evidence of a close binary companion. We suggest that the progenitor of K648 experienced mass augmentation in a close-binary merger, allowing it to evolve to a remnant of higher mass than those of the single stars in the cluster.

astro-ph↗

arXiv1999

Binarity of Central Stars of Planetary Nebulae

I list the 16 planetary nebulae (PNe) known to contain close-binary nuclei, and show that the nebulae generally have axisymmetric structures, including elliptical, bipolar, or ring morphologies. The orbital periods range from 2.7 hr to 16 days, and close binaries constitute ~10% of all central stars. Since the known binaries were found mainly from photometric variability, which depends on heating effects at very small stellar separations, radial-velocity surveys will be necessary to find the large predicted population of binary nuclei with periods of about 10-100 days. Other PN phenomena that may arise from binary-star interactions include jets and point-symmetry, the periodically spaced arcs revealed by HST in the faint halos around several PNe and proto-PNe, and the existence of PNe in globular clusters. There is thus considerable circumstantial evidence that binary-star processes play a major role in the formation and shaping of many or even most PNe.

astro-ph↗

arXiv1999

A Hubble Space Telescope Survey for Resolved Companions of Planetary-Nebula Nuclei

We report results of an HST "snapshot" survey aimed at finding resolved binary companions of the central stars of Galactic planetary nebulae (PNe). Using WF/PC and WFPC2, we searched the fields of 113 PNe for stars whose close proximity to the central star suggests a physical association. We find 10 binary nuclei that are very likely to be physically associated, and another six that are possible binary associations. By correcting for interstellar extinction and placing the central stars' companions on the main sequence, we derive distances to the objects, and thereby significantly increase the number of PNe with reliable distances. Comparison of our derived distances with those obtained from various statistical methods shows that all of the latter have systematically overestimated the distances, by factors ranging up to a factor of two or more. We show that this error is most likely due to the fact that the properties of our PNe with binary nuclei are systematically different from those of PNe used heretofore to calibrate statistical methods. Specifically, our PNe tend to have lower surface brightnesses at the same physical radius than the traditional calibration objects. This difference may arise from a selection effect: the PNe in our survey are typically nearby, old nebulae, whereas most of the objects that calibrate statistical techniques are low-latitude, high-surface-brightness, and more distant nebulae. As a result, the statistical methods that seem to work well with samples of distant PNe, e.g., those in the Galactic bulge or external galaxies, may not be applicable to the more diverse population of local PNe.

astro-ph↗

arXiv1998

Ages for Globular Clusters in the Outer Galactic Halo: The Second Parameter Clusters Palomar~3, Palomar~4 and Eridanus

We have used the WFPC2 camera on the Hubble Space Telescope to obtain photometry of the outer-halo globular clusters Palomar 3, Palomar 4, and Eridanus. These three are classic examples of the "second parameter" anomaly because of their red horizontal-branch morphologies in combination with their low-to-intermediate metallicities. Our color-magnitude diagrams in (V, V-I) reach $V_{lim} \simeq 27.0$, clearly delineating the subgiant and turnoff regions and about three magnitudes of the unevolved main sequences. The slopes and dereddened colors of the giant branches are consistent with published [Fe/H] estimates that rank the clusters (Pal 3, Eridanus, Pal 4) in order of increasing metallicity, with all three falling near or between the abundance values of the classic nearby halo clusters M3 and M5. Differential fits of their color-magnitude diagrams are made to each other and to M3 and M5 for relative age determinations. We find that the three outer-halo cluster CMDs differ from the nearby clusters in a way that is consistent with their being younger by $\sim 1.5 - 2 $Gyr, if we have correctly estimated the clusters' chemical-abundance ratios. Conversely, the inferred age difference could be smaller ($\ltsim 1 $Gyr) if either [Fe/H] or [$α$/Fe] for the outer-halo clusters is significantly lower than we have assumed. Possible age spreads of order 1$ $Gyr among both the nearby and outer-halo clusters may also be present.

astro-ph↗

arXiv1998

HST Photometry of the Globular Cluster M4

This paper presents a detailed description of the acquisition and processing of a large body of imaging data for three fields in the globular cluster M4 taken with the Wide Field and Planetary Camera 2 aboard the Hubble Space Telescope. Analysis with the ALLFRAME package yielded the deepest photometry yet obtained for this cluster. The resulting data-set for 4708 stars (positions and calibrated photometry in V, I, and, in two fields, U) spanning approximately six cluster core radii is available on the AAS CD-ROM (or email a request to RAI). The scientific analysis is deferred to three companion papers, which investigate the significant white dwarf population discovered and the main sequence population.

astro-ph↗