arXiv subjects

Howard E. Bond

Publications and source records attributed to Howard E. Bond.

At least 127 records · Page 7Linked to original sources

arXiv2008

The WFC3 Galactic Bulge Treasury Program: A First Look at Resolved Stellar Population Tools

[Abridged] When WFC3 is installed on HST, the community will have powerful new tools for investigating resolved stellar populations. The WFC3 Galactic Bulge Treasury program will obtain deep imaging on 4 low-extinction fields. These non-proprietary data will enable a variety of science investigations not possible with previous data sets. To aid in planning for the use of these data and for future proposals, we provide an introduction to the program, its photometric system, and the associated calibration effort. The observing strategy is based upon a new 5-band photometric system spanning the UV, optical, and near-infrared. With these broad bands, one can construct reddening-free indices of Teff and [Fe/H]. Besides the 4 bulge fields, the program will target 6 fields in well-studied star clusters, spanning a wide range of [Fe/H]. The cluster data serve to calibrate the indices, provide population templates, and correct the transformation of isochrones into the WFC3 photometric system. The bulge data will shed light on the bulge formation history, and will also serve as population templates for other studies. One of the fields includes 12 candidate hosts of extrasolar planets. CMDs are the most popular tool for analyzing resolved stellar populations. However, due to degeneracies among Teff, [Fe/H], and reddening in traditional CMDs, it can be difficult to draw robust conclusions from the data. The 5-band system used for the bulge Treasury observations will provide indices that are roughly orthogonal in Teff and [Fe/H], and we argue that model fitting in an index-index diagram will make better use of the information than fitting separate CMDs. We provide simulations to show the expected data quality and the potential for differentiating between different star-formation histories.

astro-ph↗

arXiv2008

Comments on "The long-period Galactic Cepheid RS Puppis. I. A geometric distance from its light echoes"

The luminous Galactic Cepheid RS Puppis is unique in being surrounded by a dust nebula illuminated by the variable light of the Cepheid. In a recent paper in this journal, Kervella et al. (2008) report a very precise geometric distance to RS Pup, based on measured phase lags of the light variations of individual knots in the reflection nebula. In this commentary, we examine the validity of the distance measurement, as well as the reality of the spatial structure of the nebula determined by Feast (2008) based upon the phase lags of the knots. {Kervella et al. assumed that the illuminated dust knots lie, on average, in the plane of the sky (otherwise it is not possible to derive a geometric distance from direct imaging of light echoes). We consider the biasing introduced by the high efficiency of forward scattering. We conclude that most of the knots are in fact likely to lie in front of the plane of the sky, thus invalidating the Kervella et al. result. We also show that the flat equatorial disk structure determined by Feast is unlikely; instead, the morphology of the nebula is more probably bipolar, with a significant tilt of its axis with respect to the plane of the sky. Although the Kervella et al. distance result is invalidated, we show that high-resolution polarimetric imaging has the potential to yield a valid geometric distance to this important Cepheid.

astro-ph↗

arXiv2008

Stellar Proper Motions in the Galactic Bulge from deep HST ACS/WFC Photometry

We present stellar proper motions in the Galactic bulge from the Sagittarius Window Eclipsing Extrasolar Search (SWEEPS) project using ACS/WFC on HST. Proper motions are extracted for more than 180,000 objects, with >81,000 measured to accuracy better than 0.3 mas/yr in both coordinates. We report several results based on these measurements: 1. Kinematic separation of bulge from disk allows a sample of >15,000 bulge objects to be extracted based on >6-sigma detections of proper motion, with <0.2% contamination from the disk. This includes the first detection of a candidate bulge Blue Straggler population. 2. Armed with a photometric distance modulus on a star by star basis, and using the large number of stars with high-quality proper motion measurements to overcome intrinsic scatter, we dissect the kinematic properties of the bulge as a function of distance along the line of sight. This allows us to extract the stellar circular speed curve from proper motions alone, which we compare with the circular speed curve obtained from radial velocities. 3. We trace the variation of the {l,b} velocity ellipse as a function of depth. 4. Finally, we use the density-weighted {l,b} proper motion ellipse produced from the tracer stars to assess the kinematic membership of the sixteen transiting planet candidates discovered in the Sagittarius Window; the kinematic distribution of the planet candidates is consistent with that of the disk and bulge stellar populations.

astro-ph↗

arXiv2008

A Three-Decade Outburst of the LMC Luminous Blue Variable R127 Draws to a Close

The paradigmatic Luminous Blue Variable R127 in the Large Magellanic Cloud has been found in the intermediate, peculiar early-B state, and substantially fainter in visual light, signaling the final decline from its major outburst that began between 1978 and 1980. This transformation was detected in 2008 January, but archival data show that it began between early 2005 and early 2007. In fact, significant changes from the maximum, peculiar A-type spectrum, which was maintained from 1986 through 1998, had already begun the following year, coinciding with a steep drop in visual light. We show detailed correspondences between the spectrum and light, in which the decline mimics the rise. Moreover, these trends are not monotonic but are characterized by multiple spikes and dips, which may provide constraints on the unknown outburst mechanism. Intensive photometric and spectroscopic monitoring of R127 should now resume, to follow the decline presumably back to the quiescent Ofpe/WN9 state, in order to fully document the remainder of this unique observational opportunity.

astro-ph↗

arXiv2008

Direct Detection of the Close Companion of Polaris with the Hubble Space Telescope

Polaris, the nearest and brightest classical Cepheid, is a single-lined spectroscopic binary with an orbital period of 30 years. Using the High Resolution Channel of the Advanced Camera for Surveys onboard the Hubble Space Telescope (HST) at a wavelength of ~2255Å, we have directly detected the faint companion at a separation of 0\farcs17. A second HST observation 1.04 yr later confirms orbital motion in a retrograde direction. By combining our two measures with the spectroscopic orbit of Kamper and an analysis of the Hipparcos and FK5 proper motions by Wielen et al., we find a mass for Polaris Aa of 4.5^{+2.2}_{-1.4} M_\odot--the first purely dynamical mass determined for any Cepheid. For the faint companion Polaris Ab we find a dynamical mass of 1.26^{+0.14}_{-0.07} M_\odot, consistent with an inferred spectral type of F6 V and with the flux difference of 5.4 mag observed at 2255Å. The magnitude difference at the V band is estimated to be 7.2 mag. Continued HST observations will significantly reduce the mass errors, which are presently still too large to provide critical constraints on the roles of convective overshoot, mass loss, rotation, and opacities in the evolution of intermediate-mass stars. Our astrometry, combined with two centuries of archival measurements, also confirms that the well-known, more distant (18") visual companion, Polaris B, has a nearly common proper motion with that of the Aa,Ab pair. This is consistent with orbital motion in a long-period bound system. The ultraviolet brightness of Polaris B is in accordance with its known F3 V spectral type if it has the same distance as Polaris Aa,Ab.

astro-ph↗

arXiv2007

Planets in the Galactic Bulge: Results from the SWEEPS Project

The exoplanets discovered so far have been mostly around relatively nearby and bright stars. As a result, the host stars are mostly (i) in the Galactic disk, (ii) relatively massive, and (iii) relatively metal rich. The aim of the SWEEPS project is to extend our knowledge to stars which (i) are in a different part of the Galaxy, (ii) have lower masses, and (iii) have a large range of metallicities. To achieve this goal, we used the Hubble Space Telescope to search for transiting planets around F, G, K, and M dwarfs in the Galactic bulge. We photometrically monitored 180,000 stars in a dense bulge field continuously for 7 days. We discovered 16 candidate transiting extrasolar planets with periods of 0.6 to 4.2 days, including a new class of ultra-short period planets (USPPs) with P < 1.2 days. Radial-velocity observations of the two brightest candidates support their planetary nature. These results suggest that planets are as abundant in the Galactic bulge as they are in the solar neighborhood, and they are equally abundant around low-mass stars (within a factor 2). The planet frequency increases with metallicity even for the stars in the Galactic bulge. All the USPP hosts are low-mass stars, suggesting either that close-in planets around higher-mass stars are irradiatively evaporated, or that the planets can migrate to close-in orbits only around such old and low-mass stars.

astro-ph↗

arXiv2007

Transient Mass-Loss Events in the PG 1159 Central Star of Longmore 4

A spectacular, transient mass-loss event in the hydrogen-deficient central star of the planetary nebula Longmore 4 was discovered in 1992 by Werner et al. During that event, the star temporarily changed from its normal PG 1159 spectrum to that of an emission-line [WCE] star. I have been monitoring the spectrum of Lo 4 since 2003. Out of 31 spectra, two of them reveal mass-loss outbursts similar to the one seen in 1992, showing that the phenomenon recurs. I speculate on possible mechanisms for these unique outbursts, but emphasize that we still have no fully convincing explanation.

astro-ph↗

arXiv2007

V838 Monocerotis: A Geometric Distance from Hubble Space Telescope Polarimetric Imaging of its Light Echo

Following the outburst of the unusual variable star V838 Monocerotis in 2002, a spectacular light echo appeared. A light echo provides the possibility of direct geometric distance determination, because it should contain a ring of highly linearly polarized light at a linear radius of ct, where t is the time since the outburst. We present imaging polarimetry of the V838 Mon light echo, obtained in 2002 and 2005 with the Advanced Camera for Surveys onboard the Hubble Space Telescope, which confirms the presence of the highly polarized ring. Based on detailed modeling that takes into account the outburst light curve, the paraboloidal echo geometry, and the physics of dust scattering and polarization, we find a distance of 6.1+-0.6 kpc. The error is dominated by the systematic uncertainty in the scattering angle of maximum linear polarization, taken to be theta_{max}=90^o +- 5^o. The polarimetric distance agrees remarkably well with a distance of 6.2+-1.5 kpc obtained from the entirely independent method of main-sequence fitting to a sparse star cluster associated with V838 Mon. At this distance, V838 Mon at maximum light had M_V\simeq-9.8, making it temporarily one of the most luminous stars in the Local Group. Our validation of the polarimetric method offers promise for measurement of extragalactic distances using supernova light echoes.

astro-ph↗

arXiv2007

The Bizarre Spectral Variability of Central Stars of Planetary Nebulae

A radial velocity (RV) survey to detect central stars in binary systems was carried out between 2002 and 2004. De Marco et al. (2004) reported that 10 out of 11 monitored stars exhibited strong RV variability, but periods were not detected. Since other mechanisms, such as wind variability, can cause apparent RV variations, we monitored 4 of the 10 RV-variable stars at echelle resolutions to determine the origin of the variability. Although RV changes are confirmed for all four stars, none of them can be ascribed to binarity at this time. However, only for IC4593 is wind variability able to explain most (though not all) spectral variability. For BD+332642, no wind and no pulsations appear to be the origin of the RV changes. Finally, M1-77 and M2-54, both known to be irregular photometric variables, exhibit dramatic RV and line shape variability of the hydrogen and HeI absorption lines, as well as large RV variability of weaker lines, which do not change in shape. There is no satisfactory explanation of this variability, though a combination of wind variability and pulsations is still the best guess at what makes these stars so variable. We suggest that luminous central stars are ill suited to detect spectroscopic binaries, because winds (and possibly pulsations) are pervasive and would mask even strong periodicities. It it likely that a sample of intrinsically faint central stars would more readily yield binary information.

astro-ph↗

arXiv2006

A Young Stellar Cluster Surrounding the Peculiar Eruptive Variable V838 Monocerotis

The unusual variable star V838 Monocerotis underwent an eruption in 2002. It continues to illuminate a spectacular series of light echoes, as the outburst light is scattered from circumstellar dust. V838 Mon has an unresolved B3 V companion star. We serendipitously discovered that a neighboring 16th-mag star is also of type B. We then carried out a survey of other stars in the vicinity, revealing two more B-type stars within 45" of V838 Mon. We have determined the distance to this sparse, young cluster, based on spectral classification and photometric main-sequence fitting of the three B stars. The cluster distance is found to be 6.2+-1.2 kpc, in excellent agreement with the geometric distance to V838 Mon of 5.9 kpc obtained from Hubble Space Telescope polarimetry of the light echoes. Using our distance determination, we show that the B3 V companion of V838 Mon is sufficient to account for the entire luminosity of the star measured on survey photographs before its outburst. The B3 star is currently, however, about 1 mag fainter than before the eruption, suggesting that it is now suffering extinction due to dust ejected from V838 Mon. Considerations of the pre-outburst luminosity and cluster age appear to leave stellar-collision or -merger scenarios as one of the remaining viable explanations for the outburst of V838 Mon.

astro-ph↗

arXiv2006

Transiting extrasolar planetary candidates in the Galactic bulge

More than 200 extrasolar planets have been discovered around relatively nearby stars, primarily through the Doppler line shifts owing to the reflex motions of their host stars, and more recently through transits of some planets across the face of the host stars. The detection of planets with the shortest known periods, 1.2 to 2.5 days, has mainly resulted from transit surveys which have generally targeted stars more massive than 0.75 M_sun. Here we report the results from a planetary transit search performed in a rich stellar field towards the Galactic bulge. We discovered 16 candidates with orbital periods between 0.4 and 4.2 days, five of which orbit stars of 0.44 to 0.75 M_sun. In two cases, radial-velocity measurements support the planetary nature of the companions. Five candidates have orbital periods below 1.0 day, constituting a new class of ultra-short-period planets (USPPs), which occur only around stars of less than 0.88 M_sun. This indicates that those orbiting very close to more luminous stars might be evaporatively destroyed, or that jovian planets around lower-mass stars might migrate to smaller radii.

astro-ph↗

arXiv2006

Polaris: Mass and Multiplicity

Polaris, the nearest and brightest classical Cepheid, is a member of at least a triple system. It has a wide ($18''$) physical companion, the F-type dwarf Polaris B. Polaris itself is a single-lined spectroscopic binary with an orbital period of 30 years (Kamper, 1996, JRASC, 90, 140). By combining {\it Hipparcos} measurements of the instantaneous proper motion with long-term measurements and the Kamper radial-velocity orbit, Wielen et al. (2000, A&A, 360, 399) have predicted the astrometric orbit of the close companion. Using the {\it Hubble Space Telescope} and the Advanced Camera for Surveys' High-Resolution Channel with an ultraviolet (F220W) filter, we have now directly detected the close companion. Based on the Wielen et al. orbit, the {\it Hipparcos} parallax, and our measurement of the separation ($0.176''$ $\pm$ $0.002''$), we find a preliminary mass of 5.0 $\pm$ 1.5 M$_{\odot}$ for the Cepheid and 1.38 $\pm$ 0.61 M$_{\odot}$ for the close companion. These values will be refined by additional {\it HST} observations scheduled for the next 3 years. We have also obtained a {\it Chandra} ACIS-I image of the Polaris field. Two distant companions C and D are not X-rays sources and hence are not young enough to be physical companions of the Cepheid. There is one additional stellar X-ray source in the field, located $253''$ from Polaris A, which is a possible companion. Further investigation of such a distant companion is valuable to confirm the full extent of the system.

astro-ph↗

arXiv2006

A Young Open Cluster Surrounding V838 Monocerotis

During a program of spectroscopic monitoring of V838 Mon, we serendipitously discovered that a neighboring 16th-mag star is of type B. We then carried out a spectroscopic survey of other stars in the vicinity, revealing two more B-type stars, all within 45" of V838 Mon. We have determined the distance to this sparse, young cluster, based on spectral classification and photometric main-sequence fitting of the three B stars. The distance is found to be 6.2+-1.2 kpc, in excellent agreement with the geometric distance to V838 Mon of 5.9 kpc obtained from Hubble Space Telescope polarimetry of the light echoes. The cluster's age is less than 25 Myr. The absolute luminosity of V838 Mon during its outburst, based on our distance measurement, was very similar to that of M31 RV, an object in the bulge of M31 that was also a cool supergiant throughout its eruption in 1988. However, there is no young population at the site of M31 RV. It does not appear possible to form a nova-like cataclysmic binary system within the young age of the V838 Mon cluster, and the lack of a young population surrounding M31 RV suggests that the outburst mechanism does not require a massive progenitor. These considerations appear to leave stellar-collision or merger scenarios as one of the remaining viable explanations for the outbursts of V838 Mon and M31 RV.

astro-ph↗

arXiv2006

Hubble Space Telescope Observations of the Light Echo around V838 Monocerotis

The outburst of V838 Monocerotis in early 2002, and the subsequent appearance of its light echoes, occurred just before the installation of the Advanced Camera for Surveys into the Hubble Space Telescope (HST). This fortunate sequence of events has allowed us to obtain spectacular HST images of the echoes, yielding not only pictures of extraordinary beauty, but also providing unique scientific information. Our team has used the HST images to provide a direct geometrical distance to V838 Mon, based on polarimetric imaging, and limits on the distance based on the apparent angular expansion rates. Several morphological features seen in the HST images strongly suggest that the illuminated dust was ejected from the star in a previous outburst, similar to the current one. In particular, a "double-helix" feature points exactly back to the star. Moreover, three-dimensional mapping of the outer edges of the dust suggests an overall ellipsoidal shape, centered on V838 Mon itself. And the appearance of the light echo in the most recent Hubble images is remarkably similar to that of a well-known planetary nebula, M27. Future work on the HST images will include an analysis of interstellar-dust physics, in a situation where the scattering angle and illumination are unambiguously known, and visualization of a fully three-dimensional map of the dust distribution.

astro-ph↗

arXiv2006

V838 Mon and M31-RV: The Stellar Populations Angle

Insight into the origin of unusual events like the eruption of V838 Mon can be obtained from studies of the stellar populations from which they arise. V838 Mon lies in an intriguing region of the Galaxy, toward the warped outer edge of the disk, with significant contributions from the Galactic thick disk and the recently discovered Monoceros tidal stream. The initial distance measures placed V838 Mon in a jumbled region of the Galaxy but the recent shorter distances make it highly likely that V838 Mon was a thin disk star -- likely in a spiral arm -- consistent with the recent detection of a young cluster in the vicinity. We compare V838 Mon to M31-RV, a red variable that erupted in the bulge of M31 in 1988 and had a peak luminosity and spectral evolution very similar to V838 Mon. Archival HST images show no nebulosity or unusual stars at M31-RV's projected location. Moreover, the only stellar population in the field is a canonic old bulge population. This indicates that whatever the origin of the red novae, the mechanism is likely independent of age and progenitor mass. In particular, the B3V star seen in V838 Mon it not a necessary part of the eruption mechanism.

astro-ph↗

arXiv2005

Hubble Space Telescope Imaging of the Outburst Site of M31 RV

M31 RV is a luminous red variable star that appeared for several months in the bulge of M31 during 1988. Unlike classical novae, M31 RV was cool throughout its outburst. Interest in this object has revived recently because of its strong resemblance to V838 Mon, a luminous Galactic variable star that appeared in 2002 and is illuminating a spectacular light echo, and has evolved to ever-cooler surface temperatures. V4332 Sgr is a third object which was also a red supergiant throughout its eruption. We have examined archival Hubble Space Telescope (HST) images of the site of M31 RV, obtained fortuitously in 1999 with the WFPC2 camera in parallel mode during spectroscopic observations of the nucleus of M31. We located the site of M31 RV in the HST frames precisely through astrometric registration with ground-based CCD images, including several taken during the outburst. No light echo is seen at the M31 RV site, implying that M31 RV is not surrounded by circumstellar (or interstellar) dust similar to that around V838 Mon, or that its extent is less than ~1.7 pc. The stellar population at the outburst site consists purely of old red giants; there is no young population, such as seen around V838 Mon. There are no stars of unusual color at the site, suggesting that M31 RV had faded below HST detectability in the 11 years since outburst, that it is an unresolved companion of one of the red giants in the field, or that it is one of the red giants. We suggest future observations that may help decide among these possibilities.

astro-ph↗

arXiv2005

Spectroscopic Binaries in Planetary Nebulae

It is already known that about 10% of central stars of PNe are very short-period binaries (hours to days), which are detected through photometric variations. These must have been formed through common-envelope interactions in initially wide binaries, accompanied by ejection of the envelope and its subsequent photoionization as a PN. Radial-velocity observations by ourselves and others are now suggesting that an even larger fraction of planetary nuclei may be spectroscopic binaries, making the total binary fraction very large. However, we have not as yet been able to rule out the possibility that the apparent velocity changes are actually due to stellar-wind variations. Pending follow-up spectroscopic observations with large telescopes, it presently appears plausible that binary-star ejection is the major formation channel for planetary nebulae.

astro-ph↗

arXiv2005

Hubble Space Telescope Spectroscopy of the Balmer lines in Sirius B

Sirius B is the nearest and brightest of all white dwarfs, but it is very difficult to observe at visible wavelengths due to the overwhelming scattered light contribution from Sirius A. However, from space we can take advantage of the superb spatial resolution of the Hubble Space Telescope to resolve the A and B components. Since the closest approach in 1993, the separation between the two stars has become increasingly favourable and we have recently been able to obtain a spectrum of the complete Balmer line series for Sirius B using HST?s Space Telescope Imaging Spectrograph (STIS). The quality of the STIS spectra greatly exceed that of previous ground-based spectra, and can be used to provide an important determination of the stellar temperature (Teff = 25193K) and gravity (log g = 8.556). In addition we have obtained a new, more accurate, gravitational red-shift of 80.42 +/- 4.83 km s-1 for Sirius B. Combining these results with the photometric data and the Hipparcos parallax we obtain new determinations of the stellar mass for comparison with the theoretical mass-radius relation. However, there are some disparities between the results obtained independently from log g and the gravitational redshift which may arise from flux losses in the narrow 50x0.2arcsec slit. Combining our measurements of Teff and log g with the Wood (1995) evolutionary mass-radius relation we get a best estimate for the white dwarf mass of 0.978 M. Within the overall uncertainties, this is in agreement with a mass of 1.02 M obtained by matching our new gravitational red-shift to the theoretical M/R relation.

astro-ph↗