arXiv subjects

Michael M. Shara

Publications and source records attributed to Michael M. Shara.

At least 127 records · Page 7Linked to original sources

arXiv2003

A Far-Ultraviolet Survey of 47 Tucanae.II The Long-Period Cataclysmic Variable AKO 9

We present time-resolved, far-ultraviolet (FUV) spectroscopy and photometry of the 1.1 day eclipsing binary system AKO 9 in the globular cluster 47 Tucanae. The FUV spectrum of AKO 9 is blue and exhibits prominent C IV and He II emission lines. The spectrum broadly resembles that of long-period, cataclysmic variables in the galactic field. Combining our time-resolved FUV data with archival optical photometry of 47 Tuc, we refine the orbital period of AKO 9 and define an accurate ephemeris for the system. We also place constraints on several other system parameters, using a variety of observational constraints. We find that all of the empirical evidence is consistent with AKO 9 being a long-period dwarf nova in which mass transfer is driven by the nuclear expansion of a sub-giant donor star. We therefore conclude that AKO 9 is the first spectroscopically confirmed cataclysmic variable in 47 Tuc. We also briefly consider AKO 9's likely formation and ultimate evolution. Regarding the former, we find that the system was almost certainly formed dynamically, either via tidal capture or in a 3-body encounter. Regarding the latter, we show that AKO 9 will probably end its CV phase by becoming a detached, double WD system or by exploding in a Type Ia supernova.

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arXiv2003

Searching for Variability in the Globular Cluster Messier 4

Time-series data taken with the Hubble Space Telescope of a field six core radii (~5') from the center of the globular cluster Messier 4, and covering a period of about 10 weeks in early 2001, have been analyzed in search of variable objects. Various criteria were employed to select candidate variable stars. Period searches were performed on the selected candidates using phase dispersion minimization (PDM). The reliability of the PDM search results was tested using synthetic light curves of eclipsing binary stars and sinusoidal light curves of different periods. Results from this analysis showed that there are probably no eclipsing binary stars or periodic variables in our field with periods on the order of a few hours to a few days, down to limiting magnitudes of V~25 and I~24, which is consistent with the absence of contact binaries such as W Ursae Majoris systems. However, one candidate variable star does show an increase in brightness of ~0.1 magnitudes in both bandpasses, which seems to last for a few days. Possible explanations concerning the nature of this object include a binary system with a white dwarf primary and a low-mass main sequence secondary, or a BY Draconis variable star. We are able to set an upper limit to the observed fraction of photometric variability in this dataset of 0.05%.

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arXiv2003

The Galactic Inner Halo: Searching for White Dwarfs and Measuring the Fundamental Galactic Constant, Vo/Ro

We establish an extragalactic, zero-motion frame of reference within the deepest optical image of a globular star cluster, an HST 123-orbit exposure of M4 (GO 8679, cycle 9). The line of sight beyond M4 (l,b (deg) = 351,16) intersects the inner halo (spheroid) of our Galaxy at a tangent-point distance of 7.6 kpc (for Ro = 8 kpc). We isolate these spheroid stars from the cluster based on their proper motions over the 6-year baseline between these and previous epoch HST data (GO 5461, cycle 4). Distant background galaxies are also found on the same sight line using image-morphology techniques. This fixed reference frame allows us to independently determine the fundamental Galactic constant, Vo/Ro = 25.3 +/- 2.6 km/s/kpc, thus providing a velocity of the Local Standard of Rest, v = 202.7 +/- 24.7 km/s for Ro = 8.0 +/- 0.5 kpc. Secondly, the galaxies allow a direct measurement of M4's absolute proper motion, mu_total = 22.57 +/- 0.76 mas/yr, in excellent agreement with recent studies. The clear separation of galaxies from stars in these deep data also allow us to search for inner-halo white dwarfs. We model the conventional Galactic contributions of white dwarfs along our line of sight and predict 7.9 (thin disk), 6.3 (thick disk) and 2.2 (spheroid) objects to the limiting magnitude at which we can clearly delineate stars from galaxies (V = 29). An additional 2.5 objects are expected from a 20% white dwarf dark halo consisting of 0.5 Mo objects, 70% of which are of the DA type. After considering the kinematics and morphology of the objects in our data set, we find the number of white dwarfs to be consistent with the predictions for each of the conventional populations. However, we do not find any evidence for dark halo white dwarfs.

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arXiv2003

Erupting Dwarf Novae in the Large Magellanic Cloud

We report the first likely detections of erupting Dwarf Novae (DN) in an external galaxy: the Large Magellanic Cloud. Six candidates were isolated from approximately a million stars observed every second night over 11 nights with the CTIO 8K x 8K Mosaic2 CCD imager. Artificial dwarf nova and completeness tests suggest that we are seeing only the brightest of the LMC DN, probably SS Cygni-like CVs, but possibly SU UMa-type cataclysmics undergoing superoutbursts. We derive crude but useful limits on the LMC DN surface density, and on the number of DN in the LMC. Many thousands of cataclysmic variables in the Magellanic Clouds can be discovered and characterized with 8 meter class telescopes.

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arXiv2003

New High Proper Motion Stars from the Digitized Sky Survey. II. Northern Stars with 0.5<mu<2.0 arcsec/yr at High Galactic Latitudes

In a continuation of our systematic search for high proper motion stars in the Digitized Sky Survey, we have completed the analysis of northern sky fields at galactic latitudes above 25 degrees. With the help of our SUPERBLINK software, a powerful automated blink comparator developed by us, we have identified 1146 stars in the magnitude range 8<r<20 with proper motions 0.500<mu<2.000 arcsec/yr. These include 1080 stars previously listed in Luyten's proper motion catalogs (LHS, NLTT), 9 stars not previously listed in the Luyten catalogs but reported elsewhere in the literature (including 1 previously reported by our team), and 57 new objects reported here for the first time. This paper includes a list of positions, proper motions, magnitudes, and finder charts for all the new high proper motion stars. Combined with our previous study of low galactic latitude fields (see Paper I), our survey now covers over 98% of the northern sky. We conclude that the Luyten catalogs were 90% complete in the northern sky for stars with 0.5<mu<2.0 arcsec/yr down to magnitude r=19. We discuss the incompleteness of the old Luyten proper motion survey, and estimate completeness limits for our new survey.

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arXiv2003

LSR1610-0040: the first early-type L subdwarf

We report the discovery of LSR1610-0040, a previously unreported r=17.5 star with a very high proper motion mu=1.46"/yr. This very cool star (b-i=6.3) is found to have a peculiar spectrum that does not fit into the standard sequence of late-M dwarfs and L dwarfs. Rather, the spectrum is more typical of an ultra-cool subdwarf, with weak bands of TiO and no detectable VO. But because LSR1610-0040 is so much redder than any other sdM known, and because it does not appear to fit well into the sdM sequence, we propose that LSR1610-0040 be assigned a spectral type sdL, making it the first early-type L subdwarf known. Evidence suggests that LSR1610-0040 is an old, metal-poor star whose mass is just above the limit of hydrogen burning (M ~ 0.08 M_sun).

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arXiv2003

LSR0602+3910 - Discovery of a Bright Nearby L-type Brown Dwarf

We report the discovery of LSR0602+3910, an L dwarf of class L1. The object was initially identified by Lepine et al. (2002) as a new high proper motion star lying close to the Galactic plane. Its 2MASS J-K_s=1.43 is consistent with an L dwarf, which we now confirm spectroscopically. In addition, we see a signature of Li I absorption, making LSR0602+3910 a brown dwarf, one of the brightest known (K_s=10.86). Among L dwarfs it is second in brightness to the combined light of 2MASS 0746+20, a close binary system. We see no indication that LSR0602+3910 is a binary, although high-resolution imaging will be required to confirm this. Spectroscopic and photometric distance estimates agree very well, placing LSR0602+3910 at d=10.6+-0.8 pc. LSR0602+3910 was most likely missed in previous searches because of its proximity to the plane, the region that most searches avoided. We estimate that some 40% of bright L dwarfs are missed because of this selection effect.

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arXiv2003

White Dwarf Sequences in Dense Star Clusters

We use the results of realistic N-body simulations to investigate the appearance of the white dwarf population in dense star clusters. We show that the presence of a substantial binary population in a star cluster, and the interaction of this population with the cluster environment, has serious consequences for the morphology of the observed white dwarf sequence and the derived white dwarf cooling age of the cluster. We find that over time the dynamical evolution of the cluster -- mass-segregation, stellar interactions and tidal stripping -- hampers the use of white dwarfs as tracers of the initial mass function, and also leads to a significant enhancement of the white dwarf mass fraction. Future observations of star clusters should be conducted slightly interior to the half-mass radius of the cluster in order to best obtain information about the cluster age and initial mass function from the white dwarf luminosity function. The evolution of binary stars and the cluster environment must necessarily be accounted for when studying the white dwarf populations of dynamically evolved star clusters.

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arXiv2003

Discovery of an ultra-cool subdwarf: LSR1425+7102, first star with spectral type sdM8.0

We report the discovery of the coolest subdwarf reported to date. The star LSR1425+7102 was discovered in our survey for faint high proper motion stars in the northern sky. Follow-up spectroscopy revealed the star to be a very red object with the characteristic signature of M subdwarfs: strong CaH bands but relatively weaker TiO bands. The CaH molecular band at $\approxλ$6900Å is particularly strong. By extrapolating the empirical relationship between the strength of the CaH molecular band and the subdwarf subtype, we conclude that LSR1425+7102 is the first star to be discovered with spectral type sdM8.0.

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arXiv2002

Spectroscopy of New High Proper Motion Stars in the Northern Sky. I. New Nearby Stars, New High Velocity Stars, and an Enhanced Classification Scheme for M Dwarfs

We define an enhanced spectral classification scheme for M dwarf stars, and use it to derive spectral classification of 104 northern stars with proper motions larger than 0.5"/yr which we discovered in a survey of high proper motion stars at low galactic latitudes. The final tally is as follows: 54 M dwarfs, 25 sdK and sdM subdwarfs, 14 esdK and esdM extreme subdwarfs, and 11 DA and DC white dwarfs. Among the most interesting cases, we find one star to be the coolest subdwarf ever reported (LSR2036+5059, with spectral type sdM7.5), a new M9.0 dwarf only about 6pc distant (LSR1835+3259), and a new M6.5 dwarf only 7pc from the Sun (LSR2124+4003). Spectroscopic distances suggests that 27 of the M dwarfs, 3 of the white dwarfs, and one of the subdwarfs (LSR2036+5059) are within 25pc of the Sun, making them excellent candidates for inclusion in the solar neighborhood census. Estimated sky-projected velocities suggest that most of our subdwarfs and extreme subdwarfs have halo kinematics. We find that several white dwarfs and non metal-poor M dwarfs also have kinematics consistent with the halo, and we briefly discuss their possible origin.

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arXiv2002

Discovery of an M8.5 Dwarf With Proper Motion mu=2.38 arcsec/yr

We report the discovery of LSR1826+3014, a very faint (V=19.36) star with a very large proper motion (mu=2.38 arcsec/yr). A low resolution red spectrum reveals that LSR1826+3014 is an ultra-cool red dwarf with spectral type M8.5 V and with a radial velocity v_rad=+77+/-10 km/s. LSR1826+3014 is thus the faintest red dwarf ever discovered with a proper motion larger than 2 arcsec/yr. Optical and infrared photometry suggest that the star is at a distance d=13.9+/-3.5 pc from the Sun, which implies it is moving relative to the local standard of rest with a total velocity of 175+/-25 km/s. Numerical integration of its orbit suggests that LSR1826+3014 is on a halo-like galactic orbit.

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arXiv2002

MODEST-1: Integrating Stellar Evolution and Stellar Dynamics

We summarize the main results from MODEST-1, the first workshop on MOdeling DEnse STellar systems. Our goal is to go beyond traditional population synthesis models, by introducing dynamical interactions between single stars, binaries, and multiple systems. The challenge is to define and develop a software framework to enable us to combine in one simulation existing computer codes in stellar evolution, stellar dynamics, and stellar hydrodynamics. With this objective, the workshop brought together experts in these three fields, as well as other interested astrophysicists and computer scientists. We report here our main conclusions, questions and suggestions for further steps toward integrating stellar evolution and stellar (hydro)dynamics.

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arXiv2002

New High Proper Motion Stars from the Digitized Sky Survey. I. Northern Stars with proper motion between 0.5 and 2.0 arcsec/yr at Low Galactic Latitudes

We have conducted a systematic search for high proper motion stars in the Digitized Sky Survey, in the area of the sky north of -2.8 degrees in declination and within 25 degrees of the galactic plane. Using the SUPERBLINK software, a powerful automated blink comparator developed by us, we have identified 601 stars in the magnitude range 9 2.0\arcsec/yr), and 5 were missed because they were either too bright for SUPERBLINK to handle or they are in the immediate proximity of very bright stars. Only one of Luyten's stars (LHS1657) could not be recovered at all, even by visual inspection of the POSS plates, and is now suspected to be bogus. The very high success rate in the recovery by SUPERBLINK of faint Luyten stars suggests that our new survey of high proper motion stars is at least 99 percent complete for stars with proper motions 0.5 to 2.0 arcsec/yr down to R=19. This paper includes a list of positions, proper motions, magnitudes, and finder charts for all the new high proper motion stars.

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arXiv2002

The Lower Main Sequence and Mass Function of the Globular Cluster Messier 4

The deepest optical image ever in a globular star cluster, a Hubble Space Telescope (HST) 123 orbit exposure in a single field of Messier 4, was obtained in 2 filters (F606W, F814W) over a 10 week period in early 2001. A somewhat shallower image obtained in 1995 allowed us to select out cluster and field objects via their proper motion displacement resulting in remarkably clean color-magnitude diagrams that reach to V = 30, I = 28. The cluster main sequence luminosity function contains very few stars fainter than MV = 15.0, MI = 11.8 which, in both filters, is more than 2 magnitudes brighter than our limit. This is about the faintest luminosity seen among field Population II subdwarfs of the same metallicity. However, there remains a sprinkling of potential cluster stars to lower luminosity all the way down to our limiting magnitudes. These latter objects are significantly redder than any known metal-poor field subdwarf. Comparison with the current generation of theoretical stellar models implies that the masses of the lowest luminosity cluster stars observed are near 0.09 Msun. We derive the mass function of the cluster in our field and find that it is very slowly rising towards the lowest masses with no convincing evidence of a turnover even below 0.1 Msun. The formal slope between 0.65 and 0.09 Msun is alpha = 0.75 (Salpeter of 2.35) with a 99% confidence interval 0.55 - 1.05. A consistency check between these slopes and the number of observed cluster white dwarfs yields a range of possible conclusions, one of which is that we have indeed seen the termination of the white dwarf cooling sequence in M4.

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arXiv2002

A Far-Ultraviolet Survey of 47 Tucanae. I. Imaging

We present results from the imaging portion of a far-ultraviolet (FUV) survey of the core of 47 Tucanae. We have detected 767 FUV sources, 527 of which have optical counterparts in archival HST/WFPC2 images of the same field. Most of our FUV sources are main-sequence (MS) turn-off stars near the detection limit of our survey. However, the FUV/optical color-magnitude diagram (CMD) also reveals 19 blue stragglers (BSs), 17 white dwarfs (WDs) and 16 cataclysmic variable (CV) candidates. The BSs lie on the extended cluster MS, and four of them are variable in the FUV data. The WDs occupy the top of the cluster cooling sequence, down to an effective temperature of T_{eff} \simeq 20,000 K. Our FUV source catalog probably contains many additional, cooler WDs without optical counterparts. Finally, the CV candidates are objects between the WD cooling track and the extended cluster MS. Four of the CV candidates are previously known or suspected cataclysmics. All of these are bright and variable in the FUV. Another CV candidate is associated with the semi-detached binary system V36 that was recently found by Albrow et al. (2001). V36 has an orbital period of 0.4 or 0.8 days, blue optical colors and is located within 1 arcsec of a Chandra x-ray source. A few of the remaining CV candidates may represent chance superpositions or SMC interlopers, but at least half are expected to be real cluster members with peculiar colors. However, only a few of these CV candidates are possible counterparts to Chandra x-ray sources. Thus it is not yet clear which, if any, of them are true CVs, rather than non-interacting MS/WD binaries or Helium WDs.

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arXiv2002

The White Dwarf Cooling Sequence of the Globular Cluster Messier 4

We present the white dwarf sequence of the globular cluster M4, based on a 123 orbit Hubble Space Telescope exposure, with limiting magnitude V = 30, I = 28. The white dwarf luminosity function rises sharply for I >25.5, consistent with the behaviour expected for a burst population. The white dwarfs of M4 extend to approximately 2.5 magnitudes fainter than the peak of the local Galactic disk white dwarf luminosity function. This demonstrates a clear and significant age difference between the Galactic disk and the halo globular cluster M4. Using the same standard white dwarf models (Hansen 1999) to fit each luminosity function yields ages of 7.3 +/- 1.5 Gyr for the disk and 12.7 +/- 0.7 Gyr for M4 (2-sigma statistical errors).

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arXiv2002

New distant companions to known nearby stars: I. GJ 4047B, GJ 718B, GJ 747.2C, GJ 4100B, and GJ 4153B

In an ongoing survey for high proper motion stars at low galactic latitudes, we have identified 5 previously uncatalogued common proper motion companions to stars listed in the Preliminary Version of the Third Catalogue of Nearby Stars (GJ stars). For each system, the relative proper motion between the components is less than 5% of the common proper motion of the pair. Spectra of the pairs have been obtained at the Lick 3-m Shane Telescope, confirming that the systems are indeed wide separation binaries. The systems are classified as follows: GJ 4047AB = K5 V + M5 V, GJ 718AB = K5 V + M4.5 V, GJ 747.2ABC = (K7 V + K7 V) + M4 Ve, GJ 4100AB = M1 V + M4.5 Ve, and GJ 4153AB = M0.5 V + M3.5 V. The total area surveyed contains 346 Gliese stars, which suggests that approximately 1.5% of the stars listed in the Preliminary Version of the Third Catalogue of Nearby Stars have unrecognized proper motion companions. We predict that 15-50 more new distant companions to GJ stars will be discovered in the Digitized Sky Survey.

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arXiv2002

The First Orbital Period for a Dwarf Nova in a Globular Cluster: V101 in M5

We report the first orbital period determination for a Dwarf Nova (DN) in a glubular cluster: V101 in M5 has a period of 5.796 +- 0.036 hours. We derived this period from I-band photometry acquired with the Calypso Observatory High Resolution Camera operating with tip-tilt adaptive optics correction. Observations from the South African Astronomical Observatory in the V-band were also analyzed and exhibit a periodic signal of the same period. This orbital period suggests that V101 has a secondary of mid to late K spectral type with Mv = +8.2 +- 0.5. The predicted spectral type is consistent with previous spectral observations in quiescence which show a fairly red continuum. From the observed minimum brightness of V = 22.5, we derive a distance modulus of (m - M)v = 14.3 +- 0.5 to the DN which supports V101's membership in the globular cluster M5. Measurement of the ellipsoidality effect indicates that the orbital plane of the V101 system is moderately inclined, but not enough to exhibit eclipses.

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