arXiv subjects

Mukremin Kilic

Publications and source records attributed to Mukremin Kilic.

At least 145 records · Page 8Linked to original sources

arXiv2010

Accurate Masses for the Primary and Secondary in the Eclipsing White Dwarf Binary NLTT 11748

We measure the radial velocity curve of the eclipsing detached white dwarf binary NLTT 11748. The primary exhibits velocity variations with a semi-amplitude of 273 km/s and an orbital period of 5.641 hr. We do not detect any spectral features from the secondary star, or any spectral changes during the secondary eclipse. We use our composite spectrum to constrain the temperature and surface gravity of the primary to be Teff = 8690 (140) K and log g = 6.54 (0.05), which correspond to a mass of 0.18 Msol. For an inclination angle of 89.9 deg. derived from the eclipse modeling, the mass function requires a 0.76 Msol companion. The merger time for the system is 7.2 Gyr. However, due to the extreme mass ratio of 0.24, the binary will most likely create an AM CVn system instead of a merger.

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arXiv2010

A Detailed Model Atmosphere Analysis of Cool White Dwarfs in the Sloan Digital Sky Survey

We present optical spectroscopy and near-infrared photometry of 126 cool white dwarfs in the Sloan Digital Sky Survey (SDSS). Our sample includes high proper motion targets selected using the SDSS and USNO-B astrometry and a dozen previously known ultracool white dwarf candidates. Our optical spectroscopic observations demonstrate that a clean selection of large samples of cool white dwarfs in the SDSS (and the SkyMapper, Pan-STARRS, and the Large Synoptic Survey Telescope datasets) is possible using a reduced proper motion diagram and a tangential velocity cut-off (depending on the proper motion accuracy) of 30 km/s. Our near-infrared observations reveal eight new stars with significant absorption. We use the optical and near-infrared photometry to perform a detailed model atmosphere analysis. More than 80% of the stars in our sample are consistent with either pure hydrogen or pure helium atmospheres. However, the eight stars with significant infrared absorption and the majority of the previously known ultracool white dwarf candidates are best explained with mixed hydrogen and helium atmosphere models. The age distribution of our sample is consistent with a Galactic disk age of 8 Gyr. A few ultracool white dwarfs may be as old as 12-13 Gyr, but our models have problems matching the spectral energy distributions of these objects. There are only two halo white dwarf candidates in our sample. However, trigonometric parallax observations are required for accurate mass and age determinations and to confirm their membership in the halo.

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arXiv2010

The Discovery of Binary White Dwarfs that will Merge within 500 Myr

We present radial velocity observations of four extremely low-mass (0.2 Msol) white dwarfs. All four stars show peak-to-peak radial velocity variations of 540 - 710 km/s with 1.0 - 5.9 hr periods. The optical photometry rules out main-sequence companions. In addition, no milli-second pulsar companions are detected in radio observations. Thus the invisible companions are most likely white dwarfs. Two of the systems are the shortest period binary white dwarfs yet discovered. Due to the loss of angular momentum through gravitational radiation, three of the systems will merge within 500 Myr. The remaining system will merge within a Hubble time. The mass functions for three of the systems imply companions more massive than 0.46 Msol; thus those are carbon/oxygen core white dwarfs. The unknown inclination angles prohibit a definitive conclusion about the future of these systems. However, the chance of a supernova Ia event is only 1% to 5%. These systems are likely to form single R Coronae Borealis stars, providing evidence for a white dwarf + white dwarf merger mechanism for these unusual objects. One of the systems, SDSS J105353.89+520031.0 has a 70% chance of having a low-mass white dwarf companion. This system will probably form a single helium-enriched subdwarf O star. All four white dwarf systems have unusal mass ratios of < 0.2-0.8 that may also lead to the formation of AM CVn systems.

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arXiv2010

Visitors from the Halo: 11 Gyr old White Dwarfs in the Solar Neighborhood

We report the discovery of three nearby old halo white dwarf candidates in the Sloan Digital Sky Survey (SDSS), including two stars in a common proper motion binary system. These candidates are selected from our 2800 square degree proper motion survey on the Bok and U.S. Naval Observatory Flagstaff Station 1.3m telescopes, and they display proper motions of 0.4-0.5 arcsec/yr. Follow-up MMT spectroscopy and near-infrared photometry demonstrate that all three objects are hydrogen-dominated atmosphere white dwarfs with Teff = 3700 - 4100 K. For average mass white dwarfs, these temperature estimates correspond to cooling ages of 9-10 Gyr, distances of 70-80 pc, and tangential velocities of 140-200 km/s. Based on the UVW space velocities, we conclude that they most likely belong to the halo. Furthermore, the combined main-sequence and white dwarf cooling ages are 10-11 Gyr. Along with SDSS J1102+4113, they are the oldest field white dwarfs currently known. These three stars represent only a small fraction of the halo white dwarf candidates in our proper motion survey, and they demonstrate that deep imaging surveys like the Pan-STARRS and Large Synoptic Survey Telescope should find many old thick disk and halo white dwarfs that can be used to constrain the age of the Galactic thick disk and halo.

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arXiv2009

A Spitzer Search for Substellar Companions to Low Mass White Dwarfs

The formation scenarios for single low-mass (M < 0.45 Msol) white dwarfs include enhanced mass loss from a metal-rich progenitor star or a common envelope phase of a solar-like star with a close-in massive planet or a brown dwarf. Both scenarios suggest that low-mass white dwarfs may have planets. Here, we present a Spitzer IRAC search for substellar and planetary mass companions to 14 low-mass white dwarfs. One of our targets, HS 1653+7753, displays near- and mid-infrared flux excess. However, follow-up MMT observations show that this excess is due to a nearby resolved source, which is mostly likely a background object. Another target, PG 2257+162, shows flux excess compatible with a late-type stellar companion. We do not detect substellar companions to any of the remaining targets. In addition, eight of these stars do not show any radial velocity variations, ruling out stellar mass companions including other white dwarfs. We conclude that a significant fraction of the low-mass white dwarfs in our sample do not have stellar or massive brown dwarf companions.

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arXiv2009

Limits on Unresolved Planetary Companions to White Dwarf Remnants of 14 Intermediate-Mass Stars

We present Spitzer IRAC photometry of white dwarf remnants of 14 stars with M = 3-5 Msol. We do not detect mid-infrared excess around any of our targets. By demanding a 3 sigma photometric excess at 4.5 micron for unresolved companions, we rule out planetary mass companions down to 5, 7, or 10 M_J for 13 of our targets based on the Burrows et al. (2003) substellar cooling models. Combined with previous IRAC observations of white dwarf remnants of intermediate-mass stars, we rule out \geq 10 M_J companions around 40 white dwarfs and \geq 5 M_J companions around 10 white dwarfs.

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arXiv2009

No Neutron Star Companion To The Lowest Mass SDSS White Dwarf

SDSS J091709.55+463821.8 (hereafter J0917+4638) is the lowest surface gravity white dwarf (WD) currently known, with log g = 5.55 +/- 0.05 (M ~ 0.17 M_sun; Kilic et al. 2007a,b). Such low-mass white dwarfs (LMWDs) are believed to originate in binaries that evolve into WD/WD or WD/neutron star (NS) systems. An optical search for J0917+4638's companion showed that it must be a compact object with a mass >= 0.28 M_sun (Kilic 2007b). Here we report on Green Bank Telescope 820 MHz and XMM-Newton X-ray observations of J0917+4638 intended to uncover a potential NS companion to the LMWD. No convincing pulsar signal is detected in our radio data. Our X-ray observation also failed to detect X-ray emission from J0917+4638's companion, while we would have detected any of the millisecond radio pulsars in 47 Tuc. We conclude that the companion is almost certainly another WD.

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arXiv2009

The Runaway White Dwarf LP400-22 Has a Companion

We report the detection of a radial velocity companion to the extremely low mass white dwarf LP400-22. The radial velocity of the white dwarf shows variations with a semi-amplitude of 119 km/s and a 0.98776 day period, which implies a companion mass of M > 0.37 Msun. The optical photometry rules out a main sequence companion. Thus the invisible companion is another white dwarf or a neutron star. Using proper motion measurements and the radial velocity of the binary system, we find that it has an unusual Galactic orbit. LP400-22 is moving away from the Galactic center with a velocity of 396 km/s, which is very difficult to explain by supernova runaway ejection mechanisms. Dynamical interactions with a massive black hole like that in the Galactic center can in principle explain its peculiar velocity, if the progenitor was a triple star system comprised of a close binary and a distant tertiary companion. Until better proper motions become available, we consider LP400-22 to be most likely a halo star with a very unusual orbit.

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arXiv2009

Spitzer Observations of the Oldest White Dwarfs in the Solar Neighborhood

We present Spitzer 5-15 micron spectroscopy of one cool white dwarf and 3.6-8 micron photometry of 51 cool white dwarfs with T_eff < 6000 K. The majority of our targets have accurate BVRIJHK photometry and trigonometric parallax measurements available, which enables us to perform a detailed model atmosphere analysis using their optical, near- and mid-infrared photometry with state- of-the-art model atmospheres. We demonstrate that the optical and infrared spectral energy distributions of cool white dwarfs are well reproduced by our grid of models. Our best fit models are consistent with the observations within 5% in all filters except the IRAC 8 micron band, which has the lowest signal- to-noise ratio photometry. Excluding the ultracool white dwarfs, none of the stars in our sample show significant mid-infrared flux deficits or excesses. The non-detection of mid-infrared excess flux around our 2-9 Gyr old targets constrain the fraction of cool white dwarfs with warm debris disks to 0.8% (+1.5% -0.8%).

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arXiv2008

Direct Distance Measurement to the Dusty White Dwarf GD 362

We present trigonometric parallax observations of GD 362 obtained over seven epochs using the MDM 2.4m Hiltner Telescope. The existence of a dust disk around this possibly massive white dwarf makes it an interesting target for parallax observations. The measured parallax for GD 362 places it at a distance of 50.6 pc, which implies that its radius and mass are ~ 0.0106 Rsun and 0.71 Msun, respectively. GD 362 is not as massive as initially thought (1.2Msun). Our results are entirely consistent with the distance and mass estimates (52.2 pc and 0.73 Msun) by Zuckerman et al., who demonstrated that GD 362 has a helium dominated atmosphere. Dropping GD 362 from the list of massive white dwarfs, there are no white dwarfs with M > 0.9 Msun that are known to host circumstellar dust disks.

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arXiv2008

Near-Infrared Constraints on the Presence of Warm Dust at Metal-Rich, Helium Atmosphere White Dwarfs

Here, we present near-infrared spectroscopic observations of 15 helium atmosphere, metal-rich white dwarfs obtained at the NASA Infrared Telescope Facility. While a connection has been demonstrated between the most highly polluted, hydrogen atmosphere white dwarfs and the presence of warm circumstellar dust and gas, their frequency at the helium atmosphere variety is poorly constrained. None of our targets show excess near-infrared radiation consistent with warm orbiting material. Adding these near-infrared constraints to previous near- and mid-infrared observations, the frequency of warm circumstellar material at metal-bearing white dwarfs is at least 20% for hydrogen-dominated photospheres, but could be less than 5% for those effectively composed of helium alone. The lower occurrence of dust disks around helium atmosphere white dwarfs is consistent with Myr timescales for photospheric metals in massive convection zones. Analyzing the mass distribution of 10 white dwarfs with warm circumstellar material, we search for similar trends between the frequency of disks and the predicted frequency of massive planets around intermediate mass stars, but find the probability that disk-bearing white dwarfs are more massive than average is not significant.

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arXiv2008

A Nearby Old Halo White Dwarf Candidate from the Sloan Digital Sky Survey

We report the discovery of a nearby, old, halo white dwarf candidate from the Sloan Digital Sky Survey. SDSS J110217.48+411315.4 has a proper motion of 1.75 arcsec/year and redder optical colors than all other known featureless (type DC) white dwarfs. We present SDSS imaging and spectroscopy of this object, along with near-infrared photometry obtained at the United Kingdom Infra-Red Telescope. Fitting its photometry with up-to-date model atmospheres, we find that its overall spectral energy distribution is fit reasonably well with a pure hydrogen composition and T_eff~3800 K (assuming log g=8). That temperature and gravity would place this white dwarf at 35 pc from the Sun with a tangential velocity of 290 km/s and space velocities consistent with halo membership; furthermore, its combined main sequence and white dwarf cooling age would be ~11 Gyr. However, if this object is a massive white dwarf, it could be a younger object with a thick disk origin. Whatever its origin, the optical colors of this object are redder than predicted by any current pure hydrogen, pure helium or mixed hydrogen-helium atmospheric model, indicating that there remain problems in our understanding of the complicated physics of the dense atmospheres of cool white dwarfs.

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arXiv2007

The First Mid-Infrared Spectra of Cool White Dwarfs

We present the first mid-infrared spectra of two cool white dwarfs obtained with the Spitzer Space Telescope. We also present 3.5-8 micron photometry for 19 cool white dwarfs with 5000K < Teff < 9000K. We perform a detailed model atmosphere analysis of these white dwarfs by fitting their UBVRIJHK and Spitzer photometry with state-of-the-art model atmospheres, and demonstrate that the optical and infrared spectral energy distributions of cool white dwarfs are well reproduced by our grid of models. Our mid-IR photometry and 7.5-14.5 micron spectrum of WD0018-267 are consistent with a Teff = 5720K, pure hydrogen white dwarf model atmosphere. On the other hand, LHS 1126 remains peculiar with significant mid-IR flux deficits in all IRAC bands and a featureless spectrum in the 5.2-7.5 micron range. Even though this deficit is attributed to collision induced absorption (CIA) due to molecular hydrogen, the shape of the deficit cannot be explained with current CIA opacity calculations. The infrared portion of the LHS 1126 spectral energy distribution is best-fit with a power law index of -1.99; identical to a Rayleigh-Jeans spectrum. This argues that the deficit may be due to an unrecognized grey-like opacity source in the infrared.

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arXiv2007

The Future is Now: the Formation of Single Low Mass White Dwarfs in the Solar Neighborhood

Low mass helium-core white dwarfs (M < 0.45 Msun) can be produced from interacting binary systems, and traditionally all of them have been attributed to this channel. However, a low mass white dwarf could also result from a single star that experiences severe mass loss on the first ascent giant branch. A large population of low mass He-core white dwarfs has been discovered in the old metal-rich cluster NGC 6791. There is therefore a mechanism in clusters to produce low mass white dwarfs without requiring binary star interactions, and we search for evidence of a similar population in field white dwarfs. We argue that there is a significant field population (of order half of the detected systems) that arises from old metal rich stars which truncate their evolution prior to the helium flash from severe mass loss. There is a consistent absence of evidence for nearby companions in a large fraction of low mass white dwarfs. The number of old metal-rich field dwarfs is also comparable with the apparently single low mass white dwarf population, and our revised estimate for the space density of low mass white dwarfs produced from binary interactions is also compatible with theoretical expectations. This indicates that this channel of stellar evolution, hitherto thought hypothetical only, has been in operation in our own Galaxy for many billions of years. One strong implication of our model is that single low mass white dwarfs should be good targets for planet searches because they are likely to arise from metal-rich progenitors. We also discuss other observational tests and implications, including the potential impact on SN Ia rates and the frequency of planetary nebulae.

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arXiv2007

Finding Planets Around White Dwarf Remnants of Massive Stars

Planet frequency shows a strong positive correlation with host mass from the hydrogen-burning limit to M ~ 2Msun. No search has yet been conducted for planets of higher-mass hosts because all existing techniques are insensitive to these planets. We show that infrared observations of the white-dwarf (WD) remnants of massive stars 3Msun < M < 7Msun would be sensitive to these planets for reasons that are closely connected to the insensitivity of other methods. We identify 49 reasonably bright, young, massive WDs from the Palomar-Green survey and discuss methods for detecting planets and for distinguishing between planet and disk explanations for any excess flux observed. The young, bright, massive WD sample could be expanded by a factor 4-5 by surveying the remainder of the sky for bright UV-excess objects.

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arXiv2007

The Discovery of a Companion to the Lowest Mass White Dwarf

We report the detection of a radial velocity companion to SDSS J091709.55+463821.8, the lowest mass white dwarf currently known with M~0.17Msun. The radial velocity of the white dwarf shows variations with a semi-amplitude of 148.8 km/s and a period of 7.5936 hours, which implies a companion mass of M > 0.28Msun. The lack of evidence of a companion in the optical photometry forces any main-sequence companion to be smaller than 0.1Msun, hence a low mass main sequence star companion is ruled out for this system. The companion is most likely another white dwarf, and we present tentative evidence for an evolutionary scenario which could have produced it. However, a neutron star companion cannot be ruled out and follow-up radio observations are required to search for a pulsar companion.

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arXiv2007

The New Class of Dusty DAZ White Dwarfs

Our mid-infrared survey of 124 white dwarfs with the Spitzer Space Telescope and the IRAC imager has revealed an infrared excess associated with the white dwarf WD 2115-560 naturally explained by circumstellar dust. This object is the fourth white dwarf observed to have circumstellar dust. All four are DAZ white dwarfs, i.e. they have both photospheric Balmer lines and photospheric metal lines. We discuss these four objects as a class, which we abbreviate "DAZd", where the "d" stands for "dust". Using an optically-thick, geometrically-thin disk model analogous to Saturn's rings, we find that the inner disk edges are at >~0.1 to 0.2 Ro and that the outer disk edges are ~0.3 to 0.6 Ro. This model naturally explains the accretion rates and lifetimes of the detected WD disks and the accretion rates inferred from photospheric metal abundances.

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arXiv2007

The Lowest Mass White Dwarf

Extremely low mass white dwarfs are very rare objects likely formed in compact binary systems. We present MMT optical spectroscopy of 42 low mass white dwarf candidates serendipitously discovered in a survey for hypervelocity B-type stars. One of these objects, SDSS J0917+46, has Teff= 11,288 \pm 72 K and log g = 5.48 \pm 0.03; with an estimated mass of 0.17 M_sun, it is the lowest gravity/mass white dwarf currently known. However, 40 of the low mass candidates are normal DA white dwarfs with apparently inaccurate SDSS g magnitudes. We revisit the identification of low mass white dwarf candidates previously found in the SDSS, and conclude that four objects have M < 0.2 M_sun. None of these white dwarfs show excess emission from a binary companion, and radial velocity searches will be necessary to constrain the nature of the unseen companions.

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