arXiv subjects

Mukremin Kilic

Publications and source records attributed to Mukremin Kilic.

At least 55 records · Page 3Linked to original sources

arXiv2023

Asteroseismological analysis of the polluted ZZ Ceti star G29-38 with TESS

G\,29$-$38 (TIC~422526868) is one of the brightest ($V=13.1$) and closest ($d = 17.51$\,pc) pulsating white dwarfs with a hydrogen-rich atmosphere (DAV/ZZ Ceti class). It was observed by the {\sl TESS} spacecraft in sectors 42 and 56. The atmosphere of G~29$-$38 is polluted by heavy elements that are expected to sink out of visible layers on short timescales. The photometric {\sl TESS} data set spans $\sim 51$ days in total, and from this, we identified 56 significant pulsation frequencies, that include rotational frequency multiplets. In addition, we identified 30 combination frequencies in each sector. The oscillation frequencies that we found are associated with $g$-mode pulsations, with periods spanning from $\sim$ 260 s to $\sim$ 1400 s. We identified %three distinct rotational frequency triplets with a mean separation $δν_{\ell=1}$ of 4.67 $μ$Hz and a quintuplet with a mean separation $δν_{\ell=2}$ of 6.67 $μ$Hz, from which we estimated a rotation period of about $1.35 \pm 0.1$ days. We determined a constant period spacing of 41.20~s for $\ell= 1$ modes and 22.58\,s for $\ell= 2$ modes. We performed period-to-period fit analyses and found an asteroseismological model with $M_{\star}/M_{\odot}=0.632 \pm 0.03$, $T_{\rm eff}=11\, 635\pm 178$ K, and $\log{g}=8.048\pm0.005$ (with a hydrogen envelope mass of $M_{\rm H}\sim 5.6\times 10^{-5}M_{\star}$), in good agreement with the values derived from spectroscopy. We obtained an asteroseismic distance of 17.54 pc, which is in excellent agreement with that provided by {\sl Gaia} (17.51 pc).

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arXiv2023

The ubiquity of carbon dredge-up in hydrogen-deficient white dwarfs as revealed by GALEX

The convective dredge-up of carbon from the interiors of hydrogen-deficient white dwarfs has long been invoked to explain the presence of carbon absorption features in the spectra of cool DQ stars ($T_{\rm eff} < 10{,}000\,{\rm K}$). It has been hypothesized that this transport process is not limited to DQ white dwarfs and also operates, albeit less efficiently, in non-DQ hydrogen-deficient white dwarfs within the same temperature range. This non-DQ population is predominantly composed of DC white dwarfs, which exhibit featureless optical spectra. However, no direct observational evidence of ubiquitous carbon pollution in DC stars has thus far been uncovered. In this Letter, we analyze data from the Galaxy Evolution Explorer (GALEX) to reveal the photometric signature of ultraviolet carbon lines in most DC white dwarfs in the $8500\,{\rm K} \leq T_{\rm eff} \leq 10{,}500\,{\rm K}$ temperature range. Our results show that the vast majority of hydrogen-deficient white dwarfs experience carbon dredge-up at some point in their evolution.

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arXiv2023

General relativistic pulsations of ultra-massive ZZ Ceti stars

Ultra-massive white dwarf stars are currently being discovered at a considerable rate, thanks to surveys such as the {\it Gaia} space mission. These dense and compact stellar remnants likely play a major role in type Ia supernova explosions. It is possible to probe the interiors of ultra-massive white dwarfs through asteroseismology. In the case of the most massive white dwarfs, General Relativity could affect their structure and pulsations substantially. In this work, we present results of relativistic pulsation calculations employing relativistic ultra-massive ONe-core white dwarf models with hydrogen-rich atmospheres and masses ranging from $1.29$ to $1.369 M_{\odot}$ with the aim of assessing the impact of General Relativity on the adiabatic gravity ($g$)-mode period spectrum of very-high mass ZZ Ceti stars. Employing the relativistic Cowling approximation for the pulsation analysis, we find that the critical buoyancy (Brunt-Väisälä) and acoustic (Lamb) frequencies are larger for the relativistic case, compared to the Newtonian case, due to the relativistic white dwarf models having smaller radii and higher gravities for a fixed stellar mass. In addition, the $g$-mode periods are shorter in the relativistic case than in the Newtonian computations, with relative differences of up to $\sim 50$ \% for the highest-mass models ($1.369 M_{\odot}$) and for effective temperatures typical of the ZZ Ceti instability strip. Hence, the effects of General Relativity on the structure, evolution, and pulsations of white dwarfs with masses larger than $\sim 1.29 M_{\odot}$ cannot be ignored in the asteroseismological analysis of ultra-massive ZZ Ceti stars.

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arXiv2023

Signs of Binary Evolution in 7 Magnetic DA White Dwarfs

We present our findings on the spectral analysis of seven magnetic white dwarfs that were presumed to be double degenerates. We obtained time-resolved spectroscopy at the Gemini Observatory to look for evidence of binarity or fast rotation. We find three of our targets have rotation periods of less than an hour based on the shifting positions of the Zeeman-split H$α$ components: 13, 35, and 39 min, and we find one more target with a ~hour long period that is currently unconstrained. We use offset dipole models to determine the inclination, magnetic field strength, and dipole offset of each target. The average surface field strengths of our fast rotators vary by 1-2 MG between different spectra. In all cases, the observed absorption features are too shallow compared to our models. This could be due to extra flux from a companion for our three low-mass targets, but the majority of our sample likely requires an inhomogeneous surface composition. Including an additional magnetic white dwarf with similar properties presented in the literature, we find that 5 of the 8 targets in this sample show field variations on minute/hour timescales. A crystallization driven dynamo can potentially explain the magnetic fields in three of our targets with masses above $0.7~M_{\odot}$ but another mechanism is still needed to explain their rapid rotation. We suggest that rapid rotation or low-masses point to binary evolution as the likely source of magnetism in 7 of these 8 targets.

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arXiv2023

A GALEX view of the DA White Dwarf Population

We present a detailed model atmosphere analysis of 14001 DA white dwarfs from the Montreal White Dwarf Database with ultraviolet photometry from the GALEX mission. We use the 100 pc sample, where the extinction is negligible, to demonstrate that there are no major systematic differences between the best-fit parameters derived from optical only data and the optical + UV photometry. GALEX FUV and NUV data improve the statistical errors in the model fits, especially for the hotter white dwarfs with spectral energy distributions that peak in the UV. Fitting the UV to optical spectral energy distributions also reveals UV-excess or UV-deficit objects. We use two different methods to identify outliers in our model fits. Known outliers include objects with unusual atmospheric compositions, strongly magnetic white dwarfs, and binary white dwarfs, including double degenerates and white dwarf + main-sequence systems. We present a list of 89 newly identified outliers based on GALEX UV data; follow-up observations of these objects will be required to constrain their nature. Several current and upcoming large scale spectroscopic surveys are targeting $>10^5$ white dwarfs. In addition, the ULTRASAT mission is planning an all-sky survey in the NUV band. A combination of the UV data from GALEX and ULTRASAT and optical data on these large samples of spectroscopically confirmed DA white dwarfs will provide an excellent opportunity to identify unusual white dwarfs in the solar neighborhood.

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arXiv2023

The ELM Survey South. II. Two dozen new low mass white dwarf binaries

We present the results from our ongoing spectroscopic survey targeting low mass white dwarf binaries, focusing on the southern sky. We used a Gaia DR2 and eDR3 based selection and identified 28 new binaries, including 19 new extremely low mass white dwarfs, one short period, likely eclipsing, DABZ, and two potential LISA binaries. We present orbital and atmospheric parameters for each new binary based on our spectroscopic follow-up. Four of our new binaries show periodic photometric variability in the TESS 2-minute cadence data, including one new eclipsing double-lined spectroscopic binary. Three others show periodic photometric variability in ZTF, including one new eclipsing binary. We provide estimates for the inclinations and scaled component radii for these ZTF variables, based on light curve modeling to our high-speed photometric follow-up observations. Our observations have increased the sample of ELM Survey binaries identified in the southern sky to 41, an increase of 64%. Future time domain surveys, such as BlackGEM and the Vera C. Rubin Observatory Legacy Survey of Space and Time, will efficiently identify the photometric variables in the southern sky and significantly increase the population of southern sky low mass white dwarf binaries, leading to a more complete all-sky population of these systems.

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arXiv2023

WD J004917.14$-$252556.81, the Most Massive Pulsating White Dwarf

We present APO and Gemini time-series photometry of WD J004917.14$-$252556.81, an ultramassive DA white dwarf with $T_{\rm eff} = 13020$ K and $\log{g} = 9.34$. We detect variability at two significant frequencies, making J0049$-$2525 the most massive pulsating white dwarf currently known with $M_\star=1.31~M_{\odot}$ (for a CO core) or $1.26~M_{\odot}$ (for an ONe core). J0049$-$2525 does not display any of the signatures of binary mergers, there is no evidence of magnetism, large tangential velocity, or rapid rotation. Hence, it likely formed through single star evolution and is likely to have an ONe core. Evolutionary models indicate that its interior is $\gtrsim99$% crystallized. Asteroseismology offers an unprecedented opportunity to probe its interior structure. However, the relatively few pulsation modes detected limit our ability to obtain robust seismic solutions. Instead, we provide several representative solutions that could explain the observed properties of this star. Extensive follow-up time-series photometry of this unique target has the potential to discover a significant number of additional pulsation modes that would help overcome the degeneracies in the asteroseismic fits, and enable us to probe the interior of an $\approx1.3~M_{\odot}$ crystallized white dwarf.

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arXiv2022

The Merger Fraction of Ultramassive White Dwarfs

We search for merger products among the 25 most massive white dwarfs in the Montreal White Dwarf Database 100 pc sample through follow-up spectroscopy and high-cadence photometry. We find an unusually high fraction, 40%, of magnetic white dwarfs among this population. In addition, we identify four outliers in transverse velocity and detect rapid rotation in five objects. Our results show that $56^{+9}_{-10}$\% of the $M\approx1.3~M_{\odot}$ ultramassive white dwarfs form through mergers. This fraction is significantly higher than expected from the default binary population synthesis calculations using the $α$-prescription (with $αλ= 2$), and provides further support for efficient orbital shrinkage, such as with low values of the common envelope efficiency.

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arXiv2022

The ELM Survey. IX. A Complete Sample of Low Mass White Dwarf Binaries in the SDSS Footprint

We present the discovery of 17 double white dwarf (WD) binaries from our on-going search for extremely low mass (ELM) <0.3 Msun WDs, objects that form from binary evolution. Gaia parallax provides a new means of target selection that we use to evaluate our original ELM Survey selection criteria. Cross-matching the Gaia and Sloan Digital Sky Survey (SDSS) catalogs, we identify an additional 36 ELM WD candidates with 17<g<19 mag and within the 3-sigma uncertainties of our original color selection. The resulting discoveries imply the ELM Survey sample was 90% complete in the color range -0.4 < (g-r)_0 < -0.1 mag (approximately 9,000 K < Teff < 22,000 K). Our observations complete the sample in the SDSS footprint. Two newly discovered binaries, J123950.370-204142.28 and J232208.733+210352.81, have orbital periods of 22.5 min and 32 min, respectively, and are future LISA gravitational wave sources.

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arXiv2022

On the Nature of Ultracool White Dwarfs: Not so Cool Afterall

A recent analysis of the 100 pc white dwarf sample in the SDSS footprint demonstrated for the first time the existence of a well defined ultracool -- or IR-faint -- white dwarf sequence in the Hertzsprung-Russell diagram. Here we take advantage of this discovery to enlarge the IR-faint white dwarf sample threefold. We expand our selection to the entire Pan-STARRS survey footprint as well as the Montreal White Dwarf Database 100 pc sample, and identify 37 candidates with strong flux deficits in the optical. We present follow-up Gemini optical spectroscopy of 30 of these systems, and confirm all of them as IR-faint white dwarfs. We identify an additional set of 33 objects as candidates based on their colors and magnitudes. We present a detailed model atmosphere analysis of all 70 newly identified IR-faint white dwarfs together with 35 previously known objects reported in the literature. We discuss the physics of model atmospheres and show that the key physical ingredient missing in our previous generation of model atmospheres was the high-density correction to the He-minus free-free absorption coefficient. With new model atmospheres calculated for the purpose of this analysis, we now obtain significantly higher effective temperatures and larger stellar masses for these IR-faint white dwarfs than the Teff and M values reported in previous analyses, thus solving a two decade old problem. In particular, we identify in our sample a group of ultramassive white dwarfs in the Debye cooling phase with stellar parameters never measured before.

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arXiv2021

An Isolated White Dwarf with a 70 second Spin Period

We report the discovery of an isolated white dwarf with a spin period of 70 s. We obtained high speed photometry of three ultramassive white dwarfs within 100 pc, and discovered significant variability in one. SDSS J221141.80+113604.4 is a $1.27~M_{\odot}$ (assuming a CO core) magnetic white dwarf that shows 2.9\% brightness variations in the BG40 filter with a $70.32 \pm 0.04$ s period, becoming the fastest spinning isolated white dwarf currently known. A detailed model atmosphere analysis shows that it has a mixed hydrogen and helium atmosphere with a dipole field strength of $B_d = 15$ MG. Given its large mass, fast rotation, strong magnetic field, unusual atmospheric composition, and relatively large tangential velocity for its cooling age, J2211+1136 displays all of the signatures of a double white dwarf merger remnant. Long term monitoring of the spin evolution of J2211+1136 and other fast spinning isolated white dwarfs opens a new discovery space for substellar and planetary mass companions around white dwarfs. In addition, the discovery of such fast rotators outside of the ZZ Ceti instability strip suggests that some should also exist within the strip. Hence, some of the mono-periodic variables found within the instability strip may be fast spinning white dwarfs impersonating ZZ Ceti pulsators.

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arXiv2021

The Discovery of Two LISA Sources within 0.5 kpc

We report the discovery of the brightest detached binary white dwarfs with periods less than an hour, which provide two new gravitational wave verification binaries for the Laser Interferometer Space Antenna (LISA). The first one, SMSS J033816.16$-$813929.9 (hereafter J0338), is a 30.6 min orbital period, $g=17.2$ mag detached double white dwarf binary with a Gaia parallax measurement that places it at a distance of 533 pc. The observed radial velocity and photometric variability provide precise constraints on the system parameters. J0338 contains a $0.230 \pm 0.015~M_{\odot}$ white dwarf with a $0.38_{-0.03}^{+0.05}~M_{\odot}$ companion at an inclination of $69 \pm 9^{\circ}$. The second system, SDSS J063449.92+380352.2 (hereafter J0634), is a 26.5 min orbital period, $g=17.0$ mag detached double white dwarf binary at a distance of 435 pc. J0634 contains a $0.452^{+0.070}_{-0.062}~M_{\odot}$ white dwarf with a $0.209^{+0.034}_{-0.021}~M_{\odot}$ companion at an inclination of $37 \pm 7^{\circ}$. The more massive white dwarf in J0634 is hotter than its companion, even though tidal dissipation is predicted to be relatively inefficient at such periods. This suggests that the more massive white dwarf formed last. J0338 and J0634 will be detected by LISA with a signal-to-noise ratio of 5 and 19, respectively, after four years. We identified these two systems based on their overluminosity and $u$-band photometry. Follow-up of $u$-band selected Gaia targets will likely yield additional LISA verification binaries.

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arXiv2021

The Most Massive White Dwarfs in the Solar Neighborhood

We present an analysis of the most massive white dwarf candidates in the Montreal White Dwarf Database 100 pc sample. We identify 25 objects that would be more massive than $1.3~M_{\odot}$ if they had pure H atmospheres and CO cores, including two outliers with unusually high photometric mass estimates near the Chandrasekhar limit. We provide follow-up spectroscopy of these two white dwarfs and show that they are indeed significantly below this limit. We expand our model calculations for CO core white dwarfs up to $M=1.334\ M_\odot$, which corresponds to the high-density limit of our equation-of-state tables, $ρ= 10^9$ g cm$^{-3}$. We find many objects close to this maximum mass of our CO core models. A significant fraction of ultramassive white dwarfs are predicted to form through binary mergers. Merger populations can reveal themselves through their kinematics, magnetism, or rapid rotation rates. We identify four outliers in transverse velocity, four likely magnetic white dwarfs (one of which is also an outlier in transverse velocity), and one with rapid rotation, indicating that at least 8 of the 25 ultramassive white dwarfs in our sample are likely merger products.

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arXiv2021

The White Dwarfs of the Old, Solar Metallicity Open Star Cluster Messier 67: Properties and Progenitors

The old, solar metallicity open cluster Messier 67 has long been considered a lynchpin in the study and understanding of the structure and evolution of solar-type stars. The same is arguably true for stellar remnants - the white dwarf population of M67 provides crucial observational data for understanding and interpreting white dwarf populations and evolution. In this work, we determine the white dwarf masses and derive their progenitor star masses using high signal-to-noise spectroscopy of warm ($\gtrsim10,000$ K) DA white dwarfs in the cluster. From this we are able to derive each white dwarf's position on the initial-final mass relation, with an average $M_{\mathrm WD} = 0.60\pm 0.01 M_{\odot}$ and progenitor mass $M_i = 1.52\pm 0.04 M_{\odot}$. These values are fully consistent with recently published linear and piecewise linear fits to the semi-empirical initial-final mass relation and provide a crucial, precise anchor point for the initial-final mass relation for solar-metallicity, low-mass stars. The mean mass of M67 white dwarfs is also consistent with the sharp narrow peak in the local field white dwarf mass distribution, indicating that a majority of recently-formed field white dwarfs come from stars with progenitor masses of $\approx 1.5 M_{\odot}$. Our results enable more precise modeling of the Galactic star formation rate encoded in the field WD mass distribution.

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arXiv2021

Hidden in Plain Sight: A Double-lined White Dwarf Binary 26 pc Away and a Distant Cousin

We present high-resolution spectroscopy of two nearby white dwarfs with inconsistent spectroscopic and parallax distances. The first one, PG 1632+177, is a 13th magnitude white dwarf only 25.6 pc away. Previous spectroscopic observations failed to detect any radial velocity changes in this star. Here, we show that PG 1632+177 is a 2.05 d period double-lined spectroscopic binary (SB2) containing a low-mass He-core white dwarf with a more-massive, likely CO-core white dwarf companion. After L 870-2, PG 1632+177 becomes the second closest SB2 white dwarf currently known. Our second target, WD 1534+503, is also an SB2 system with an orbital period of 0.71 d. For each system, we constrain the atmospheric parameters of both components through a composite model-atmosphere analysis. We also present a new set of NLTE synthetic spectra appropriate for modeling high-resolution observations of cool white dwarfs, and show that NLTE effects in the core of the H$α$ line increase with decreasing effective temperature. We discuss the orbital period and mass distribution of SB2 and eclipsing double white dwarfs with orbital constraints, and demonstrate that the observed population is consistent with the predicted period distribution from the binary population synthesis models. The latter predict more massive CO + CO white dwarf binaries at short ($<1$ d) periods, as well as binaries with several day orbital periods; such systems are still waiting to be discovered in large numbers.

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arXiv2020

Multi-band light curve analysis of the 40.5-minute period eclipsing double-degenerate binary SDSS J082239.54$+$304857.19

We present the Apache Point Observatory BG40 broadband and simultaneous Gemini $r$-band and $i$-band high-speed follow-up photometry observations and analysis of the 40.5 minute period eclipsing detached double-degenerate binary SDSS J082239.54$+$304857.19. Our APO data spans over 318 days and includes 13 primary eclipses, from which we precisely measure the system's orbital period and improve the time of mid-eclipse measurement. We fit the light curves for each filter individually and show that this system contains a low-mass DA white dwarf with radius $R_A=0.031\pm0.006~{\rm R_\odot}$ and a $R_B=0.013\pm0.005~{\rm R_\odot}$ companion at an inclination of $i=87.7\pm0.2^\circ$. We use the best-fitting eclipsing light curve model to estimate the temperature of the secondary star as $T_{\rm eff}=5200\pm100~{\rm K}$. Finally, while we do not record significant offsets to the expected time of mid-eclipse caused by the emission of gravitational waves with our 1-year baseline, we show that a $3σ$ significant measurement of the orbital decay due to gravitational waves will be possible in 2023, at which point the eclipse will occur about $8$ seconds earlier than expected.

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arXiv2020

The 100 pc White Dwarf Sample in the SDSS Footprint

We present follow-up spectroscopy of 711 white dwarfs within 100 pc, and present a detailed model atmosphere analysis of the 100 pc white dwarf sample in the SDSS footprint. Our spectroscopic follow-up is complete for 83% of the white dwarfs hotter than 6000 K, where the atmospheric composition can be constrained reliably. We identify 1508 DA white dwarfs with pure hydrogen atmospheres. The DA mass distribution has an extremely narrow peak at $0.59~M_{\odot}$, and reveals a shoulder from relatively massive white dwarfs with $M=0.7$-$0.9~M_{\odot}$. Comparing this distribution with binary population synthesis models, we find that the contribution from single stars that form through mergers cannot explain the over-abundance of massive white dwarfs. In addition, the mass distribution of cool DAs shows a near absence of $M>1~M_{\odot}$ white dwarfs. The pile-up of 0.7-$0.9~M_{\odot}$ and the disappearance of $M>1~M_{\odot}$ white dwarfs is consistent with the effects of core crystallization. Even though the evolutionary models predict the location of the pile-up correctly, the delay from the latent heat of crystallization by itself is insufficient to create a significant pile-up, and additional cooling delays from related effects like phase separation are necessary. We also discuss the population of infrared-faint (ultracool) white dwarfs, and demonstrate for the first time the existence of a well defined sequence in color and magnitude. Curiously, this sequence is connected to a region in the color-magnitude diagrams where the number of helium-dominated atmosphere white dwarfs is low. This suggests that the infrared-faint white dwarfs likely have mixed H/He atmospheres.

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arXiv2020

The ELM Survey South. I. An Effective Search for Extremely Low Mass White Dwarfs

We begin the search for extremely-low mass ($M\leq0.3M_{\odot}$, ELM) white dwarfs (WDs) in the southern sky based on photometry from the VST ATLAS and SkyMapper surveys. We use a similar color-selection method as the Hypervelocity star survey. We switched to an astrometric selection once Gaia Data Release 2 became available. We use the previously known sample of ELM white dwarfs to demonstrate that these objects occupy a unique parameter space in parallax and magnitude. We use the SOAR 4.1m telescope to test the Gaia-based selection, and identify more than two dozen low-mass white dwarfs, including 6 new ELM white dwarf binaries with periods as short as 2 h. The better efficiency of the Gaia-based selection enables us to extend the ELM Survey footprint to the southern sky. We confirm one of our candidates, J0500$-$0930, to become the brightest ($G=12.6$ mag) and closest ($d=72$ pc) ELM white dwarf binary currently known. Remarkably, the Transiting Exoplanet Survey Satellite (TESS) full-frame imaging data on this system reveals low-level ($<0.1$%) but significant variability at the orbital period of this system ($P=9.5$ h), likely from the relativistic beaming effect. TESS data on another system, J0642$-$5605, reveals ellipsoidal variations due to a tidally distorted ELM WD. These demonstrate the power of TESS full-frame images in confirming the orbital periods of relatively bright compact object binaries.

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