Helium-core White Dwarf Research Articles

The Formation of Blue Large-amplitude Pulsators from White-dwarf Main-sequence Star Mergers

Blue large-amplitude pulsators (BLAPs) are hot low-mass stars that show large-amplitude light variations likely due to radial oscillations driven by iron group opacities. Period changes provide evidence of both secular contraction and expansion among the class. Various formation histories have been proposed, but none are completely satisfactory. Zhang et al. proposed that the merger of a helium-core white dwarf with a low-mass main-sequence star (HeWD+MS) can lead to the formation of some classes of hot subdwarfs. We have analyzed these HeWD+MS merger models in more detail. Between helium-shell ignition and full helium-core burning, the models pass through the volume of luminosity–gravity–temperature space occupied by BLAPs. Periods of expansion and contraction associated with helium-shell flashes can account for the observed rates of period change. We argue that the HeWD+MS merger model provides at least one BLAP formation channel.

Asteroseismic Study of KUV03442+0719 with Parallax Constraints

Hydrogen atmosphere white dwarf KUV03442+0719 was first reported as a pulsator by Gianninas et al. in 2006. Follow-up campaigns by Su et al. revealed more periods. Some spectroscopic results suggest that KUV03442+0719 has a slightly below-average mass and an effective temperature of 11,000 K. However, Gaia data (parallax and magnitude) suggest that it may be a low-mass white dwarf. Such an object would have a helium core. We perform asteroseismic fitting of KUV03442+0719, modeling it both as a carbon/oxygen normal mass white dwarf, and a helium-core low-mass white dwarf. To perform the study, we perform a grid search with WDEC models, refined by simplex minimization of the best fits. Both analyses result in best-fit models that are comparable in terms of quality of fit. More pulsation data would be required to allow us to distinguish between the two scenarios. We present and contrast our results with expectations from stellar evolution. We also provide analytic formulae for a temperature-dependent mass–radius relationship for helium-core white dwarfs.

Hot subdwarfs in close binaries observed from space

Context. Hot subdwarfs in close binaries with either M dwarf, brown dwarf, or white dwarf companions show unique light variations. In hot subdwarf binaries with M dwarf or brown dwarf companions, we can observe the so-called reflection effect, while in hot subdwarfs with close white dwarf companions, we find ellipsoidal modulation and/or Doppler beaming. Aims. Analyses of these light variations can be used to derive the mass and radius of the companion and determine its nature. Thereby, we can assume the most probable sdB mass and the radius of the sdB derived by the fit of the spectral energy distribution and the Gaia parallax. Methods. In the high signal-to-noise space-based light curves from the Transiting Exoplanet Survey Satellite and the K2 space mission, several reflection effect binaries and ellipsoidal modulation binaries have been observed with much better quality than with ground-based observations. The high quality of the light curves allowed us to analyze a large sample of sdB binaries with M dwarf or white dwarf companions using LCURVE. Results. For the first time, we can constrain the absolute parameters of 19 companions of reflection effect systems, covering periods from 2.5 to 19 h and with companion masses from the hydrogen-burning limit to early M dwarfs. Moreover, we were able to determine the mass of eight white dwarf companion to hot subdwarf binaries showing ellipsoidal modulations, covering the as-yet unexplored period range of 7 to 19 h. The derived masses of the white dwarf companions show that all but two of the white dwarf companions are most likely helium-core white dwarfs. Combining our results with previously measured rotation velocities allowed us to derive the rotation period of seven sdBs in short-period binaries. In four of those systems, the rotation period of the sdB agrees with a tidally locked orbit, whereas in the other three systems, the sdB rotates significantly more slowly.

A Young White Dwarf Orbiting PSR J1835−3259B in the Bulge Globular Cluster NGC 6652

We report on the discovery of the companion star to the millisecond pulsar PSR J1835−3259B in the Galactic globular cluster NGC 6652. Taking advantage of deep photometric archival observations acquired through the Hubble Space Telescope in near-UV and optical bands, we identified a bright and blue object at a position compatible with that of the radio pulsar. The companion is located along the helium-core white dwarf cooling sequence, and the comparison with binary evolution models provides a mass of 0.17 ± 0.02 M ⊙, a surface temperature of 11,500 ± 1900 K, and a very young cooling age of only 200 ± 100 Myr. The mass and the age of the companion are compatible with a progenitor star of about 0.87 M ⊙, which started transferring mass to the primary during its evolution along the subgiant branch and stopped during the early red giant branch phase. Combining together the pulsar mass function and the companion mass, we found that this system is observed at an almost edge-on orbit and hosts a neutron star with a mass of 1.44 ± 0.06 M ⊙, thus suggesting a highly nonconservative mass accretion phase. The young age of the WD companion is consistent with the scenario of a powerful, relatively young MSP indicated by the earlier detection of gamma-rays from this system.

A candidate magnetic helium-core white dwarf in the globular cluster NGC 6397

ABSTRACT We report a peculiar variable blue star in the globular cluster (GC) NGC 6397, using Hubble Space Telescope optical imaging. Its position in the colour–magnitude diagrams and its spectrum are consistent with this star being a helium-core white dwarf (He WD) in a binary system. The optical light curve shows a periodicity at 18.5 h. We argue that this periodicity is due to the rotation of the WD and possibly due to magnetic spots on the surface of the WD. This would make this object the first candidate magnetic He WD in any GC, and the first candidate magnetic WD in a detached binary system in any GC and one of the few He WDs with a known rotation period and of magnetic nature. Another possibility is that this system is an He WD in a binary system with another WD or another degenerate object, which would make this object one of the few candidate non-accreting double-degenerate binaries in any GC.

ELM of ELM-WD: An Extremely-low-mass Hot Star Discovered in LAMOST Survey

The extremely-low-mass white dwarfs (ELM WDs) and pre-ELM WDs are helium-core white dwarfs with mass <∼ 0.3M ⊙. Evolution simulations show that a lower mass limit for ELM WDs exists at ≈0.14M ⊙ and no ELM WD is proposed by observation to be less massive than that. Here we report the discovery of a binary system, LAMOST J224040.77-020732.8 (J2240 in short), which consists of a very low-mass hot star and a compact companion. Multiepoch spectroscopy shows an orbital period P orb = 0.219658 ± 0.000002 days and a radial-velocity semiamplitude K1 = 318.5 ± 3.3 km s−1, which gives the mass function of 0.74M ⊙, indicating the companion is a compact star. The F-type low-resolution spectra illustrate no emission features, and the temperature (∼7400 K) is consistent with that from spectral energy distribution fitting and multicolor light-curve solution. The optical light curves, in ZTF g, r, and i bands and the Catalina V band, show ellipsoidal variability with amplitudes of ∼30%, suggesting that the visible component is heavily tidally distorted. Combining the distance from Gaia survey, the ZTF light curves are modeled with Wilson–Devinney code and the result shows that the mass of the visible component is , and the mass of the invisible component is . The radius of the visible component is . The inclination angle is approximately between 60° and 90°. The observations indicate the system is most likely a pre-ELM WD + WD/NS binary, and the mass of pre-ELM is possibly lower than the 0.14M ⊙ theoretical limit.

Probing TYC 3315-1807-1, An sdB+dM Binary Displaying Strong Period Variation and Reflection Effect

Subdwarf (sdB) stars include core helium-burning stars with a very thin hydrogen envelope that lies at the blue end of the horizontal branch (or extreme horizontal branch). Among them, short-period sdB binaries especially with cool companions are significant to test and constrain binary evolution. We discuss one such sdB+dM type binary, TYC 3315-1807-1 (V1), which was first reported by Kawka. Results of the photometric analysis on Transiting Exoplanet Survey Satellite data are being reported. Light variation in the light curve suggests that the system displays a large reflection effect with no eclipses. Spectroscopic observations of the object were also carried out to probe into the nature of secondary companion as well as to understand the post-common-envelope evolution of such objects. The variability in Balmer, He, and Na line profiles as a function of phase, probably caused by observed reflection effect was identified and studied. Period variation study of the object was done using times of minima obtained from the literature and the O − C plot was produced, which points to a decrease in the period (dp/dt = −1.36315 × 10−7 day yr−1), and the possible scenario of evolution is discussed. From the evolutionary models, we constrain the possible mass of the sdB to be 0.274 M ⊙ and that of the secondary is 0.113 M ⊙, and we conclude that V1 may evolve directly as a helium-core white dwarf.

The pre-He white dwarfs in eclipsing binaries – III. WASP 1625−04

ABSTRACT 1SWASP J162545.15−043027.9 (WASP 1625−04) has been announced as one of EL CVn candidates showing total primary eclipses and ellipsoidal variations. This paper presents the absolute properties of the binary star, based on our high-resolution spectroscopy conducted from 2015 through 2020. From the spectral analysis, the radial velocities (RVs) for both components were obtained with the effective temperature Teff, 1 = 8990 ± 200 K and the rotational rate $v_1\sin \, i=53\pm 5$ km s−1 for the more massive primary. The RV measurements were analysed with archival WASP photometry. From the modelling we obtained: M1 = 1.745 ± 0.013 M⊙, M2 = 0.187 ± 0.002 M⊙, R1 = 1.626 ± 0.008 M⊙, R2 = 0.290 ± 0.003 M⊙, L1 = 15.5 ± 1.4 L⊙, and L2 = 1.84 ± 0.16 L⊙. In the Hertzsprung–Russell diagram, WASP 1625-04 A lies on the zero-age main sequence and its companion accords well with the helium-core white dwarf models of 0.19 M⊙ in the constant luminosity phase. Our improved results demonstrate that WASP 1625−04 is a typical EL CVn-type binary with a low-mass ratio and M2 combination in the thin-disc population and is the product of the stable, non-conservative mass transfer of the precursor binary.

No Pulsar Companion Around the Nearest Low Mass White Dwarf

Abstract 2MASS J050051.85−093054.9 is the closest known low-mass helium-core white dwarf in a binary system. We used three high-band international Low-Frequency Array stations to perform a targeted search for a pulsar companion, reaching sensitivities of ∼3 mJy for a 10 ms pulsar at a DM = 1 pc cm−3. No pulsed signal was detected, confidently excluding the presence of a detectable radio pulsar in the system.

Discovery, TESS Characterization, and Modeling of Pulsations in the Extremely Low-mass White Dwarf GD 278

Abstract We report the discovery of pulsations in the extremely low-mass (ELM), likely helium-core white dwarf GD 278 via ground- and space-based photometry. GD 278 was observed by the Transiting Exoplanet Survey Satellite (TESS) in Sector 18 at a 2 minute cadence for roughly 24 days. The TESS data reveal at least 19 significant periodicities between 2447 and 6729 s, one of which is the longest pulsation period ever detected in a white dwarf. Previous spectroscopy found that this white dwarf is in a 4.61 hr orbit with an unseen &gt;0.4 M ⊙ companion and has T eff = 9230 ± 100 K and log g = 6.627 ± 0.056 , which corresponds to a mass of 0.191 ± 0.013 M ⊙. Patterns in the TESS pulsation frequencies from rotational splittings appear to reveal a stellar rotation period of roughly 10 hr, making GD 278 the first ELM white dwarf with a measured rotation rate. The patterns inform our mode identification for asteroseismic fits, which, unfortunately, do not reveal a global best-fit solution. Asteroseismology reveals two main solutions roughly consistent with the spectroscopic parameters of this ELM white dwarf, but with vastly different hydrogen-layer masses; future seismic fits could be further improved by using the stellar parallax. GD 278 is now the tenth known pulsating ELM white dwarf; it is only the fifth known to be in a short-period binary, but is the first with extended, space-based photometry.

Constraining the evolution of cataclysmic variables via the masses and accretion rates of their underlying white dwarfs

ABSTRACT We report on the masses (MWD), effective temperatures ($\rm{T_\mathrm{eff}}$), and secular mean accretion rates ($\langle \dot{M} \rangle$) of 43 cataclysmic variable (CV) white dwarfs, 42 of which were obtained from the combined analysis of their Hubble Space Telescope ultraviolet data with the parallaxes provided by the Early Third Data Release of the Gaia space mission, and one from the white dwarf gravitational redshift. Our results double the number of CV white dwarfs with an accurate mass measurement, bringing the total census to 89 systems. From the study of the mass distribution, we derive $\langle M_\mathrm{WD} \rangle = 0.81^{+0.16}_{-0.20}\, \mathrm{M_\odot }$, in perfect agreement with previous results, and find no evidence of any evolution of the mass with orbital period. Moreover, we identify five systems with MWD &amp;lt; 0.5 M⊙, which are most likely representative of helium-core white dwarfs, showing that these CVs are present in the overall population. We reveal the presence of an anticorrelation between the average accretion rates and the white dwarf masses for the systems below the $2\!-\!3\,$ h period gap. Since $\langle \dot{M} \rangle$ reflects the rate of system angular momentum loss, this correlation suggests the presence of an additional mechanism of angular momentum loss that is more efficient at low white dwarf masses. This is the fundamental concept of the recently proposed empirical prescription of consequential angular momentum loss (eCAML) and our results provide observational support for it, although we also highlight how its current recipe needs to be refined to better reproduce the observed scatter in $\rm{T_\mathrm{eff}}$ and $\langle \dot{M} \rangle$, and the presence of helium-core white dwarfs.

Convection and rotation boosted prescription of magnetic braking: application to the formation of extremely low-mass white dwarfs

ABSTRACT Extremely low-mass white dwarfs (ELM WDs) are the result of binary evolution in which a low-mass donor star is stripped by its companion leaving behind a helium-core white dwarf (WD). We explore the formation of ELM WDs in binary systems considering the Convection And Rotation Boosted magnetic braking treatment. Our evolutionary sequences were calculated using the Modules for Experiments in Stellar Astrophysics code, with initial masses of 1.0 and 1.2 M⊙ (donor), and 1.4 (accretor), compatible with low-mass X-ray binary (LMXB) systems. We obtain ELM models in the range 0.15–0.27 M⊙ from a broad range of initial orbital periods, 1–25 d. The bifurcation period, where the initial period is equal to the final period, ranges from 20 to 25 d. In addition to LMXBs, we show that ultracompact X-ray binaries (UCXBs) and wide-orbit binary millisecond pulsars can also be formed. The relation between mass and orbital period obtained is compatible with the observational data from He WD companions to pulsars.

A new instability domain of CNO-flashing low-mass He-core stars on their early white-dwarf cooling branches

Context. Before reaching their quiescent terminal white-dwarf cooling branch, some low-mass helium-core white dwarf stellar models experience a number of nuclear flashes which greatly reduce their hydrogen envelopes. Just before the occurrence of each flash, stable hydrogen burning may be able to drive global pulsations that could be relevant in shedding some light on the internal structure of these stars through asteroseismology, similarly to what occurs with other classes of pulsating white dwarfs. Aims. We present a pulsational stability analysis applied to low-mass helium-core stars on their early white-dwarf cooling branches going through CNO flashes in order to study the possibility that the ε mechanism is able to excite gravity-mode pulsations. We assess the ranges of unstable periods and the corresponding instability domain in the log g − Teff plane. Methods. We carried out a nonadiabatic pulsation analysis for low-mass helium-core white-dwarf models with stellar masses between 0.2025 and 0.3630 M⊙ going through CNO flashes during their early cooling phases. Results. We found that the ε mechanism due to stable hydrogen burning can excite low-order (ℓ = 1, 2) gravity modes with periods between ∼80 and 500 s for stars with 0.2025 ≲ M⋆/M⊙ ≲ 0.3630 located in an extended region of the log g − Teff diagram, with effective temperature and surface gravity in the ranges 15 000 ≲ Teff ≲ 38 000 K and 5.8 ≲ log g ≲ 7.1, respectively. For the sequences that experience multiple CNO flashes, we found that with every consecutive flash, the region of instability becomes wider and the modes are more strongly excited. The magnitudes of the rate of period change for these modes are in the range of ∼10−10–10−11 [s/s]. Conclusions. Since the timescales required for these modes to reach amplitudes large enough to be observable are shorter than their corresponding evolutionary timescales, the detection of pulsations in these stars is feasible. Given the current problems in distinguishing some stars that populate the same region of the log g − Teff plane, the eventual detection of short-period pulsations may help in the classification of such stars. Furthermore, if a low-mass white dwarf star were found to pulsate with low-order gravity modes in this region of instability, it would confirm our result that such pulsations can be driven by the ε mechanism. In addition, confirming a rapid rate of period change in these pulsations would support the idea that these stars actually experience CNO flashes, as has been predicted by evolutionary calculations.

The Pre-He White Dwarfs in Eclipsing Binaries. I. WASP 0131+28

Abstract We report the first BV light curves and high-resolution spectra of the post-mass transfer binary star WASP 0131+28 to study the absolute properties of extremely low-mass white dwarfs. From the observed spectra, the double-lined radial velocities were derived, and the effective temperature and rotational velocity of the brighter, more massive primary were found to be K and = 55 ± 10 km s−1, respectively. The combined analysis of the Transiting Exoplanet Survey Satellite (TESS) archive data and ours yielded the accurate fundamental parameters of the program target. The masses were derived to about 1.0% accuracy and the radii to 0.6%, or better. The secondary component's parameters of , , = 11,186 ± 235 K, and are in excellent agreement with the evolutionary sequence for a helium-core white dwarf of mass 0.203 , and indicates that this star is halfway through the constant luminosity phase. The results presented in this article demonstrate that WASP 0131+28 is an EL CVn eclipsing binary in a thin disk, which is formed from the stable Roche-lobe overflow channel and composed of a main-sequence dwarf with a spectral type A0 and a pre-He white dwarf.

PSR J1012+5307: a millisecond pulsar with an extremely low-mass white dwarf companion

ABSTRACT Binaries harbouring millisecond pulsars (MSPs) enable a unique path to determine neutron star (NS) masses: radio pulsations reveal the motion of the NS, while that of the companion can be characterized through studies in the optical range. PSR J1012+5307 is an MSP in a 14.5-h orbit with a helium-core white dwarf (WD) companion. In this work we present the analysis of an optical spectroscopic campaign, where the companion star absorption features reveal one of the lightest known WDs. We determine a WD radial velocity semi-amplitude of $K_2 = 218.9 \pm 2.2\, \rm km\, s^{-1}$, which combined with that of the pulsar derived from the precise radio timing, yields a mass ratio of q = 10.44 ± 0.11. We also attempt to infer the WD mass from observational constraints using new binary evolution models for extremely low-mass (ELM) WDs, but find that they cannot reproduce all observed parameters simultaneously. In particular, we cannot reconcile the radius predicted from binary evolution with the measurement from the photometric analysis ($R_{\rm WD}=0.047_{-0.002}^{+0.003}\, \mathrm{ R}_{\odot }$). Our limited understanding of ELM WD evolution, which results from binary interaction, therefore comes as the main factor limiting the precision with which we can measure the mass of the WD in this system. Our conservative WD mass estimate of $M_{\rm WD} = 0.165 \pm 0.015\, \mathrm{ M}_{\rm \odot }$, along with the mass ratio enables us to infer a pulsar mass of $M_{\rm NS} = 1.72 \pm 0.16\, \mathrm{ M}_{\rm \odot }$. This value is clearly above the canonical $\sim 1.4\, \mathrm{ M}_{\rm \odot }$, therefore adding PSR J1012+5307 to the growing list of massive MSPs.

A 1201 s Orbital Period Detached Binary: The First Double Helium Core White Dwarf LISA Verification Binary

Abstract We report the discovery of a 1201 s orbital period binary, the third shortest-period detached binary known. Sloan Digital Sky Survey J232230.20 + 050942.06 contains two He-core white dwarfs orbiting with a 27° inclination. Located 0.76 kpc from the Sun, the binary has an estimated Laser Interferometer Space Antenna (LISA) 4 yr signal-to-noise ratio of 40. J2322 + 0509 is the first He + He white dwarf LISA verification binary, a source class that is predicted to account for one-third of resolved LISA ultra-compact binary detections.

Population Synthesis of Helium White Dwarf–Red Giant Star Mergers and the Formation of Lithium-rich Giants and Carbon Stars

Abstract The formation histories of lithium-rich and carbon-rich red giants are not yet understood. It has been proposed that the merger of a helium-core white dwarf with a red giant branch (RGB) star might provide a solution. We have computed an extended grid of post-merger evolution models and combined these with predictions of binary-star population synthesis. The results strongly support the proposal that the merger of a helium white dwarf with an RGB star can provide the progenitors of both lithium-rich red clump stars and early-R carbon stars. The distribution of post-merger models in T eff, log g, log L, the surface abundances of lithium and carbon, and the predicted space densities agree well with the observed distributions of these parameters for Li-rich and early-R stars in the Galaxy.

EVR-CB-001: An Evolving, Progenitor, White Dwarf Compact Binary Discovered with the Evryscope

Abstract We present EVR-CB-001, the discovery of a compact binary with an extremely low-mass (0.21 ± 0.05M ⊙) helium core white dwarf progenitor (pre-He WD) and an unseen low-mass (0.32 ± 0.06M ⊙) helium white dwarf (He WD) companion. He WDs are thought to evolve from the remnant helium-rich core of a main-sequence star stripped during the giant phase by a close companion. Low-mass He WDs are exotic objects (only about 0.2% of WDs are thought to be less than 0.3 M ⊙), and are expected to be found in compact binaries. Pre-He WDs are even rarer, and occupy the intermediate phase after the core is stripped, but before the star becomes a fully degenerate WD and with a larger radius (≈0.2R ⊙) than a typical WD. The primary component of EVR-CB-001 (the pre-He WD) was originally thought to be a hot subdwarf (sdB) star from its blue color and under-luminous magnitude, characteristic of sdBs. The mass, temperature (T eff = 18,500 ± 500 K), and surface gravity ( ) solutions from this work are lower than values for typical hot subdwarfs. The primary is likely to be a post-red-giant branch, pre-He WD contracting into a He WD, and at a stage that places it nearest to sdBs on color–magnitude and T eff–log(g) diagrams. EVR-CB-001 is expected to evolve into a fully double degenerate, compact system that should spin down and potentially evolve into a single hot subdwarf star. Single hot subdwarfs are observed, but progenitor systems have been elusive.

EVR-CB-001: An Evolving, Progenitor, White Dwarf Compact Binary Discovered with the Evryscope

We present EVR-CB-001, the discovery of a compact binary with an extremely low mass ($.21 \pm 0.05 M_{\odot}$) helium core white dwarf progenitor (pre-He WD) and an unseen low mass ($.32 \pm 0.06 M_{\odot}$) helium white dwarf (He WD) companion. He WDs are thought to evolve from the remnant helium-rich core of a main-sequence star stripped during the giant phase by a close companion. Low mass He WDs are exotic objects (only about .2$\%$ of WDs are thought to be less than .3 $M_{\odot}$), and are expected to be found in compact binaries. Pre-He WDs are even rarer, and occupy the intermediate phase after the core is stripped, but before the star becomes a fully degenerate WD and with a larger radius ($\approx .2 R_{\odot}$) than a typical WD. The primary component of EVR-CB-001 (the pre-He WD) was originally thought to be a hot subdwarf (sdB) star from its blue color and under-luminous magnitude, characteristic of sdBs. The mass, temperature ($T_{\rm eff}=18,500 \pm 500 K$), and surface gravity ($\log(g)=4.96 \pm 0.04$) solutions from this work are lower than values for typical hot subdwarfs. The primary is likely to be a post-RGB, pre-He WD contracting into a He WD, and at a stage that places it nearest to sdBs on color-magnitude and $T_{\rm eff}$-$\log(g)$ diagrams. EVR-CB-001 is expected to evolve into a fully double degenerate, compact system that should spin down and potentially evolve into a single hot subdwarf star. Single hot subdwarfs are observed, but progenitor systems have been elusive.

Gaia Data Release 2 catalogue of extremely low-mass white dwarf candidates

ABSTRACT Extremely low-mass white dwarf stars (ELMs) are M &amp;lt; 0.3 M⊙ helium-core white dwarfs born either as a result of a common-envelope phase or after a stable Roche lobe overflow episode in a multiple system. The Universe is not old enough for ELMs to have formed through single-star evolution channels. As remnants of binary evolution, ELMs can shed light onto the poorly understood phase of common-envelope evolution and provide constraints to the physics of mass accretion. Most known ELMs will merge in less than a Hubble time, providing an important contribution to the signal to be detected by upcoming space-based gravitational wave detectors. There are currently less than 150 known ELMs; most were selected by colour, focusing on hot objects, in a magnitude-limited survey of the Northern hemisphere only. Recent theoretical models have predicted a much larger space density for ELMs than estimated observationally based on this limited sample. In order to perform meaningful comparisons with theoretical models and test their predictions, a larger well-defined sample is required. In this work, we present a catalogue of ELM candidates selected from the second data release of Gaia (DR2). We have used predictions from theoretical models and analysed the properties of the known sample to map the space spanned by ELMs in the Gaia Hertzsprung–Russell diagram. Defining a set of colour cuts and quality flags, we have obtained a final sample of 5762 ELM candidates down to Teff ≈ 5000 K.