Extremely Low Mass Research Articles

Low-mass contact binaries: orbital stability at extreme low mass ratios

ABSTRACT With the ever-increasing number of light-curve solutions of contact binary systems, an increasing number of potential bright red nova progenitors are being reported. There remains, however, only one confirmed event. In this study, we undertake a comprehensive review of the orbital stability of contact binary systems, considering the effects of the stellar internal composition (metallicity) and age on the evolution of the gyration radius and its effect on the instability mass ratio of contact binaries. We find that both metallicity and age have an independent effect on orbital stability, with metal-poor and older systems being more stable. The combined effects of age and metallicity are quite profound, such that for most systems with primaries of solar mass or greater, which are halfway or more through the main-sequence lifespans have instability mass ratios at levels where the secondary component would be below the hydrogen fusion mass limit. We find that from the currently available solutions we cannot confidently assign any system as unstable. Although we identify eight potential red nova progenitors, all have methodological or astrophysical concerns, which lowers our confidence in designating any of them as potential merger candidates.

The Ancient Star Formation History of the Extremely Low-mass Galaxy Leo P: An Emerging Trend of a Post-reionization Pause in Star Formation

Isolated, low-mass galaxies provide the opportunity to assess the impact of reionization on their star formation histories (SFHs) without the ambiguity of environmental processes associated with massive host galaxies. There are very few isolated, low-mass galaxies that are close enough to determine their SFHs from resolved star photometry reaching below the oldest main-sequence turnoff. The James Webb Space Telescope (JWST) has increased the volume for which this is possible, and here we report on JWST observations of the low-mass, isolated galaxy Leo P. From NIRCam imaging in F090W, F150W, and F277W, we derive an SFH that shows early star formation followed by a pause subsequent to the Epoch of Reionization, which is then later followed by a reignition of star formation. This is very similar to the SFHs from previous studies of other dwarf galaxies in the “transition zone” between quenched very-low-mass galaxies and the more massive galaxies that show no evidence of the impact of reionization on their SFHs; this pattern is rarely produced in simulations of SFHs. The lifetime SFH reveals that Leo P’s stellar mass at the Epoch of Reionization was in the range that is normally associated with being totally quenched. The extended pause in star formation from z ∼ 5 to 1 has important implications for the contribution of low-mass galaxies to the ultraviolet photon budget at intermediate redshifts. We also demonstrate that, due to higher sensitivity and angular resolution, observing in two NIRCam short-wavelength filters is superior to observing in a combination of a short- and a long-wavelength filter.

Photometric and Spectroscopic Study of Ten Low Mass Ratio Contact Binary Systems: Orbital Stability, O’Connell Effect and Infrared Calcium Line Filling

Low mass ratio contact binary systems are more likely to have unstable orbits and potentially merge. In addition, such systems exhibit characteristics such as starspots and high energy emissions (UV) suggestive of chromospheric and magnetic activity. Light curve modeling of ten contact binary systems is reported. All were found to be of extreme low mass ratio ranging from 0.122 to 0.24 and three were found to be potentially unstable and possible merger candidates. Filling of the infrared calcium absorption lines is a marker of increased chromospheric activity. We use the available Large Sky Area Multi-Object Fiber Spectroscopic Telescope spectra along with matched standard spectra (broadened for rotation) to measure the excess filling of the central core depression flux of the two main infrared calcium absorption lines λ8542 and λ8662. We find that all reported contact binaries have excess filling of the core flux in the infrared calcium lines. Three of the systems reported were also observed by the Galaxy Evolution Explorer mission and we find that all three have features of excess ultraviolet emissions further adding evidence for increased chromospheric activity in low mass ratio contact binaries. Analysis of both orbital stability and absorption line filling is dependent on the determination of geometric and absolute parameters from light curve modeling. Not an insignificant number of contact binary light curves exhibit the O’Connell effect, usually attributed to starspots. We discuss the inclusion of starspots in light curve solutions and how they influence the geometric and absolute parameters.

Investigation of the stability of the extreme low mass ratio contact binaries SX Crv and XX Sex, which are analysed photometrically

The photometric analyses of the extreme low mass ratio contact binaries SX Crv and XX Sex were conducted in detail. Light curves of SX Crv and XX Sex were retrieved from TESS, and light curve analyses were performed simultaneously with radial velocities. The mass ratios for SX Crv and XX Sex were calculated as 0.077(1) and 0.099(2). The masses of the primary components account for about 90% of the total masses of the systems and were determined to be 1.256(48) M⊙ and 1.337(37) M⊙ for SX Crv and XX Sex, respectively. It has been observed that the orbital periods of both systems have increased. The orbital period change rate of SX Crv has been calculated to be 0.39 s per century, while that of XX Sex has been calculated to be 2.68 s per century. The orbital angular momentum (Jo) determined for both systems is more than 3 times the spin angular momentum (Js), and it can be said that the SX Crv and XX Sex systems are stable. The calculated instability separation (ains.) and instability mass ratio (qins.) values for both systems are smaller than those obtained in the analyses, which supports the stability. The estimated ages of the systems are 7.4 Gyr for SX Crv and 3.9 Gyr for XX Sex. logMtot – logJ and logTeff – logL diagrams were also interpreted to better understand the evolution of the extreme low mass ratio contact binaries.

Discovery of a Proto–White Dwarf with a Massive Unseen Companion

We report the discovery of SDSS J022932.28+713002.7, a nascent extremely low-mass (ELM) white dwarf (WD) orbiting a massive (>1 M ⊙ at 2σ confidence) companion with a period of 36 hr. We use a combination of spectroscopy, including data from the ongoing fifth-generation Sloan Digital Sky Survey (SDSS-V), and photometry to measure the stellar parameters of the primary pre-ELM WD. The lightcurve of the primary WD exhibits ellipsoidal variation, which we combine with radial velocity data and PHOEBE binary simulations to estimate the mass of the invisible companion. We find that the primary WD has mass M 1 = 0.18−0.02+0.02 M ⊙ and the unseen secondary has mass M 2 = 1.19−0.14+0.21 M ⊙. The mass of the companion suggests that it is most likely a near-Chandrasekhar-mass WD or a neutron star. It is likely that the system recently went through a Roche lobe overflow from the visible primary onto the invisible secondary. The dynamical configuration of the binary is consistent with the theoretical evolutionary tracks for such objects, and the primary is currently in its contraction phase. The measured orbital period puts this system on a stable evolutionary path which, within a few gigayears, will lead to a contracted ELM WD orbiting a massive compact companion.

UOCS. XIII. Study of the Far-ultraviolet Bright Stars in the Open Cluster NGC 2420 Using AstroSat

We present the study of four far-ultraviolet (FUV) bright stars in the field of the open cluster NGC 2420 using the Ultra Violet Imaging Telescope mounted on AstroSat. The three stars 525, 527, and 560 are members, while star 646 is a nonmember of the cluster. To characterize and determine the parameters of these stars, multiwavelength spectral energy distributions (SEDs) are analyzed using UV, optical, and IR data sets. For all four FUV bright stars, a two-component SED model fits well. Our findings indicate that two stars, 525 and 560, are binary blue straggler star (BSS) systems. These BSS systems may have formed in a tertiary system due to mass transfer from an evolved outer tertiary companion. Star 527 is a binary system of a BSS and an extremely low-mass (ELM) white dwarf, while star 646 is a binary system of a horizontal branch star and an ELM white dwarf. The effective temperatures, radii, luminosities, and masses of the two ELMs are (10250, 11500) K (0.42, 0.12) R ⊙, (1.61, 0.23) L ⊙, and (0.186, 0.170) M ⊙, respectively. The star 527 could be a post-mass-transfer system and may have originated through the Case A/B mass-transfer process in a low-density environment. The cooling age of the ELMs is < 1 Myr, indicating that they have only recently formed.

A Study of Six Extreme Low Mass Ratio Contact Binary Systems

Multi-band (B, V and R) photometric and spectroscopic observations of six poorly studied contact binaries carried out at the Western Sydney University and Las Cumbres Observatory were analyzed using a recent version of the Wilson–Devenney code. All six were found to be of extreme low mass ratio ranging from 0.073 to 0.149. All are of F spectral class with the mass of the primary component ranging from 1.05M ☉ to 1.48M ☉. None show light curve features of enhanced choromospheric activity (O’Connel Effect) however five of the six do have significant ultraviolet excess indicating the presence of increased magnetic and chromospheric activity. Period analysis based on available survey data suggests two systems have a slowly increasing period suggesting mass transfer from the secondary to the primary, two have a slow declining period with likely mass transfer from primary to the secondary, while one shows a steady period, and one undergoing transition from a declining to increasing period suggesting possible mass transfer reversal. We also compare light curve solutions against theoretical markers of orbital stability and show that three of the six systems have mass ratios within the theoretical instability limit and may be regarded as potential merger candidates.

IP Lyn: A Totally Eclipsing Contact Binary with an Extremely Low Mass Ratio

We present the first photometric and orbital period investigations for a neglected totally eclipsing contact binary IP Lyn. The photometric solutions derived from both ground-based and several surveys’ observations suggest that it is a shallow contact binary with an extremely low mass ratio of 0.055. The weak asymmetry observed in our multiple band light curves can be interpreted as a result of an active cool spot on the primary. The absolute physical parameters were determined with the Gaia-distance-based method and checked by an empirical relation. Combining the eclipse timings collected from the literature and those derived from our and variable surveys’ observations, we find that IP Lyn has been undergoing a secular orbital period increase for the past two decades, implying a mass transfer from the less massive secondary to the primary. By comparing the current parameters with the critical instability ones, we infer that IP Lyn is currently stable in spite of its relatively low mass ratio and orbital angular momentum. Finally, from a catalog of 117 extremely low mass ratio contact binaries, we find that their orbital angular momenta are significantly lower than those of the contact binaries with a relatively high mass ratio, suggesting they should be at the late evolutionary stage of a contact binary.

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 (ELM) white dwarfs, one short period, likely eclipsing, DABZ, and two potential LISA binaries. We present the orbital and atmospheric parameters for each new binary based on our spectroscopic follow up. Four of our new binaries show periodic photometric variability in TESS 2 minutes 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 of 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 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.

Effective temperature and the light curve solution of contact binary systems

With an increasing number of contact binary discoveries and the recognition that luminous red novae are the result of contact binary merger events, there has been a significant increase in the number of light curve solutions appearing in the literature. One key element of such solutions is assigning and fixing the effective temperature of the primary component (T1). Sometimes the expectation that the assigned temperature will significantly alter light curve solution is exaggerated even though theoretical considerations suggest that the absolute value of T1 has little influence on the geometric elements of the light curve solution. In this study, we show that assigning T1 over a range of 1000 K has no significant influence on the light curve solutions of two extreme low mass ratio contact binary systems. In addition, we explore the use of photometric spectral energy distribution as a potential standard for assigning T1 in the absence of spectroscopic observations.

Extremely Low-mass White Dwarf Stars Observed in Gaia DR2 and LAMOST DR8

We present the first results from our ongoing project to study extremely low-mass (ELM) white dwarfs (WDs) (M ≤ 0.3M ☉) with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) spectra. Based on the LAMOST DR8 spectral database, we analyzed 136 ELM WD candidates selected from Gaia DR2 data and 12 known objects previously identified by the ELM Survey. The atmospheric parameters and radial velocities of these stars were obtained by fitting the LAMOST low-resolution spectra. After comparing the atmospheric parameters of the 12 known objects from this work to the results reported by the ELM Survey, we demonstrated the potential of LAMOST spectra in probing into the nature of ELM WDs. Based on the atmospheric parameters and Gaia EDR3 data, we identified 21 new high-probability ELM WDs with masses M ≤ 0.3M ☉ and parallax estimates that agree to within a factor of 3. Two of them, J0338+4134 and J1129+4715, show significant radial velocity variability and are very likely to be binary systems containing at least one ELM WD.

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 ongoing search for extremely low mass (ELM) < 0.3 M ⊙ 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 3σ 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 9000 K < T eff < 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 and 32 minutes, respectively, and are future Laser Interferometer Space Antenna gravitational-wave sources.

Absolute parameters of four W UMa stars with extreme low mass ratios

Photometric observations of four extreme low mass ratio W UMa binaries with wide total eclipses, and their absolute parameters are presented. The main results are as follows: (i) ASAS J015429+2042.8 is of A subtype while V755 Cep, VESPA V22 and GSC 02800-01387 are of W subtype; (ii) The mass ratios are close to the lower mass-ratio limit and can be classified as Extreme Low Mass Ratio Binaries (ELMRB). (iii) The fill-out factors of ASAS J015429+2042.8 and VESPA V22 are 0.16 and 0.21 respectively, the one of GSC 02800-01387 is 0.63 while the one of V755 Cep is 0.83; (iv) The smaller in size (and less massive) components of all targets are located above the MS band of the evolutional diagrams while the bigger (and more massive) components are located around ZAMS; (v) Our ELMRB systems fall into the stability zone with Jspin/Jorb lower than the Darwin’s instability ∼1/3.

UOCS – VII. Blue straggler populations of open cluster NGC 7789 with UVIT/AstroSat

ABSTRACT NGC 7789 is a ∼1.6 Gyr old, populous open cluster located at ∼2000 pc. We characterize the blue straggler stars (BSS) of this cluster using the Ultraviolet (UV) data from the UVIT/AstroSat. We present spectral energy distributions (SED) of 15 BSS candidates constructed using multiwavelength data ranging from UV to IR wavelengths. In 8 BSS candidates, a single temperature SED is found to be satisfactory. We discover hot companions in 5 BSS candidates. The hot companions with Teff ∼ 11750–15500 K, R ∼ 0.069–0.242 R⊙, and L ∼ 0.25–1.55 L⊙, are most likely extremely low mass (ELM) white dwarfs (WDs) with masses smaller than ∼0.18 M⊙, and thereby confirmed post mass transfer systems. We discuss the implication of this finding in the context of BSS formation mechanisms. Two additional BSS show excess in one or more UV filters, and may have a hot companion, however we are unable to characterize them. We suggest that at least 5 of the 15 BSS candidates (33 per cent) studied in this cluster have formed via the mass-transfer mechanism.

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.

Photometric analysis of two extreme low mass ratio contact binary systems

Multi-band photometry and light curve analysis for two newly recognized contact binary systems, TYC 6995–813–1 and NSVS 13602901, are presented. Both were found to be of extreme low mass ratios 0.11 and 0.17, respectively. The secondary components of both systems show evidence of considerable evolution with elevated densities as well as both luminosity and radii well above their main sequence counterparts. Even in the absence of significant spot activity, at least one of the systems, TYC 6995–813–1, displays features of magnetic and chromospheric activity. TYC 6995–813–1 is also determined to be a potential merger candidate with its current separation near the theoretical instability separation.

LAMOST J0140355 + 392651: an evolved cataclysmic variable donor transitioning to become an extremely low-mass white dwarf

ABSTRACT We present LAMOST J0140355 + 392651 (hereafter J0140), a close (Porb = 3.81 h) binary containing a bloated, low-mass ($M \approx 0.15 \, \mathrm{M}_{\odot }$) proto-white dwarf (WD) and a massive ($M \approx 0.95\, \mathrm{M}_{\odot }$) WD companion. The system’s optical light curve is dominated by large-amplitude ellipsoidal variability but also exhibits additional scatter, likely driven by pulsations. The proto-WD is cooler (Teff = 6800 ± 100 K) and more puffy ($\log \left[g/\left({\rm cm\, s^{-2}}\right)\right]=4.74\pm 0.07$) than any known extremely low-mass (ELM) WD, but hotter than any known cataclysmic variable (CV) donor. It either completely or very nearly fills its Roche lobe ($R/R_{{\rm Roche\, lobe}}=0.99\pm 0.01$), suggesting ongoing or recently terminated mass transfer. No dwarf nova-like outbursts have been observed. The spectrum is dominated by the proto-WD but shows tentative hints of H α emission, perhaps due to accretion on to the massive WD. The properties of the system are well-matched by mesa binary evolution models of CVs with donors that underwent significant nuclear evolution before the onset of mass transfer. In these models, the bloated proto-WD is either still losing mass via stable Roche lobe overflow or was doing so until very recently. In either case, it is evolving towards higher temperatures at near-constant luminosity to become an ELM WD. If the system is detached, mass transfer likely ended when the donor became too hot for magnetic braking to remain efficient. Evolutionary models predict that the binary will shrink to Porb ≲ 10 min within a few Gyr, when it will either merge or become an AM CVn binary. J0140 provides an observational link between the formation channels of CVs, ELM WDs, detached ultracompact WD binaries, and AM CVn systems.

Blow-away in the Extreme Low-mass Starburst Galaxy Pox 186

Abstract Pox 186 is an exceptionally small dwarf starburst galaxy hosting a stellar mass of ∼105 M ⊙. Undetected in H i (M &lt; 106 M ⊙) from deep 21 cm observations and with an [O iii]/[O ii] (5007/3727) ratio of 18.3 ± 0.11, Pox 186 is a promising candidate Lyman continuum emitter. It may be a possible analog of low-mass reionization-era galaxies. We present a spatially resolved kinematic study of Pox 186 and identify two distinct ionized gas components: a broad one with σ &gt; 400 km s−1 and a narrow one with σ &lt; 30 km s−1. We find strikingly different morphologies between the two components and direct evidence of outflows as seen in the high-velocity gas. Possible physical mechanisms driving the creation of high-velocity gas seen in [O iii] are discussed, from outflow geometry to turbulent mixing between a hot (106 K) star-cluster wind and cooler (104 K) gas clouds. We find a modest mass-outflow rate of 0.022 M ⊙ yr−1 with a small mass-loading factor of 0.5, consistent with other low-mass galaxies. Finally, we compare the mass-loading factor of Pox 186 with extrapolations from numerical simulations and discuss possible reasons for the apparent discrepancy between them.

UVIT/AstroSat studies of blue straggler stars and post-mass transfer systems in star clusters: detection of one more blue lurker in M67

The blue straggler stars (BSSs) are main-sequence (MS) stars, which have evaded stellar evolution by acquiring mass while on the MS. The detection of extremely low mass (ELM) white dwarf (WD) companions to two BSSs and one yellow straggler star (YSS) from our earlier study using UVIT/ASTROSAT, as well as WD companions to main-sequence stars (known as blue lurkers) suggest a good fraction of post-mass transfer binaries in M67. Using deeper UVIT observations, here we report the detection of another blue lurker in M67, with an ELM WD companion. The post-mass transfer systems with the presence of ELM WDs, including BSSs, are formed from Case A/B mass transfer and are unlikely to show any difference in surface abundances. We find a correlation between the temperature of the WD and the $v\ sin i$ of the BSSs. We also find that the progenitors of the massive WDs are likely to belong to the hot and luminous group of BSSs in M67. The only detected BSS+WD system by UVIT in the globular cluster NGC 5466, has a normal WD and suggests that open cluster like environment might be present in the outskirts of low density globular clusters.

Non-linear dynamical tides in white dwarf binaries

ABSTRACT Compact white dwarf (WD) binaries are important sources for space-based gravitational-wave (GW) observatories, and an increasing number of them are being identified by surveys like Extremely Low Mass (ELM) and Zwicky Transient Facility (ZTF). We study the effects of non-linear dynamical tides in such binaries. We focus on the global three-mode parametric instability and show that it has a much lower threshold energy than the local wave-breaking condition studied previously. By integrating networks of coupled modes, we calculate the tidal dissipation rate as a function of orbital period. We construct phenomenological models that match these numerical results and use them to evaluate the spin and luminosity evolution of a WD binary. While in linear theory the WD’s spin frequency can lock to the orbital frequency, we find that such a lock cannot be maintained when non-linear effects are taken into account. Instead, as the orbit decays, the spin and orbit go in and out of synchronization. Each time they go out of synchronization, there is a brief but significant dip in the tidal heating rate. While most WDs in compact binaries should have luminosities that are similar to previous traveling-wave estimates, a few per cent should be about 10 times dimmer because they reside in heating rate dips. This offers a potential explanation for the low luminosity of the CO WD in J0651. Lastly, we consider the impact of tides on the GW signal and show that the Laser Interferometer Space Antenna (LISA) and TianGO can constrain the WD’s moment of inertia to better than $1{{\ \rm per\ cent}}$ for centi-Hz systems.