Nova Outbursts Research Articles

The path to Z And-type outbursts: The case of V426 Sagittae (HBHA 1704-05)

Context. The star V426 Sge (HBHA 1704-05), originally classified as an emission-line object and a semi-regular variable, brightened at the beginning of August 2018, showing signatures of a symbiotic star outburst. Aims. We aim to confirm the nature of V426 Sge as a classical symbiotic star, determine the photometric ephemeris of the light minima, and suggest the path from its 1968 symbiotic nova outburst to the following 2018 Z And-type outburst. Methods. We re-constructed an historical light curve (LC) of V426 Sge from approximately the year 1900, and used original low- (R ∼ 500–1500; 330–880 nm) and high-resolution (R ∼ 11 000–34 000; 360–760 nm) spectroscopy complemented with Swift-XRT and UVOT, optical UBVRCIC and near-infrared JHKL photometry obtained during the 2018 outburst and the following quiescence. Results. The historical LC reveals no symbiotic-like activity from ∼1900 to 1967. In 1968, V426 Sge experienced a symbiotic nova outburst that ceased around 1990. From approximately 1972, a wave-like orbitally related variation with a period of 493.4 ± 0.7 days developed in the LC. This was interrupted by a Z And-type outburst from the beginning of August 2018 to the middle of February 2019. At the maximum of the 2018 outburst, the burning white dwarf (WD) increased its temperature to ≳2 × 105 K, generated a luminosity of ∼7 × 1037 (d/3.3 kpc)2 erg s−1 and blew a wind at the rate of ∼3 × 10−6 M⊙ yr−1. Our spectral energy distribution models from the current quiescent phase reveal that the donor is a normal M4-5 III giant characterised with Teff ∼ 3400 K, RG ∼ 106 (d/3.3 kpc) R⊙ and LG ∼ 1350 (d/3.3 kpc)2 L⊙ and the accretor is a low-mass ∼0.5 M⊙ WD. Conclusions. During the transition from the symbiotic nova outburst to the quiescent phase, a pronounced sinusoidal variation along the orbit develops in the LC of most symbiotic novae. The following eventual outburst is of Z And-type, when the accretion by the WD temporarily exceeds the upper limit of the stable burning. At this point the system becomes a classical symbiotic star.

ASASSN-16oh: A Nova Outburst with No Mass Ejection—A New Type of Supersoft X-Ray Source in Old Populations

Abstract ASASSN-16oh is a peculiar transient supersoft X-ray source without a mass-ejection signature in the field of the Small Magellanic Cloud. Maccarone et al. concluded that ASASSN-16oh is the first dwarf nova with supersoft X-ray that originated from an equatorial accretion belt on a white dwarf (WD). Hillman et al. proposed a thermonuclear runaway model that both the X-rays and V/I photons are emitted from the hot WD. We propose a nova model induced by a high rate of mass accretion during a dwarf nova outburst, i.e., the X-rays originate from the surface of the hydrogen-burning WD whereas the V/I photons are from the irradiated disk. Our model explains the main observational properties of ASASSN-16oh. We also obtained thermonuclear runaway models with no mass ejection for a wide range of parameters of the WD mass and mass-accretion rates including both natural and forced novae in low-metal environments of Z = 0.001 and Z = 0.0001. They are a new type of periodic supersoft X-ray sources with no mass ejection and also a bright transient in V/I bands if they have a large disk. We suggest that such objects are candidates for Type Ia supernova progenitors because its mass is increasing at a very high efficiency (∼100%).

Determination of the superoutburst cycle lengths of 206 SU UMa-type dwarf novae

ABSTRACT SU UMa stars are characterized by ‘superoutbursts’, which are brighter at maximum light and which last much longer than the more frequent ‘ordinary’ outbursts of these dwarf novae. Although there are now more than 1180 SU UMa-type dwarf novae catalogued, our knowledge on their superoutburst cycle length Cso has hitherto been limited to about 6 ${{\ \rm per\ cent}}$ of the entire sample of known SU UMa stars. Using public data bases we have determined new Cso values for a total of 206 additional SU UMa stars in the range 17 d < Cso < 4590 d (including some ER UMa and WZ Sge-type representants) within total time intervals between 2 and 57 yr, and with an estimated uncertainty of $\pm 11{{\ \rm per\ cent}}$. This way, we are increasing our present knowledge of Cso values by a factor of ∼3.8. Their distribution is characterized by a broad maximum around Cso ≈ 270 d, and slowly decreasing numbers up to Cso ≈ 800 d. The domain Cso > 450 d was unexplored until now; we add here 106 cases ($\sim \! 51{{\ \rm per\ cent}}$ of our total sample) in this range of long cycles, implying a better statistical basis for future studies of their distribution. Our sample contains 16 known WZ Sge stars, and we propose WZ Sge membership for five others hitherto classified as ordinary SU UMa stars. Individual superoutburst timings deviate on average by about $\pm 7{{\ \rm per\ cent}}$ of the cycle length from their overall linear ephemeris, confirming the pronounced quasi-periodic repeatability of superoutbursts. All relevant parameters are listed with their errors, and a table with individual superoutburst epochs of our targets is given, enabling future researchers to combine our results with other (past or future) observations.

X-ray grating spectra of novae in outburst, and the cases of two luminous novae in the Magellanic Clouds

High resolution X-ray spectra of classical and recurrent novae in outburst have opened a new window into the nova phenomenon, allowing to observe both the hydrogen burning white dwarf (WD) with only a very thin atmosphere on top, and violent phenomena occurring in the ejecta. By means of these spectra, the mass and chemical composition of the WD can be estimated and insight is obtained into the mass ejection process. I present here the cases of two Magellanic Clouds novae, the recurrent LMC 2009a, and SMC 2016a, the most luminous nova observed in the Clouds. The bolometric and X-ray luminosity of LMC 2009a peaked at only about 10% of the Eddington luminosity; our interpretation is that the WD was partially obscured, probably by the accretion disk, or by large clumps in the ejecta. SMC 2016a instead reached super-Eddington luminosity and was observed at or above the Eddington level for about 6 months. The WD was more massive than 1.2 M⊙ in both novae, but the short recurrence time and the (inferred) much higher accretion rate caused a less energetic outburst in LMC 2009a. While the SMC 2016a spectrum is dominated by the characteristic continuum and features of the WD atmosphere, the LMC 2009a spectrum is much more complex and has an overlap of absorption and emission features. The main contribution to the continuum in both novae originated in the WD atmosphere, but the spectral features of N LMC 2009a were produced also in different regions, and most likely were due to more than one mechanism.

Discovering novae in early-type galaxies with MUSE: A chance find in NGC 1404, and 12 more candidates from an archival search

ABSTRACT I report the discovery of a transient broad-Hα point source in the outskirts of the giant elliptical galaxy NGC 1404, discovered in archival observations taken with the Multi-Unit Spectroscopic Explorer (MUSE) integral field spectrograph. The Hα line width of 1950 km s−1 full width at half-maximum, and luminosity of (4.1 ± 0.1) × 1036 erg s−1, are consistent with a nova outburst, and the source is not visible in MUSE data obtained 9 months later. A transient soft X-ray source was detected at the same position (within <1 arcsec), 14 yr before the Hα transient. If the X-ray and Hα emission are from the same object, the source may be a short-time-scale recurrent nova with a massive white dwarf accretor, and hence a possible Type-Ia supernova progenitor. Selecting broad-Hα point sources in MUSE archival observations for a set of nearby early-type galaxies, I discovered 12 more nova candidates with similar properties to the NGC 1404 source, including five in NGC 1380 and four in NGC 4365. Multi-epoch data are available for four of these twelve sources; all four are confirmed to be transient on ∼1 yr time-scales, supporting their identification as novae.

The $${}^{\mathbf{7}}$$Be Abundance in Nova V5668 Sgr Is Consistent with the Theory

By now the resonance lines of the isotope $${}^{7}$$ Be have been detected in five novae. The known estimates of the relative abundance of this isotope from the equivalent widths of $${}^{7}$$ Be II and Ca II lines lead to the conclusion that the $${}^{7}$$ Be abundance is considerably higher than the predictions of thermonuclear burning models during a nova outburst. In an attempt to understand the reason for the discrepancy, we have investigated the possibility of an increase in the $${}^{7}$$ Be yield during nova outbursts based on nuclear burning kinetics within the framework of a two-zone model and shown that the mass fraction of this isotope does not exceed $$3\times 10^{-5}$$ even for the most favorable set of parameters. This agrees with the known theoretical results and leaves the contradiction between the theory and observations unresolved. The reason for the contradiction is shown to be the assumption that the relative fractions of the ions Be II/Be and Ca II/Ca are equal, which was made when determining the $${}^{7}$$ Be abundance. For Nova V5668 Sgr the relative fraction of the Be II ions in the ejecta turns out to be higher than the relative fraction of the Ca II ions at least by an order of magnitude because of the difference between the ionization potentials. Our new estimate of the $${}^{7}$$ Be mass fraction is consistent with the theory. The calculated gamma-ray flux at 478 keV from the $${}^{7}$$ Be decay agrees with the upper limit from INTEGRAL satellite observations.

123–321 models of classical novae

Context.High-resolution spectroscopy has revealed large concentrations of CNO and sometimes other intermediate-mass elements (e.g., Ne, Na, Mg, or Al, for ONe novae) in the shells ejected during nova outbursts, suggesting that the solar composition material transferred from the secondary mixes with the outermost layers of the underlying white dwarf during thermonuclear runaway.Aims.Multidimensional simulations have shown that Kelvin-Helmholtz instabilities provide self-enrichment of the accreted envelope with material from the outermost layers of the white dwarf, at levels that agree with observations. However, the Eulerian and time-explicit nature of most multidimensional codes used to date and the overwhelming computational load have limited their applicability, and no multidimensional simulation has been conducted for a full nova cycle.Methods.This paper explores a new methodology that combines 1D and 3D simulations. The early stages of the explosion (i.e., mass-accretion and initiation of the runaway) were computed with the 1D hydrodynamic codeSHIVA. When convection extended throughout the entire envelope, the structures for each model were mapped into 3D Cartesian grids and were subsequently followed with the multidimensional codeFLASH. Two key physical quantities were extracted from the 3D simulations and were subsequently implemented intoSHIVA, which was used to complete the simulation through the late expansion and ejection stages: the time-dependent amount of mass dredged-up from the outer white dwarf layers, and the time-dependent convective velocity profile throughout the envelope.Results.This work explores for the first time the effect of the inverse energy cascade that characterizes turbulent convection in nova outbursts. More massive envelopes have been found that are those reported from previous models with pre-enrichment. These result in more violent outbursts, characterized by higher peak temperatures and greater ejected masses, with metallicity enhancements in agreement with observations.

Discovery of deep eclipses in the cataclysmic variable IPHAS J013031.89+622132.3

I performed photometric observations of the poorly studied cataclysmic variable IPHAS J013031.89+622132.3 and discovered very deep eclipses. I obtained observations over 14 nights for a total time of 50 hours during a time span of 6 months. Thanks to the long observation interval, I determined the orbital period with high precision, Porb=0.14935014±0.00000020 d. I derived the eclipse ephemeris, which, thanks to the precision of the orbital period, has a formal validity of 300 years. The average eclipse depth was 1.88 ± 0.07 mag. The prominent parts of the eclipses were smooth and symmetrical. The average eclipse width, including extended asymmetric eclipse wings, was 0.18 ± 0.01 phases or 40 ± 2 min. The average orbital light curve did not show a prominent orbital hump. Because no dwarf nova outburst occurred during the 6 months of monitoring, this cataclysmic variable is likely to be a nova-like variable.

A review of the disc instability model for dwarf novae, soft X-ray transients and related objects

I review the basics of the disc instability model (DIM) for dwarf novae and soft-X-ray transients and its most recent developments, as well as the current limitations of the model, focusing on the dwarf nova case. Although the DIM uses the Shakura-Sunyaev prescription for angular momentum transport, which we know now to be at best inaccurate, it is surprisingly efficient in reproducing the outbursts of dwarf novae and soft X-ray transients, provided that some ingredients, such as irradiation of the accretion disc and of the donor star, mass transfer variations, truncation of the inner disc, etc., are added to the basic model. As recently realized, taking into account the existence of winds and outflows and of the torque they exert on the accretion disc may significantly impact the model. I also discuss the origin of the superoutbursts that are probably due to a combination of variations of the mass transfer rate and of a tidal instability. I finally mention a number of unsolved problems and caveats, among which the most embarrassing one is the modelling of the low state. Despite significant progresses in the past few years both on our understanding of angular momentum transport, the DIM is still needed for understanding transient systems.

Discovery of a new WZ Sagittae-type cataclysmic variable in the Kepler/K2 data

ABSTRACT We identify a new, bright transient in the K2/Kepler Campaign 11 field. Its light curve rises over 7 mag in a day and then declines 3 mag over a month before quickly fading another 2 mag. The transient was still detectable at the end of the campaign. The light curve is consistent with a WZ Sge-type dwarf nova outburst. Early superhumps with a period of 82 min are seen in the first 10 days and suggest that this is the orbital period of the binary, which is typical for the WZ Sge class. Strong superhump oscillations develop 10 days after peak brightness with periods ranging between 83 and 84 min. At 25 days after the peak brightness a bump in the light curve appears to signal a subtle rebrightening phase implying that this was an unusual type-A outburst. This is the only WZ Sge-type system observed by K2/Kepler during an outburst. The early rise of this outburst is well fitted with a broken power law. In first 10 h, the system brightened linearly and then transitioned to a steep rise with a power-law index of 4.8. Looking at archival K2/Kepler data and new TESS observations, a linear rise in the first several hours at the initiation of a superoutburst appears to be common in SU Ursa Majoris stars.

Accreting pulsating white dwarfs: Probing heating and rotation

AbstractThe white dwarfs in close, interacting binaries provide a natural laboratory for exploring the effects of heating and angular momentum from the accreting material arriving on the surface from the companion. This study is even more fruitful when it involves a pulsating white dwarf, which allows an exploration of the effects of the accretion on the interior as well as in the atmosphere. The last decade has seen the accomplishment of UV (HST) and optical (ground) studies of several accreting white dwarfs that have undergone a dwarf nova outburst that heated the white dwarf and subsequently returned to its quiescent temperature. The most recent study involves V386 Ser, which underwent its first known outburst in January 2019, after 19 years at quiescence. V386 Ser is unique in that its quiescent pulsation shows a triplet, with spacing indicating a rotation period of 4.8 days, extremely slow for accreting white dwarfs. This paper presents the result of HST ultraviolet spectra obtained 7 months after its outburst that shows the first clear confirmation of shorter period modes being driven following the heating from a dwarf nova outburst.

Comprehensive models of novae at metallicity Z = 0.02 and Z = 10−4

ABSTRACT Novae are the observational manifestations of thermonuclear runaways on the surface of accreting white dwarfs (WDs). Although novae are an ubiquitous phenomenon, their properties at low metallicity are not well understood. Using the publicly-available stellar evolution code Modules for Experiments in Stellar Astrophysics (mesa), we model the evolution of accreting carbon–oxygen WDs and consider models that accrete matter with metallicity Z = 0.02 or 10−4. We consider both models without mixing and with matter enriched by CO-elements assuming that mixing occurs in the process of accretion (with mixing fraction 0.25). We present and contrast ignition mass, ejected mass, recurrence period, and maximum luminosity of novae for different WD masses and accretion rates for these metallicities and mixing cases. We find that models with Z = 0.02 have ignition masses and recurrence periods smaller than models with low Z, while the ejected mass and maximum luminosity are larger. Retention efficiency during novae outbursts decreases with increasing metallicity. In our implementation, inclusion of mixing at the H/He interface reduces accreted mass, ejected mass, and recurrence period as compared to the no-mixing case, while the maximum luminosity becomes larger. Retention efficiency is significantly reduced, becoming negative in most of our models. For ease of use, we provide a tabular summary of our results.

A 9-h CV with one outburst in 4 yr of Kepler data

Abstract During a visual search through the Kepler main-field light curves, we have discovered a cataclysmic variable (CV) that experienced only a single 4-d long outburst over four years, rising to three times the quiescent flux. During the four years of non-outburst data the Kepler photometry of KIC 5608384 exhibits ellipsoidal light variations (‘ELVs’) with a ∼12 per cent amplitude and period of 8.7 h. Follow-up ground-based spectral observations have yielded a high-quality radial velocity curve and the associated mass function. Additionally, H α emission lines were present in the spectra even though these were taken while the source was presumably in quiescence. These emission lines are at least partially eclipsed by the companion K star. We utilize the available constraints of the mass function, the ELV amplitude, Roche lobe filling condition, and inferred radius of the K star to derive the system masses and orbital inclination angle: $M_{\rm wd} \simeq 0.46 \pm 0.02 \, \mathrm{M}_\odot$, $M_{\rm K} \simeq 0.41 \pm 0.03 \, \mathrm{M}_\odot$, and i ≳ 70°. The value of Mwd is the lowest reported for any accreting WD in a CV. We have also run binary evolution models using mesa to infer the most likely parameters of the pre-cataclysmic binary. Using the mass-transfer rates from the model evolution tracks we conclude that although the rates are close to the critical value for accretion disc stability, we expect KIC 5608384 to exhibit dwarf nova outbursts. We also conclude that the accreting white dwarf most likely descended from a hot subdwarf and, most notably, that this binary is one of the first bona fide examples of a progenitor of AM CVn binaries to have evolved through the CV channel.

What is wrong with steady accretion discs?

In a standard, steady, thin accretion disc, the radial distribution of the dissipation of the accretion energy is determined simply by energy considerations. Here we draw attention to the fact that while the (quasi-)steady discs in dwarf novae in outburst are in agreement with the expected emission distribution, the steady discs in the nova-like variables are not. We note that essentially the only difference between these two sets of discs is the time for which they have been in the high viscosity, high accretion rate state. In such discs, the major process by which angular momentum is transported outwards is MHD turbulence. We speculate that such turbulence gives rise to corona-like structures (here called magnetically controlled zones, or MCZs) which are also able to provide non-negligible angular momentum transport, the magnitude of which depends on the spatial scale L of the magnetic field structures in such zones. For short-lived, high accretion rate discs (such as those in dwarf novae) we expect L ∼ H and the MCZ to have little effect. But, with time (such as in the nova-like variables) an inverse cascade in the MHD turbulence enables L, and the net effect of the MCZ, to grow. We present a simple toy model which demonstrates that such ideas can provide an explanation for the difference between the dwarf novae and the nova-like variable discs.

Dissecting a Disk-instability Outburst in a Symbiotic Star: NuSTAR and Swift Observations of T Coronae Borealis during the Rise to the “Superactive” State

Abstract The current superactive state of the recurrent nova T Coronae Borealis (T CrB) has been observed with unprecedented detail. Previously published observations provide strong evidence that this state is due to an enhancement of the flow of material through the accretion disk, which increased the optical depth of its most internal region, the boundary layer. NuSTAR and Swift observed T CrB in 2015 September, roughly halfway through the rise to optical maximum. In our analysis of these data, we have found that: (i) the UV emission, as observed with Swift/UVOT in 2015, was already as bright as it became in 2017, after the optical peak; (ii) the soft X-ray emission (E ≲ 0.6 keV) observed in 2017 after the optical peak, on the other hand, had not yet developed during the rising phase in 2015; (iii) the hard X-ray emitting plasma (E ≳ 2 keV) had the same temperature and about half the flux of that observed during quiescence in 2006. This phenomenology is akin to that observed during dwarf novae in outburst, but with the changes in the spectral energy distribution happening on a far longer timescale.

Mixed messages from a nova outburst

Multi-technique analyses of a stardust grain from a CO nova outburst show that carbon- and oxygen-rich phases co-condensed in the stellar ejecta, confirming previous spectroscopy observations.

Multiwavelength Modeling of the SED of Nova V339 Del: Stopping the Wind and Long-lasting Super-Eddington Luminosity with Dust Emission

Abstract During the classical nova outburst, the radiation generated by the nuclear burning of hydrogen in the surface layer of a white dwarf (WD) is reprocessed by the outer material into different forms at softer energies, whose distribution in the spectrum depends on the nova age. Using the method of multiwavelength modeling the spectral energy distribution (SED), we determined physical parameters of the stellar, nebular, and dust component of radiation isolated from the spectrum of the classical nova V339 Del from day 35 to day 636 after its explosion. The transition from the iron-curtain phase to the supersoft source phase (days 35–72), when the optical brightness dropped by 3–4 mag, the absorbing column density fell by its circumstellar component from ∼1 × 1023 to ∼1 × 1021 cm−2, and the emission measure decreased from ∼2 × 1062 to ∼8.5 × 1060 cm−3, was caused by stopping down the mass loss from the WD. The day 35 model SED indicated an oblate shape of the WD pseudophotosphere and the presence of the dust located in a slow equatorially concentrated outflow. The dust emission peaked around day 59. Its coexistence with the strong supersoft X-ray source in the day 100 model SED constrained the presence of the disk-like outflow, where the dust can spend a long time. Both the SED models revealed a super-Eddington luminosity of the burning WD at a level of (1–2) × 1039 (d/4.5 kpc)2 erg s−1, lasting from ∼day 2 to at least day 100.

Laboratory evidence for co-condensed oxygen- and carbon-rich meteoritic stardust from nova outbursts

Although their parent stars no longer exist, the isotopic and chemical compositions and microstructure of individual stardust grains identified in meteorites provide unique constraints on dust formation and thermodynamic conditions in stellar outflows1–5. Novae are stellar explosions that take place in the hydrogen-rich envelope accreted onto the surface of a white dwarf in a close binary system6. The energy released by a suite of nuclear processes operating in the envelope powers a thermonuclear runaway, resulting in the ejection of processed material into the interstellar medium. Spectral fitting of features observed in the infrared spectra of dust-forming novae provided evidence of the co-condensation of both carbonaceous and silicate dust in stellar outflows within 50 to 100 days after explosion7–9. Although novae appear as prolific producers of both carbon- and oxygen-rich dust, very few presolar grains that can be attributed to novae have been found in meteorites thus far10–16. Here, we report the identification of an oxygen-rich inclusion, composed of both silicate and oxide nanoparticles, inside a graphite spherule that originated in the ejecta of a low-mass carbon- and oxygen-rich (CO) nova. This observation establishes laboratory evidence of the co-condensation of oxygen- and carbon-rich dust in nova outbursts and is consistent with large-scale transport and mixing of materials between chemically distinct clumps in the nova ejecta. Carbon-rich and oxygen-rich material coexist in a presolar grain originated by an outburst of a CO nova and found within the LaPaz Icefield 031117 meteorite. Condensation of both carbon and oxygen can thus happen in the same circumstellar environment, contrary to previous assumptions.

LMC S154: the first Magellanic symbiotic recurrent nova

Classical nova outburst has been suggested for a number of extragalactic symbiotic stars, but in none of the systems has it been proven. In this work we study the nature of one of these systems, LMC S154. We gathered archival photometric observations in order to determine the timescales and nature of variability in this system. Additionally we carried out photometric and spectroscopic monitoring of the system and fitted synthetic spectra to the observations. Carbon abundance in the photosphere of the red giant is significantly higher than that derived for the nebula, which confirms pollution of the circumbinary material by the ejecta from nova outburst. The photometric and spectroscopic data show that the system reached quiescence in 2009, which means that for the first time all of the phases of a nova outburst were observed in an extragalactic symbiotic star. The data indicate that most probably there were three outbursts observed in LMC S154, which would make this system a member of a rare class of symbiotic recurrent novae. The recurrent nature of the system is supported by the discovery of coronal lines in the spectra, which are observed only in symbiotic stars with massive white dwarfs and with short-recurrence-time outbursts. Gathered evidence is sufficient to classify LMC S154 as the first bona fide extragalactic symbiotic nova, which is likely a recurrent nova. It is also the first nova with a carbon-rich donor.

The UBV Color Evolution of Classical Novae. III. Time-stretched Color–Magnitude Diagram of Novae in Outburst

Abstract We propose a modified color–magnitude diagram for novae in outburst, i.e., (B − V)0 versus (M V − 2.5 log f s), where f s is the time-scaling factor of a (target) nova against a comparison (template) nova, (B − V)0 is the intrinsic B − V color, and M V is the absolute V magnitude. We dub it the time-stretched color–magnitude diagram. We carefully reanalyzed 20 novae based on the time-stretching method and revised their extinctions E(B − V), distance moduli in the V-band (m − M) V , distances d, and time-scaling factors f s against the template nova LV Vul. We have found that these 20 nova outburst tracks broadly follow one of the two template tracks, the LV Vul/V1668 Cyg or V1500 Cyg/V1974 Cyg group, in the time-stretched color–magnitude diagram. In addition, we estimate the white dwarf masses and (m − M) V of the novae by directly fitting the absolute V model light curves (M V ) with observational apparent V magnitudes (m V ). A good agreement of the two estimates of (m − M) V confirms the consistency of the time-stretched color–magnitude diagram. Our distance estimates are in good agreement with the results of Gaia Data Release 2.