Emission-line Stars Research Articles

Morphologies of Wolf–Rayet Planetary Nebulae Based on IFU Observations

Integral field unit (IFU) spectroscopy of planetary nebulae (PNe) provides a plethora of information about their morphologies and ionization structures. An IFU survey of a sample of PNe around hydrogen-deficient stars has been conducted with the Wide Field Spectrograph (WiFeS) on the ANU 2.3-m telescope. In this paper, we present the Hα kinematic observations of the PN M 2-42 with a weak emission-line star (wels), and the compact PNe Hen 3-1333 and Hen 2-113 around Wolf–Rayet ([WR]) stars from this WiFeS survey. We see that the ring and point-symmetric knots previously identified in the velocity [N ii] channels of M 2-42 are also surrounded by a thin exterior ionized Hα halo, whose polar expansion is apparently faster than the low-ionization knots. The velocity-resolved Hα channel maps of Hen 3-1333 and Hen 2-113 also suggest that the faint multipolar lobes may get to a projected outflow velocity of ∼100 ± 20 km s−1 far from the central stars. Our recent kinematic studies of the WiFeS/IFU survey of other PNe around [WR] and wels mostly hint at elliptical morphologies, while collimated outflows are present in many of them. As the WiFeS does not have adequate resolution for compact (≤6 arcsec) PNe, future high-resolution spatially-resolved observations are necessary to unveil full details of their morpho-kinematic structures.

Follow-Up of Extended Shells around B[e] Stars

B[e] stars are massive B type emission line stars in different evolutionary stages ranging from pre-main sequence to post-main sequence. Due to their mass loss and ejection events these objects deposit huge amounts of mass and energy into their environment and enrich it with chemically processed material, contributing significantly to the chemical and dynamical evolution of their host galaxies. However, the large-scale environments of these enigmatic objects have not attracted much attention. The first and so far only catalog reporting the detection of extended shells around a sample of B[e] stars was an Hα imaging survey carried out in the year 2001, and was limited to bright targets in the northern hemisphere. We have recently started a follow-up of those targets to detect possible evolution of their nebulae in the plane of the sky over a baseline of two decades. Furthermore, we extend our survey to southern targets and fainter northern ones to complement and complete our knowledge on large-scale ejecta surrounding B[e] stars. Besides imaging in Hα and selected nebular lines, we utilize long-slit and 3D spectral observations across the nebulae to derive their physical properties. We discovered pronounced nebula structures around 15 more objects, resulting in a total of 27 B[e] stars with a large-scale nebula. Here we present our (preliminary) results for three selected objects: the two massive supergiants MWC137 and MWC 314, and the unclassified B[e] star MWC 819.

Dwarf Irregular Galaxy Leo A. II. Suprime-Cam R and Hα Stellar Photometry

Abstract We have surveyed the complete extent of Leo A, which is an apparently isolated Local Group dwarf irregular galaxy, in B, V, R, I (the Johnson–Cousins system), and NA656 (centered on Hα) passbands with the Subaru Telescope equipped with the Suprime-Cam mosaic camera. B, V, and I photometry results were published earlier by Stonkutė et al. The recently published Stetson’s high-quality photometric standards in the R passband encouraged us to calibrate and make available to the community deep (R ∼ 25 mag) R and NA656 photometry results in the field of Leo A ( 20 ′ × 24 ′ ). We present the photometry catalog of 28,224 objects in R and 25,800 objects in NA656 passbands. This catalog could serve for future imaging and spectroscopic observation programs of Leo A, especially targeted to study emission-line objects. Also, we demonstrate the capability of the B, V, R, and NA656 passband system to separate Milky Way M-type dwarfs and late-type evolved stars in external galaxies, as well as to recognize early-type (Be, B[e]) emission-line stars.

HIGH-RESOLUTION SPECTROSCOPY OF THE B[e] STAR MWC 645

Optical high-resolution spectroscopic observations of the emission-line star MWC645 are presented. The spectrum exhibits strong variable double-peaked Balmer emission lines as well as low-excitation emission lines of FeII, [FeII], and [OI] which are signatures of the B[e] phenomenon, while lines of helium have not been found. In addition to the emission lines, for the first time we identified absorption lines of neutral metals (e.g., LiI 6708 A, CaI 6717 A, and a number of FeI and TiI lines) that indicate the presence of a cool component in the system. The heliocentric radial velocity measured in our best spectrum was found to be −65.1±1.0 kms −1 for the emission lines and −23.2±0.4 kms −1 for the absorption lines. Using a combination of photometric and spectroscopic data as well as the Gaia EDR3 distance (D=6.5±0.9 kpc), we disentangled the component contributions and estimated their temperatures and luminosities (∼15000 K and ∼4000 K, log L/L ? = 3.8±0.2 and 2.8±0.2 for the hot and cool component, respectively).

Discovery of 2716 hot emission-line stars from LAMOST DR5

We present a catalog of 3339 hot emission-line stars (ELSs) identified from 451 695 O, B and A type spectra, provided by LAMOST Data Release 5 (DR5). We developed an automated Python routine that identified 5437 spectra having a peak between 6561 and 6568 Å. False detections and bad spectra were removed, leaving 4138 good emission-line spectra of 3339 unique ELSs. We re-estimated the spectral types of 3307 spectra as the LAMOST Stellar Parameter Pipeline (LASP) did not provide accurate spectral types for these emission-line spectra. As Herbig Ae/Be stars exhibit higher excess in near-infrared and mid-infrared wavelengths than classical Ae/Be stars, we relied on 2MASS and WISE photometry to distinguish them. Finally, we report 1089 classical Be, 233 classical Ae and 56 Herbig Ae/Be stars identified from LAMOST DR5. In addition, 928 B[em]/A[em] stars and 240 CAe/CBe potential candidates are identified. From our sample of 3339 hot ELSs, 2716 ELSs identified in this work do not have any record in the SIMBAD database and they can be considered as new detections. Identification of such a large homogeneous set of emission-line spectra will help the community study the emission phenomenon in detail without worrying about the inherent biases when compiling from various sources.

Probabilistic orbits and dynamical masses of emission-line binaries

ABSTRACT The observed orbits of emission-line stars may be affected by systematics owing to their broad emission lines being formed in complex and extended environments. This is problematic when orbital parameter probability distributions are estimated assuming radial-velocity data are solely comprised of Keplerian motion plus Gaussian white noise, leading to overconfident and inaccurate orbital solutions, with implications for the inferred dynamical masses and hence evolutionary models. We present a framework in which these systems can be meaningfully analysed. We synthesize benchmark data sets, each with a different and challenging noise formulation, for testing the performance of different algorithms. We make these data sets freely available with the aim of making model validation an easy and standardized practice in this field. Next, we develop an application of Gaussian processes to model the radial-velocity systematics of emission-line binaries, named marginalized $\mathcal {GP}$. We benchmark this algorithm, along with current standardized algorithms, on the synthetic data sets and find our marginalized $\mathcal {GP}$ algorithm performs significantly better than the standard algorithms for data contaminated by systematics. Finally, we apply the marginalized $\mathcal {GP}$ algorithm to four prototypical emission-line binaries: Eta Carinae, GG Carinae, WR 140, and WR 133. We find systematics to be present in several of these case studies; and consequently, the predicted orbital parameter distributions, and dynamical masses, are modified from those previously determined.

Identification of a rare class of emission-line stars in transition between pre-main sequence to Main Sequence phase

Over time, pre-main-sequence (PMS) stars evolve into main-sequence (MS) stars. Stars evolving from PMS to MS phase is a significant subject of research that aims to better understand stellar phases and their properties. Existing literature shows a lack of study of stars in between PMS and MS phase. 'We focused on what belongs in the midst of these two phases. Classical Be (CBe) and Herbig Ae/Be (HAeBe) stars, corresponding to MS and PMS phases, are two well known categories of emission-line stars. Through optical and infrared photometric analysis of a sample of 2167 CBe and 225 HAeBe stars, we identified 98 such rare stars which are in transition between PMS and MS phase. Those rare stars are termed as 'Transition Phase' (TP) candidates in our study. Using the machine learning approach, the potential of the identified TP candidates was verified. 'This article provides a brief overview

Identification of emission-line stars in transition phase from pre-main sequence to main sequence

ABSTRACT Pre-main-sequence (PMS) stars evolve into main-sequence (MS) phase over a period of time. Interestingly, we found a scarcity of studies in existing literature that examine and attempt to better understand the stars in PMS to MS transition phase. The purpose of this study is to detect such rare stars, which we named as ‘transition phase’ (TP) candidates – stars evolving from the PMS to the MS phase. We identified 98 TP candidates using photometric analysis of a sample of 2167 classical Be (CBe) and 225 Herbig Ae/Be (HAeBe) stars. This identification is done by analysing the near- and mid-infrared excess and their location in the optical colour–magnitude diagram. The age and mass of 58 of these TP candidates are determined to be between 0.1–5 Myr and 2–10.5 M⊙, respectively. The TP candidates are found to possess rotational velocity and colour excess values in between CBe and HAeBe stars, which is reconfirmed by generating a set of synthetic samples using the machine learning approach.

First high-resolution optical spectra of the distant emission-line star VES 723 (IRAS 02110+6212)

ABSTRACT The first high-resolution spectra (resolving powers of R ≥ 60 000) of the emission-line star VES 723, which has an unknown evolutionary status, were taken at the 6-m Big Telescope Alt-Azimuthal and the 3.6-m Canada–France–Hawaii Telescope. The spectrum is dominated with powerful emission lines of neutral hydrogen and helium, and forbidden singly ionized nitrogen. The Hα and He i lines were found to exhibit broad foundations of the emission profiles at least 250 km s−1 wide. The strongest emission lines exhibit no noticeable variations during the observing period (2011–2017) except for the He i lines. The spectra contain multicomponent interstellar absorptions of Na i and K i (mult. 1) and several strongest diffuse interstellar bands, while photospheric absorptions have not been detected. The average radial velocity of the forbidden [N ii] emission line centres was adopted as the systemic velocity (Vsys ≈ −52 km s−1), which indicates a large distance of ≈6 kpc. Our luminosity estimates of log L/L⊙ = 3.7−4.0 for two different Gaia distance measurements, along with an effective temperature of Teff = 25 000 ± 2000 K and a fast decrease of the infrared flux longward of $\lambda \sim 10\, \mu{\rm m}$, allow us to suggest that VES 723 is neither a pre-main-sequence Herbig B[e] star nor a B[e] supergiant but rather a member of the FS CMa group of objects with the B[e] phenomenon.

Red Be stars in the Magellanic Clouds?

ABSTRACT We revisit the subject of Be candidate stars towards the Magellanic Clouds, previously studied by the authors using SPM4 proper motions. We obtain Gaia DR2 parallaxes and proper motions for 2357 and 994 LMC and SMC Be candidates, respectively. Parallaxes and proper motions versus colour V − I easily reveal the presence of the redder galactic contaminant foreground, as concluded in our previous work, but this time we do find a few red Be candidates consistent with being true Magellanic objects. A membership assessment to each Magellanic Cloud is done for each Be candidate, based on the distribution of their parallaxes and proper motions. From a compilation of published catalogues of spectroscopically confirmed emission line stars,we find that 41 (LMC) and 56 (SMC) of the Be candidates have shown the Hα line in emission. Near-infrared IRSF JHKs magnitudes are obtained for about 70 per cent of the Be candidates with Gaia DR2 astrometric data. Mid-infrared SAGE IRAC 3.6, 4.5, 5.8, and 8.0 $\mu$m magnitudes are obtained for about 85 per cent as well. After discarding possible Herbig Ae/Be stars, one LMC and three SMC B emission line confirmed stars show significantly redder optical, near- and mid-infrared colours than what has been typically measured for Classical Be stars – they are called red Be stars. Taking into account that classical Be stars do not show these red colours, further studies about these four stars are needed in order to establish their true nature and explain the observed red excess.

Photometric Classifications of Evolved Massive Stars: Preparing for the Era of Webb and Roman with Machine Learning

Abstract In the coming years, next-generation space-based infrared observatories will significantly increase our samples of rare massive stars, representing a tremendous opportunity to leverage modern statistical tools and methods to test massive stellar evolution in entirely new environments. Such work is only possible if the observed objects can be reliably classified. Spectroscopic observations are infeasible with more distant targets, and so we wish to determine whether machine-learning methods can classify massive stars using broadband infrared photometry. We find that a Support Vector Machine classifier is capable of coarsely classifying massive stars with labels corresponding to hot, cool, and emission-line stars with high accuracy, while rejecting contaminating low-mass giants. Remarkably, 76% of emission-line stars can be recovered without the need for narrowband or spectroscopic observations. We classify a sample of ∼2500 objects with no existing labels and identify 14 candidate emission-line objects. Unfortunately, despite the high precision of the photometry in our sample, the heterogeneous origins of the labels for the stars in our sample severely inhibit our classifier from distinguishing classes of stars with more granularity. Ultimately, no large and homogeneously labeled sample of massive stars currently exists. Without significant efforts to robustly classify evolved massive stars—which is feasible given existing data from large all-sky spectroscopic surveys—shortcomings in the labeling of existing data sets will hinder efforts to leverage the next generation of space observatories.

The Detection and Characterization of Be+sdO Binaries from HST/STIS FUV Spectroscopy

Abstract The B emission-line stars are rapid rotators that were probably spun up by mass and angular momentum accretion through mass transfer in an interacting binary. Mass transfer will strip the donor star of its envelope to create a small and hot subdwarf remnant. Here we report on Hubble Space Telescope/STIS far-ultraviolet spectroscopy of a sample of Be stars that reveals the presence of the hot sdO companion through the calculation of cross-correlation functions of the observed and model spectra. We clearly detect the spectral signature of the sdO star in 10 of the 13 stars in the sample, and the spectral signals indicate that the sdO stars are hot, relatively faint, and slowly rotating as predicted by models. A comparison of their temperatures and radii with evolutionary tracks indicates that the sdO stars occupy the relatively long-lived, He-core burning stage. Only 1 of the 10 detections was a known binary prior to this investigation, which emphasizes the difficulty of finding such Be+sdO binaries through optical spectroscopy. However, these results and others indicate that many Be stars probably host hot subdwarf companions.

Data-driven Spectroscopic Estimates of Absolute Magnitude, Distance, and Binarity: Method and Catalog of 16,002 O- and B-type Stars from LAMOST

Abstract We present a data-driven method to estimate absolute magnitudes for O- and B-type stars from the LAMOST spectra, which we combine with Gaia DR2 parallaxes to infer distance and binarity. The method applies a neural network model trained on stars with precise Gaia parallax to the spectra and predicts K s-band absolute magnitudes with a precision of 0.25 mag, which corresponds to a precision of 12% in spectroscopic distance. For distant stars (e.g., >5 kpc), the inclusion of constraints from spectroscopic significantly improves the distance estimates compared to inferences from Gaia parallax alone. Our method accommodates for emission-line stars by first identifying them via principal component analysis reconstructions and then treating them separately for the estimation. We also take into account unresolved binary/multiple stars, which we identify through deviations in the spectroscopic from the geometric inferred from Gaia parallax. This method of binary identification is particularly efficient for unresolved binaries with near equal-mass components and thus provides a useful supplementary way to identify unresolved binary or multiple-star systems. We present a catalog of spectroscopic , extinction, distance, flags for emission lines, and binary classification for 16,002 OB stars from LAMOST DR5. As an illustration, we investigate the of the enigmatic LB-1 system, which Liu et al. had argued consists of a B star and a massive stellar-mass black hole. Our results suggest that LB-1 is a binary system that contains two luminous stars with comparable brightness, and the result is further supported by parallax from the Gaia eDR3.

Line Identification and Excitation of Autoionizing States in a Late-type, Low-mass Wolf–Rayet Star

Abstract Identifications of a large fraction of previously unidentified lines in the complex spectrum of the low mass, late-type LMC [WC11] star J060819.93−715737.4 have been made utilizing electronic databases. There are an exceptionally large number of C ii emission lines originating from autoionizing (AI) levels. Resonance fluorescence between the C ii ground state and excited AI levels is shown to be an important photoabsorption process that is competitive with dielectronic recombination in exciting AI emission lines in stellar winds, and has broad application to many types of emission-line stars. In addition, numerous C ii quartet multiplets appear in emission that are not excited directly by recombination or resonance fluorescence, signifying high wind densities in the emission region that enhance collisional transfer between doublet and quartet states.

Identification of new classical Ae stars in the Galaxy using LAMOST DR5

ABSTRACTWe report the first systematic study to identify and characterize a sample of classical Ae stars in the Galaxy. The spectra of these stars were retrieved from the A-star catalogue using the Large sky Area Multi-Object fibre Spectroscopic Telescope (LAMOST) survey. We identified the emission-line stars in this catalogue from which 159 are confirmed as classical Ae stars. This increases the sample of known classical Ae stars by about nine times from the previously identified 21 stars. The evolutionary phase of classical Ae stars in this study is confirmed from the relatively small mid- and far-infrared excess and from their location in the optical colour–magnitude diagram. We estimated the spectral type using MILES spectral templates and identified classical Ae stars beyond A3, for the first time. The prominent emission lines in the spectra within the wavelength range 3700–9000 Å are identified and compared with the features present in classical Be stars. The H α emission strength of the stars in our sample show a steady decrease from late-B type to Ae stars, suggesting that the disc size may be dependent on the spectral type. Interestingly, we noticed emission lines of Fe ii, O i, and Paschen series in the spectrum of some classical Ae stars. These lines are supposed to fade out by late B-type and should not be present in Ae stars. Further studies, including spectra with better resolution, is needed to correlate these results with the rotation rates of classical Ae stars.

Serendipitous Discovery of Nine White Dwarfs with Gaseous Debris Disks

Abstract Optical spectroscopic observations of white dwarf stars selected from catalogs based on the Gaia DR2 database reveal nine new gaseous debris disks that orbit single white dwarf stars, about a factor of 2 increase over the previously known sample. For each source we present gas emission lines identified and basic stellar parameters, including abundances for lines seen with low-resolution spectroscopy. Principle discoveries include (1) the coolest white dwarf (T eff ≈ 12,720 K) with a gas disk; this star, WD0145+234, has been reported to have undergone a recent infrared outburst; (2) co-location in velocity space of gaseous emission from multiple elements, suggesting that different elements are well mixed; (3) highly asymmetric emission structures toward SDSS J0006+2858, and possibly asymmetric structures for two other systems; (4) an overall sample composed of approximately 25% DB and 75% DA white dwarfs, consistent with the overall distribution of primary atmospheric types found in the field population; and (5) never-before-seen emission lines from Na in the spectra of Gaia J0611−6931, semi-forbidden Mg, Ca, and Fe lines toward WD 0842+572, and Si in both stars. The currently known sample of gaseous debris disk systems is significantly skewed toward northern hemisphere stars, suggesting a dozen or so emission line stars are waiting to be found in the southern hemisphere.

Multiple populations of Hβ emission line stars in the Large Magellanic Cloud cluster NGC 1971

We revisited the young Large Magellanic Cloud star cluster NGC 1971 with the aim of providing additional clues to our understanding of its observed extended main-sequence turnoff (eMSTO), a feature commonly seen in young star clusters that has recently been argued to be caused by a real age spread similar to the cluster age (∼160 Myr). We combined accurate Washington and Strömgren photometry of stars with high membership probability to explore the nature of this eMSTO. From different ad hoc defined pseudo-colors, we found that bluer and redder stars distributed throughout the eMSTO do not show any inhomogeneities of light- and heavy-element abundances. These blue and red stars are split into two clearly different groups only when the Washington M magnitudes are employed, which delimites the number of spectral features that cause the appearance of the eMSTO. We speculate that Be stars populate the eMSTO of NGC 1971 because (i) Hβ contributes to the M passband, (ii) Hβ emissions are common features of Be stars, and (iii) the Washington M and T1 magnitudes are tightly correlated; the latter measuring the observed contribution of Hα emission line in Be stars, which is in turn correlated with Hβ emissions. This is the first observational result to our knowledge that indicates that Hβ emissions are the origin of eMSTOs observed in young star clusters. Our results certainly open new possibilities of studying eMSTO from photometric systems with passbands centered at features commonly seen in Be stars.

Gaia18aen: First symbiotic star discovered by Gaia

Context. Besides the astrometric mission of the Gaia satellite, its repeated and high-precision measurements also serve as an all-sky photometric transient survey. The sudden brightenings of the sources are published as Gaia Photometric Science Alerts and are made publicly available, allowing the community to photometrically and spectroscopically follow up on the object. Aims. The goal of this paper is to analyze the nature and derive the basic parameters of Gaia18aen, a transient detected at the beginning of 2018. This object coincides with the position of the emission-line star WRAY 15-136. The brightening was classified as a “nova?” on the basis of a subsequent spectroscopic observation. Methods. We analyzed two spectra of Gaia18aen and collected the available photometry of the object covering the brightenings in 2018 and also the preceding and following periods of quiescence. Based on this observational data, we derived the parameters of Gaia18aen and discussed the nature of the object. Results. Gaia18aen is the first symbiotic star discovered by Gaia satellite. The system is an S-type symbiotic star and consists of an M giant of a slightly super-solar metallicity, where Teff ∼ 3500 K, a radius of ∼230 R⊙, and a high luminosity L ∼ 7400 L⊙. The hot component is a hot white dwarf. We tentatively determined the orbital period of the system ∼487 d. The main outburst of Gaia18aen in 2018 was accompanied by a decrease in the temperature of the hot component. The first phase of the outburst was characterized by the high luminosity L ∼ 27 000 L⊙, which remained constant for about three weeks after the optical maximum, later followed by the gradual decline of luminosity and increase of temperature. Several re-brightenings have been detected on the timescales of hundreds of days.

Active deep learning method for the discovery of objects of interest in large spectroscopic surveys

Context.Current archives of the LAMOST telescope contain millions of pipeline-processed spectra that have probably never been seen by human eyes. Most of the rare objects with interesting physical properties, however, can only be identified by visual analysis of their characteristic spectral features. A proper combination of interactive visualisation with modern machine learning techniques opens new ways to discover such objects.Aims.We apply active learning classification methods supported by deep convolutional neural networks to automatically identify complex emission-line shapes in multi-million spectra archives.Methods.We used the pool-based uncertainty sampling active learning method driven by a custom-designed deep convolutional neural network with 12 layers. The architecture of the network was inspired by VGGNet, AlexNet, and ZFNet, but it was adapted for operating on one-dimensional feature vectors. The unlabelled pool set is represented by 4.1 million spectra from the LAMOST data release 2 survey. The initial training of the network was performed on a labelled set of about 13 000 spectra obtained in the 400 Å wide region around Hαby the 2 m Perek telescope of the Ondřejov observatory, which mostly contains spectra of Be and related early-type stars. The differences between the Ondřejov intermediate-resolution and the LAMOST low-resolution spectrographs were compensated for by Gaussian blurring and wavelength conversion.Results.After several iterations, the network was able to successfully identify emission-line stars with an error smaller than 6.5%. Using the technology of the Virtual Observatory to visualise the results, we discovered 1013 spectra of 948 new candidates of emission-line objects in addition to 664 spectra of 549 objects that are listed in SIMBAD and 2644 spectra of 2291 objects identified in an earlier paper of a Chinese group led by Wen Hou. The most interesting objects with unusual spectral properties are discussed in detail.

A Photometric Hα Survey of the Open Cluster M29

Abstract Photometric B, V, R, and Hα observations of the young open cluster M29 have been used to search for emission-line stars. Four stars with prominent Hα emission are identified. One of these may be a Herbig AeBe star that shows significant variation in emission strength during the four-month observing interval.