- ID:
- ivo://CDS.VizieR/J/A+AS/137/521
- Title:
- 8500-8750{AA} high resolution spectroscopy
- Short Name:
- J/A+AS/137/521
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present an Echelle+CCD high resolution spectroscopic atlas mapping the MKK classification system over the wavelength interval lambda 8500-8750 A, centered on the near-IR triplet of Ca II. Table 2 lists all the 131 program stars with basic informations (magnitudes, coordinates, spectral types, metallicities, rotational velocities, variabilities) and the journal of observations; Table 3 shows our mapping of the MKK scheme in terms of spectral and luminosity classes; Table 4 lists the major un-blended absorption lines which we identified in our spectra
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- ID:
- ivo://CDS.VizieR/J/A+A/656/A125
- Title:
- ACV variables in the Zwicky Transient Facility
- Short Name:
- J/A+A/656/A125
- Date:
- 16 Mar 2022 00:55:18
- Publisher:
- CDS
- Description:
- The present work presents a case study of known variables from the Zwicky Transient Facility (ZTF) survey, with the aim of investigating the survey's suitability for the detection and study of new ACV variables. Using suitable selection criteria based on the known characteristics of ACV variables, candidate ACV stars were selected from the ZTF Catalog of Periodic Variable Stars. All light curves were inspected in detail to select the most promising candidates. Where available, low-resolution spectra from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) were employed to classify the stars on the MK system and confirm their status as mCP stars. We have identified 86 new promising ACV star candidates. 15 of these stars have LAMOST spectra available, which, in all cases, confirm them as classical mCP stars, which highlights the viability of our approach. We present astrophysical parameters for all sample stars, which can be sorted into four subgroups characterized by distinct light curve shapes. Anti-phase variations in different photometric passbands, in particular, is a unique characteristic of a subset of ACV stars readily usable for their identification. The availability of data in three different passbands (g, r, and i) is a major advantage of the ZTF survey. On the basis of our experience with other photometric surveys and the analysis of light curves, we conclude that the ZTF is well suited for the search for, and the analysis of, ACV variables, which, however, are not considered in the available ZTF variable star catalogues. Further work will be concerned with the development and refinement of a search algorithm to correctly identify these stars in ZTF data and, subsequently, in massive photometric time-series databases in general.
- ID:
- ivo://CDS.VizieR/J/A+A/574/A102
- Title:
- AGB candidates in NGC 6822
- Short Name:
- J/A+A/574/A102
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The ratio of C- and M-type asymptotic giant branch (AGB) stars is commonly used to estimate the metallicity of extragalactic populations. Sources in the AGB population must therefore be accurately classified as either C- or M-type. Spectroscopic data are presented for candidate C- and M-type AGB stars, previously classified using JHK photometry, in the Local Group dwarf galaxy NGC 6822. This paper aims to evaluate the success of the JHK classification criteria used in order to determine the level of error associated with this method, and to refine the criteria for future studies. The success rate of a second independent method of source classification, the CN-TiO method, is also examined. We also review the validity of the 4kpc radial limit imposed in our previous work. Spectra of 323 sources, distributed across an area of 2deg^2^, were taken using the AAOmega multi-fibre spectrograph on the Anglo-Australian Telescope and have been classified using an automated classification system and spectral standards from the literature. Nearly half (135) of these sources were selected in common with a photometric catalogue that relied on the CN-TiO method. Within this sample we were able to classify 158 sources, including 82 C-type giants and one anomalous M-type giant, all members of NGC 6822, and 75 foreground K- and M-type dwarf sources. All but three of the giant sources are located within 3 kpc of the galactic centre. Using this spectroscopic sample, new JHK photometric criteria for the isolation and classification of C- and M-type AGB stars have been derived. The error rate in the CN-TiO method, arising from stars incorrectly classified as C-type, has been estimated to be ~7%. Based on the new JHK classification criteria, revised estimates of the global C/M ratio, 0.95+/-0.04, and iron abundance, -1.38+/-0.06dex, are presented for NGC 6822.
- ID:
- ivo://CDS.VizieR/J/A+A/612/A98
- Title:
- APOGEE full information on classes
- Short Name:
- J/A+A/612/A98
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The volume of data generated by astronomical surveys is growing rapidly. Traditional analysis techniques in spectroscopy either demand intensive human interaction or are computationally expensive. In this scenario, machine learning, and unsupervised clustering algorithms in particular, offer interesting alternatives. The Apache Point Observatory Galactic Evolution Experiment (APOGEE) offers a vast data set of near-infrared stellar spectra, which is perfect for testing such alternatives. Our research applies an unsupervised classification scheme based on K-means to the massive APOGEE data set. We explore whether the data are amenable to classification into discrete classes. We apply the K-means algorithm to 153,847 high resolution spectra (R~22,500). We discuss the main virtues and weaknesses of the algorithm, as well as our choice of parameters. We show that a classification based on normalised spectra captures the variations in stellar atmospheric parameters, chemical abundances, and rotational velocity, among other factors. The algorithm is able to separate the bulge and halo populations, and distinguish dwarfs, sub-giants, RC, and RGB stars. However, a discrete classification in flux space does not result in a neat organisation in the parameters' space. Furthermore, the lack of obvious groups in flux space causes the results to be fairly sensitive to the initialisation, and disrupts the efficiency of commonly-used methods to select the optimal number of clusters. Our classification is publicly available, including extensive online material associated with the APOGEE Data Release 12 (DR12). Our description of the APOGEE database can help greatly with the identification of specific types of targets for various applications. We find a lack of obvious groups in flux space, and identify limitations of the K-means algorithm in dealing with this kind of data.
- ID:
- ivo://CDS.VizieR/J/MNRAS/395/593
- Title:
- ASAS chromospherically active eclipsing binaries
- Short Name:
- J/MNRAS/395/593
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have started a spectroscopic survey to identify new chromospherically active components and low-mass pre-main sequence (PMS) stars in recently discovered All Sky Automated Survey (ASAS) eclipsing binaries. In this paper, we briefly describe our scientific motivation, the observing tools and the results obtained from the first phase of this survey. Using the available observing facilities in India, the spectroscopic observations of a sample of 180 candidate eclipsing binary stars selected from ASAS-I&II releases were carried out during 2004-2006. The strength of Halpha emission was used to characterize the level of chromospheric activity. Our spectroscopic survey reveals that out of 180 stars about 36 binary systems show excess Halpha emission. One of the objects in our sample, ASAS 081700-4243.8, displays very strong Halpha emission. Follow-up high-resolution spectroscopic observations reveal that this object is indeed very interesting and most likely a classical Be-type system with K0III companion.
- ID:
- ivo://CDS.VizieR/III/195
- Title:
- Atlas of Optical Spectral Classification OB Stars
- Short Name:
- III/195
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The Atlas provides digital spectra to assist the classification of OB spectra for 78 standard objects in the wavelength range 395.0-475.0 nm. Spectral types range from O3 - B3 (-B8 at Ia). The Atlas provides contemporary digital data comparable to the earlier printed Atlasses by Morgan et al. (1943), Abt et al. (1968), Yamashita et al. (1977) and Morgan et al. (1978). The digital data were obtained with the Shectman/Heathcote two- dimensional, photon-counting detector on the Casegrain spectrograph at the CTIO 1-meter telescope during October 1988 and March 1989. The 3-pixel resolution is 1.5 Angstroem, and the full wavelength coverage is 3800-5000 A. The data were extracted and rectified by using a uniform template followed by a low-order spine fit.
- ID:
- ivo://CDS.VizieR/J/MNRAS/414/2602
- Title:
- Automated classification of HIP variables
- Short Name:
- J/MNRAS/414/2602
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present an evaluation of the performance of an automated classification of the Hipparcos periodic variable stars into 26 types. The sub-sample with the most reliable variability types available in the literature is used to train supervised algorithms to characterize the type dependencies on a number of attributes. The most useful attributes evaluated with the random forest methodology include, in decreasing order of importance, the period, the amplitude, the V-I colour index, the absolute magnitude, the residual around the folded light-curve model, the magnitude distribution skewness and the amplitude of the second harmonic of the Fourier series model relative to that of the fundamental frequency.
- ID:
- ivo://CDS.VizieR/J/A+A/494/739
- Title:
- Automatic classification of OGLE variables
- Short Name:
- J/A+A/494/739
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Scientific exploitation of large variability databases can only be fully optimized if these archives contain, besides the actual observations, annotations about the variability class of the objects they contain. Supervised classification of observations produces these tags, and makes it possible to generate refined candidate lists and catalogues suitable for further investigation. We aim to extend and test the classifiers presented in a previous work against an independent dataset. We complement the assessment of the validity of the classifiers by applying them to the set of OGLE light curves treated as variable objects of unknown class. The results are compared to published classification results based on the so-called extractor methods. Two complementary analyses are carried out in parallel. In both cases, the original time series of OGLE observations of the Galactic bulge and Magellanic Clouds are processed in order to identify and characterize the frequency components. In the first approach, the classifiers are applied to the data and the results analyzed in terms of systematic errors and differences between the definition samples in the training set and in the extractor rules. In the second approach, the original classifiers are extended with colour information and, again, applied to OGLE light curves. We have constructed a classification system that can process huge amounts of time series in negligible time and provide reliable samples of the main variability classes. We have evaluated its strengths and weaknesses and provide potential users of the classifier with a detailed description of its characteristics to aid in the interpretation of classification results. Finally, we apply the classifiers to obtain object samples of classes not previously studied in the OGLE database and analyse the results. We pay specific attention to the B-stars in the samples, as their pulsations are strongly dependent on metallicity.
- ID:
- ivo://CDS.VizieR/J/A+A/538/A76
- Title:
- Automatic stellar spectral classification
- Short Name:
- J/A+A/538/A76
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- As part of a project aimed at deriving extinction-distances for thirty-five planetary nebulae, spectra of a few thousand stars were analyzed to determine their spectral type and luminosity class. We present here the automatic spectral classification process used to classify stellar spectra. This system can be used to classify any other stellar spectra with similar or higher signal-to-noise ratios. Spectral classification was performed using a system of artificial neural networks that were trained with a set of line-strength indices selected among the spectral lines most sensitive to temperature and the best luminosity tracers. The training and validation processes of the neural networks are discussed and the results of additional validation probes, designed to ensure the accuracy of the spectral classification, are presented.
- ID:
- ivo://CDS.VizieR/J/MNRAS/477/2976
- Title:
- Berkeley 51 UBV photometry
- Short Name:
- J/MNRAS/477/2976
- Date:
- 02 Mar 2022 00:35:30
- Publisher:
- CDS
- Description:
- The heavily obscured open cluster Berkeley 51 shows characteristics typical of young massive clusters, even though the few previous studies have suggested older ages. We combine optical (UBV) and 2MASS photometry of the cluster field with multi-object and long-slit optical spectroscopy for a large sample of stars. We apply classical photometric analysis techniques to determine the reddening to the cluster, and then derive cluster parameters via isochrone fitting. We find a large population of B-type stars, with a main-sequence turn-off at B3 V, as well as a large number of supergiants with spectral types ranging from F to M. We use intermediate-resolution spectra of the evolved cool stars to derive their stellar parameters and find an essentially solar iron abundance. Under the plausible assumption that our photometry reaches stars still close to the zero-age main sequence, the cluster is located at d=~5.5kpc and has an age of ~60Ma, though a slightly younger and more distant cluster cannot be ruled out. Despite the apparent good fit of isochrones, evolved stars seem to reside in positions of the colour-magnitude diagram far away from the locations where stellar tracks predict helium burning to occur. Of particular interest is the presence of four yellow supergiants, two on the ascending branch and two others close to or inside the instability strip.
