As the nearest known AGB star (d=64pc) and one of the brightest (m_K_~-2), L2 Pup is a particularly interesting benchmark object to monitor the final stages of stellar evolution. We report new lucky imaging observations of this star with the VLT/NACO adaptive optics system in twelve narrow band filters covering the 1.0-4.0 microns wavelength range. These diffraction limited images reveal an extended circumstellar dust lane in front of the star, that exhibits a high opacity in the J band and becomes translucent in the H and K bands. In the L band, extended thermal emission from the dust is detected. We reproduce these observations using Monte-Carlo radiative transfer modeling of a dust disk with the RADMC-3D code. We also present new interferometric observations with the VLTI/VINCI and MIDI instruments. We measure in the K band an upper limit to the limb-darkened angular diameter of theta_LD_=17.9+/-1.6mas, converting to a maximum linear radius of R=123+/-14R_{sun}_. Considering the geometry of the extended K band emission in the NACO images, this upper limit is probably close to the actual angular diameter of the star. The position of L2 Pup in the Herzsprung-Russell diagram indicates that this star has a mass around 2M_{sun}_ and is probably experiencing an early stage of the asymptotic giant branch. We do not detect any stellar companion of L2 Pup in our adaptive optics and interferometric observations, and we attribute its apparent astrometric wobble in the Hipparcos data to variable lighting effects on its circumstellar material. We however do not exclude the presence of a binary companion, as the large loop structure extending to more than 10AU to the North-East of the disk in our L band images may be the result of interaction between the stellar wind of L2 Pup and a hidden secondary object. The geometric configuration that we propose, with a large dust disk seen almost edge-on, appears particularly favorable to test and develop our understanding of the formation of bipolar nebulae.
In this series of papers we have presented the results of a spectroscopic survey of luminous stars in the nearby spirals M31 and M33. Here, we present spectroscopy of 132 additional stars. Most have emission-line spectra, including luminous blue variables (LBVs) and candidate LBVs, Fe II emission line stars, the B[e] supergiants, and the warm hypergiants. Many of these objects are spectroscopically similar and are often confused with each other. We examine their similarities and differences and propose the following guidelines that can be used to help distinguish these stars in future work. (1) The B[e] supergiants have emission lines of [OI] and [FeII] in their spectra. Most of the spectroscopically confirmed sgB[e] stars also have warm circumstellar dust in their spectral energy distributions (SEDs). (2) Confirmed LBVs do not have the [OI] emission lines in their spectra. Some LBVs have [FeII] emission lines, but not all. Their SEDs show free-free emission in the near-infrared but no evidence for warm dust. Their most important and defining characteristic is the S Dor-type variability. (3) The warm hypergiants spectroscopically resemble the LBVs in their dense wind state and the B[e] supergiants. However, they are very dusty. Some have [FeII] and [OI] emission in their spectra like the sgB[e] stars, but are distinguished by their A- and F-type absorption-line spectra. In contrast, the B[e] supergiant spectra have strong continua and few if any apparent absorption lines. Candidate LBVs should share the spectral characteristics of the confirmed LBVs with low outflow velocities and the lack of warm circumstellar dust.
We present surface brightness fluctuations (SBFs) in the near-IR for 191 Magellanic star clusters available in the Second Incremental and All Sky Data releases of the Two Micron All Sky Survey (2MASS, Cat. <II/246>) and compare them with SBFs of Fornax Cluster galaxies and with predictions from stellar population models as well.
We present X-ray data for all main-sequence and subgiant stars of spectral types A, F, G, and K and luminosity classes IV and V listed in the Bright Star Catalogue (Cat. <V/50>) that have been detected as X-ray sources in the ROSAT all-sky survey; In addition to count rates, source detection parameters, hardness ratios, and X-ray fluxes we also list X-ray luminosities derived from Hipparcos parallaxes.
Maser and infrared studies of oxygen-rich AGB stars
Short Name:
J/ApJ/794/81
Date:
21 Oct 2021
Publisher:
CDS
Description:
We explored an efficient method to identify evolved stars with oxygen-rich envelopes in the late asymptotic giant branch (AGB) or post-AGB phase of stellar evolution, which include a rare class of objects - the "water fountains (WF)." Our method considers the OH and H_2_O maser spectra, the near-infrared Q-parameters (these are color indices accounting for the effect of extinction), and far-infrared AKARI colors. Here we first present the results of a new survey on OH and H_2_O masers. There were 108 color-selected objects: 53 of them were observed in the three OH maser lines (1612, 1665, and 1667 MHz), with 24 detections (16 new for 1612 MHz); and 106 of them were observed in the H_2_O maser line (22 GHz), with 24 detections (12 new). We identify a new potential WF source, IRAS 19356+0754, with large velocity coverages of both OH and H_2_O maser emission. In addition, several objects with high-velocity OH maser emission are reported for the first time. The Q-parameters as well as the infrared [09]-[18] and [18]-[65] AKARI colors of the surveyed objects are then calculated. We suggest that these infrared properties are effective in isolating aspherical from spherical objects, but the morphology may not necessarily be related to the evolutionary status. Nonetheless, by considering altogether the maser and infrared properties, the efficiency of identifying oxygen-rich late/post-AGB stars could be improved.
Obtaining accurate and precise masses and ages for large numbers of giant stars is of great importance for unraveling the assemblage history of the Galaxy. In this paper, we estimate masses and ages of 6940 red giant branch (RGB) stars with asteroseismic parameters deduced from Kepler photometry and stellar atmospheric parameters derived from LAMOST spectra. The typical uncertainties of mass is a few per cent, and that of age is ~20 per cent. The sample stars reveal two separate sequences in the age-[{alpha}/Fe] relation - a high-{alpha} sequence with stars older than ~8Gyr and a low-{alpha} sequence composed of stars with ages ranging from younger than 1Gyr to older than 11Gyr. We further investigate the feasibility of deducing ages and masses directly from LAMOST spectra with a machine learning method based on kernel based principal component analysis, taking a sub-sample of these RGB stars as a training data set. We demonstrate that ages thus derived achieve an accuracy of ~24 per cent. We also explored the feasibility of estimating ages and masses based on the spectroscopically measured carbon and nitrogen abundances. The results are quite satisfactory and significantly improved compared to the previous studies.
We show that the masses of red giant stars can be well predicted from their photospheric carbon and nitrogen abundances, in conjunction with their spectroscopic stellar labels logg, Teff, and [Fe/H]. This is qualitatively expected from mass-dependent post-main-sequence evolution. We here establish an empirical relation between these quantities by drawing on 1475 red giants with asteroseismic mass estimates from Kepler that also have spectroscopic labels from Apache Point Observatory Galactic Evolution Experiment (APOGEE) DR12. We assess the accuracy of our model, and find that it predicts stellar masses with fractional rms errors of about 14 percent (typically 0.2M_{sun}_). From these masses, we derive ages with rms errors of 40 percent. This empirical model allows us for the first time to make age determinations (in the range 1-13Gyr) for vast numbers of giant stars across the Galaxy. We apply our model to ~52000 stars in APOGEE DR12, for which no direct mass and age information was previously available. We find that these estimates highlight the vertical age structure of the Milky Way disc, and that the relation of age with [{alpha}/M] and metallicity is broadly consistent with established expectations based on detailed studies of the solar neighbourhood.
We have applied the semi-empirical spectral analysis, developed by the Sloan Digital Sky Survey (SDSS)-IV/Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE2) Massive Star Team, to a large sample of new O- and B-type stars identified along the Sagittarius spiral arm, in the direction of the southern star clusters NGC3603 and NGC3576. We obtained H-band spectra for 265 point sources, using the APOGEE2-S spectrograph at the du Pont Telescope at the Las Campanas Observatory. We analyzed the associated spectral features deriving spectral types, as well as the massive star distribution along the line of sight. From a total of 265 science targets, 95 are classified as mid- to late-O-type stars (for which only 10 O-type stars are previously known in the literature), 38 are found to be early- to mid-B-type stars, and 32 are classified as either yellow or blue supergiants, completing a total of 165 massive stars.
