- ID:
- ivo://CDS.VizieR/J/A+A/629/A3
- Title:
- NGC1326A, 1425 and 4548 supergiants VI mags
- Short Name:
- J/A+A/629/A3
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Studies on the massive star population in galaxies beyond the Local Group are the key to understand the link between their numbers and modes of star formation in different environments. We present the analysis of the massive star population of the galaxies NGC1326A, NGC 1425 and NGC 4548 using archival Hubble Space Telescope Wide Field Planetary Camera 2 images in the F555W and F814W filters. Through high precision point spread function fitting photometry for all sources in the three fields we identified 7640 candidate blue supergiants, 2314 candidate yellow supergiants, and 4270 candidate red supergiants. We provide an estimation the ratio of blue to red supergiants for each field as a function of galactocentric radius. Using Modules for Experiments in Stellar Astrophysics (MESA) at solar metallicity, we defined the luminosity function and estimated the star formation history of each galaxy. We carried out a variability search in the V and I filters using three variability indexes: the median absolute deviation, the interquartile range, and the inverse von-Neumann ratio. This analysis yielded 243 new variable candidates with absolute magnitudes ranging from M_V_=-4 to -10mag. We classified the variable stars based on their absolute magnitude and their position on the color-magnitude diagram using the MESA evolutionary tracks at solar metallicity. Our analysis yielded 8 candidate variable blue supergiants, 12 candidate variable yellow supergiants, 21 candidate variable red supergiants, and 4 candidate periodic variables.
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Search Results
- ID:
- ivo://CDS.VizieR/J/AJ/106/1906
- Title:
- NGC 6611: A Cluster Caught in the Act
- Short Name:
- J/AJ/106/1906
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have combined optical CCD photometry and spectroscopy with infrared imaging photometry to study the young cluster NGC 6611. We use these data to derive improved values for the reddening law (R=3.75) and the distance modulus (m-M=11.5), and to construct a physical H-R diagram from which we can probe the ages, masses, and evolutionary states of this stellar ensemble. The H-R diagram shows a strong population of high-mass stars, the most massive of which has a mass of roughly 80 solar masses, similar to what we find in other Galactic and Magellanic Cloud clusters and associations. The age of the massive stellar population in NGC 6611 is approximately 2 million years, with an age spread of, perhaps, a million years, although the data are also consistent with there being no discernible age spread among the most massive stars. However, the H-R diagram does reveal that one star of somewhat lower mass (30 solar masses) must have formed approximately 6 million years ago. The upper end of the mass function of NGC 6611 is found to have a slope of Gamma=-1.1+/-0.3, indistinguishable from a Salpeter slope, and similar to what we have found in other Galactic associations, but shallower than what we have found in the Magellanic Clouds. Our most significant result, however, is that we catch this cluster in the act of forming intermediate mass (3-8 Solar masses) stars. This is the first well-established case where large numbers of intermediate-mass stars have been seen on their way to the zero-age main sequence. That intermediate mass pre-main sequence stars are indeed present is evidenced both by their location above the zero-age main sequence in the H-R diagram, and in some cases by their spectroscopic and infrared signatures of (possibly remnant protostellar) circumstellar material. The pre-main sequence population ranges from as young as 0.25 million years to at least 1 million years of age. We find an highly unusual number (27) of emission-line stars, which appear quite similar in their optical and infrared continuum and optical spectroscopic properties to "classical Be/Ae" stars (as opposed to Herbig Be/Ae stars). Our data are inconsistent with the traditional interpretation that these "classical Be/Ae" stars are slightly evolved stars undergoing mass loss. Instead, we offer the conjecture that these may be young stars whose circumstellar disks have become optically thin, and produce Balmer emission lines. The infrared data do indicate a number of stars, particularly amongst the embedded sample, whose colors are consistent with those of stars thought to be surrounded by optically thick circumstellar accretion disks. The identification of such disks around young massive stars continues to be rare, and implies that the disk survival times around intermediate and high mass stars are much shorter (< 0.5Myr) than those of disks surrounding lower mass stars.
- ID:
- ivo://CDS.VizieR/J/A+A/592/A66
- Title:
- NGC 2808 AGB and RGB stars Na abundance
- Short Name:
- J/A+A/592/A66
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Galactic globular clusters (GC) are known to have multiple stellar populations and be characterised by similar chemical features, e.g., O-Na anti-correlation. While second-population stars, identified by their Na overabundance, have been found from the main sequence turnoff up to the tip of the red giant branch in various Galactic GCs, asymptotic giant branch stars have rarely been targeted. Here we present the results of our high-resolution spectroscopic study of a sample of AGB and RGB stars in the Galactic globular cluster NGC 2808. We derived accurate Na abundances for 31 AGB and 40 RGB stars in this cluster. Comparable Na abundance dispersions are found for our AGB and RGB samples, and we conclude that NGC 2808 has Na-rich second-population AGB stars, which turn out to be even more numerous - in relative terms - than their Na-poor counterparts (on the AGB) and the Na-rich stars on the RGB.
- ID:
- ivo://CDS.VizieR/J/A+A/607/A135
- Title:
- NGC 104, 6121 & 6809 AGB and RGB stars Na abundance
- Short Name:
- J/A+A/607/A135
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We analyze high-resolution spectra of a large sample of asymptotic giant branch AGB and red giant branch (RGB) stars in the Galactic GCs NGC 104, NGC 6121, and NGC 6809 obtained with FLAMES/GIRAFFE at ESO/VLT, and determine their Na abundances. This is the first time that the AGB stars in NGC 6809 are targeted. We find that NGC 104 and NG 6809 have comparable AGB and RGB Na abundance distributions, while NGC 6121 shows a lack of very Na-rich AGB stars. Moreover, to investigate the dependence of AGB Na abundance dispersion on GC parameters, we compare the AGB [Na/H] distributions of a total of nine GCs, with five determined by ourselves with homogeneous method and four from literature, covering a wide range of GC parameters. Their Na abundances and multiple populations of AGB stars form complex picture. In some GCs, AGB stars have similar Na abundances and/or second-population fractions as their RGB counterparts, while some GCs do not have Na-rich second-population AGB stars, and various cases exist between the two extremes. In addition, the fitted relations between fractions of the AGB second population and GC global parameters show that the AGB second-population fraction slightly anticorrelates with GC central concentration, while no robust dependency can be confirmed with other GC parameters.
- ID:
- ivo://CDS.VizieR/J/A+A/537/A108
- Title:
- NGC 6822 AGB spectroscopic catalogue
- Short Name:
- J/A+A/537/A108
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We calibrate spectroscopically the C- versus (vs.) M-type asymptotic giant branch (AGB) star selection made using near-IR photometry, and investigate the spatial distribution of the C/M ratio in NGC 6822, based on low resolution spectroscopy and near-IR photometry.
- ID:
- ivo://CDS.VizieR/J/A+AS/114/281
- Title:
- NGC 7762: a moderate age open cluster
- Short Name:
- J/A+AS/114/281
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report CCD B and V photometry for the central region of the old open cluster NGC 7762. The cluster appears very loose and occupies a large sky area. The Colour Magnitude Diagram (CMD) shows a distinct Main Sequence (MS) down to V=~16.5, and a couple of evolved stars. The bottom of the sequence is characterized by a large spread, much greater than the expected broadness due to photometric errors, and probably related to low mass stars evaporation. The analysis here carried out allows us to conclude that NGC 7762 in an intermediate age cluster about 1.8Gyr old. The comparison with theoretical isochrones from the Padova group constrains colour excess and apparent distance modulus in the range 0.85-0.90 and 12.00-12.20, respectively. The distance to the Sun comes out to be 800 pc, somewhat lower than Chincarini's (1966MmSAI..37..423C) estimate.
- ID:
- ivo://CDS.VizieR/J/A+A/542/A68
- Title:
- NGC 3231, 7055 and 7127 BVRI photometry
- Short Name:
- J/A+A/542/A68
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Open clusters are often used as tracers for the formation and evolution of the Milky Way. But they can also be used to study distinct "local stellar populations" and all kind of stellar groups. All these studies crucially depend on their unambiguous detection and classification separating them from the fore- and background field population. Still more than one third of the catalogued galactic open clusters are unstudied to date. We have chosen three northern open cluster fields, namely NGC 3231, NGC 7055, and NGC 7127 which have been never studied before to shed more light on their true nature. We present Johnson-Cousins BVRI photometry down to V=19mag. After the transformation to the standard systems, colour-magnitude diagrams were generated. These diagrams were used to fit solar abundant isochrones to determine the distance modulus, reddening and apparent age of the main sequences. From the analysis of the colour-magnitude diagrams and the available proper motions we conclude that NGC 7055 and NGC 7127 are young, real, open clusters. NGC 3231, on the other hand, is probably a high galactic latitude open cluster remnant.
- ID:
- ivo://CDS.VizieR/J/A+AS/119/499
- Title:
- NGC 628 and companions photometric distances
- Short Name:
- J/A+AS/119/499
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Based on photometry of the brightest blue stars we derived distance moduli for NGC 628 and its dwarf irregular companions: DDO 13, UGC 1171, UGC 1104, and K 10. The mean modulus of the group is estimated to be 29.46 mag with a formal (internal) error of 0.11. The total mass to luminosity ratio of 40M_{sun}_/L_{sun}_ seems sufficient for NGC 628 to keep its companions in bound orbits.
- ID:
- ivo://CDS.VizieR/J/A+A/531/A73
- Title:
- NGC 6167 and NGC 6193 multi-photometry
- Short Name:
- J/A+A/531/A73
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The Ara OB1a association is a nearby complex in the fourth Galactic quadrant where a number of young/embedded star clusters are projected close to more evolved, intermediate age clusters. It is also rich in interstellar matter, and contains evidences of the interplay between massive stars and their surrounding medium, like the rim HII region NGC 6188. We provide more solid estimates of the fundamental parameters (age and distance) of the two most prominent stellar clusters, NGC 6167 and NGC 6193, that may be used as a basis to study the star formation history of the region. The study is based on a photometric optical survey (UBVIHa) of NGC 6167 and NGC 6193 and their nearby field, complemented with public data from 2MASS-VVV, UCAC3 and IRAC-Spitzer in this region. We produced a uniform photometric catalogue and we estimated more robustly the fundamental parameters for NGC 6167, NGC 6193 and the IRAS 16375-4854 source. As a consequence, all of them are located at approximately the same distance from the Sun in the Sagittarius-Carina Galactic arm. However, the ages we estimated differ widely: NGC 6167 results to be an intermediate-age cluster (20-30Myr), NGC 6193 a very young one (1-5Myr) with PMS, H{alpha} emitters, and class II objects; and the IRAS 16375-4854 source is revealed as the youngest of the three containing several YSOs. These results support a picture in which Ara OB1a is a region where star formation has proceeded for several tens of Myr up to the present. The difference in ages of different stellar groups can be interpreted as a consequence of a triggered star formation process. Additionally, in the specific case of NGC 6193, an indication of possible non-coeval star formation was found.
- ID:
- ivo://CDS.VizieR/J/MNRAS/430/221
- Title:
- NGC 2849 and NGC 6134 UBVI photometry
- Short Name:
- J/MNRAS/430/221
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present CCD photometry of two southern open clusters. As part of the Bologna Open Cluster Chemical Evolution project we obtained BVI and UBVI imaging for NGC 2849 and NGC 6134, respectively. By means of the synthetic colour-magnitude diagram method and using various evolutionary sets of stellar evolution tracks with various metallicities, we determined at the same time age, distance and reddening. We also determined an approximate metallicity for NGC 2849, for which the information is not available from sounder methods like high-resolution spectroscopy. NGC 2849 turned to be 0.85-1.0Gyr old with a solar metallicity. The foreground reddening is E(B-V)=0.28-0.32, and the true distance modulus (m-M)_0_=13.8-13.9. For NGC 6134 we did not obtain fully consistent answers from the V, B-V and V, V-I photometry, an unexpected problem, since both the metallicity and the reddening are known (from high-resolution spectroscopy and the U-B, B-V two colours diagram, respectively). This may either indicate a difficulty of current models (evolutionary tracks and/or models of atmosphere) to accurately reproduce colours, or be related to differences in the metal mixture assumed by the models and those of the clusters. Assuming the spectroscopic abundance and the colour excess [E(B-V)=0.35] from the U-B, B-V plot, we derived a best age between 0.82 and 0.95Gyr and a distance modulus 10.5.
