- ID:
- ivo://CDS.VizieR/J/ApJ/794/36
- Title:
- {sigma} Orionis cluster stellar population
- Short Name:
- J/ApJ/794/36
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a spectroscopic survey of the stellar population of the {sigma} Orionis cluster. We have obtained spectral types for 340 stars. Spectroscopic data for spectral typing come from several spectrographs with similar spectroscopic coverage and resolution. More than half of the stars in our sample are members confirmed by the presence of lithium in absorption, strong H{alpha} in emission or weak gravity-sensitive features. In addition, we have obtained high-resolution (R~34000) spectra in the H{alpha} region for 169 stars in the region. Radial velocities were calculated from this data set. The radial velocity distribution for members of the cluster is in agreement with previous work. Analysis of the profile of the H{alpha} line and infrared observations reveals two binary systems or fast rotators that mimic the H{alpha} width expected in stars with accretion disks. On the other hand, there are stars with optically thick disks and narrow H{alpha} profiles not expected in stars with accretion disks. This contribution constitutes the largest homogeneous spectroscopic data set of the {sigma} Orionis cluster to date.
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- ID:
- ivo://CDS.VizieR/J/AJ/151/143
- Title:
- Silicon depletion in the interstellar medium
- Short Name:
- J/AJ/151/143
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report interstellar silicon (Si) depletion and dust-phase column densities of Si along 131 Galactic sight lines using archival observations. The data were corrected for differences in the assumed oscillator strength. This is a much larger sample than previous studies but confirms the majority of results, which state that the depletion of Si is correlated with the average density of hydrogen along the line of sight (<n(H)>) as well as the fraction of hydrogen in molecular form (f(H_2_)). We also find that the linear part of the extinction curve is independent of Si depletion. Si depletion is correlated with the bump strength (c_3_/R_V_) and the FUV curvature (c_4_/R_V_) suggesting that silicon plays a significant role in both the 2175{AA} bump and the FUV rise.
- ID:
- ivo://CDS.VizieR/J/ApJ/812/147
- Title:
- Slug analysis of star clusters in NGC 628 & 7793
- Short Name:
- J/ApJ/812/147
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We investigate a novel Bayesian analysis method, based on the Stochastically Lighting Up Galaxies (slug) code, to derive the masses, ages, and extinctions of star clusters from integrated light photometry. Unlike many analysis methods, slug correctly accounts for incomplete initial mass function (IMF) sampling, and returns full posterior probability distributions rather than simply probability maxima. We apply our technique to 621 visually confirmed clusters in two nearby galaxies, NGC 628 and NGC 7793, that are part of the Legacy Extragalactic UV Survey (LEGUS). LEGUS provides Hubble Space Telescope photometry in the NUV, U, B, V, and I bands. We analyze the sensitivity of the derived cluster properties to choices of prior probability distribution, evolutionary tracks, IMF, metallicity, treatment of nebular emission, and extinction curve. We find that slug's results for individual clusters are insensitive to most of these choices, but that the posterior probability distributions we derive are often quite broad, and sometimes multi-peaked and quite sensitive to the choice of priors. In contrast, the properties of the cluster population as a whole are relatively robust against all of these choices. We also compare our results from slug to those derived with a conventional non-stochastic fitting code, Yggdrasil. We show that slug's stochastic models are generally a better fit to the observations than the deterministic ones used by Yggdrasil. However, the overall properties of the cluster populations recovered by both codes are qualitatively similar.
- ID:
- ivo://CDS.VizieR/J/ApJ/886/27
- Title:
- SNe IIP progenitors. I. LMC giant comparison sample
- Short Name:
- J/ApJ/886/27
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the evolution of massive star progenitors of supernovae of type IIP. We take the example of the nearby and well-studied SN2013ej. We explore how convective overshoot affects the stellar structure, surface abundances, and effective temperature of massive stars, using the Modules for Experiments in Stellar Astrophysics. In particular, models with moderate overshoot (f=0.02-0.031) show the presence of blue loops in the Hertzsprung-Russell diagram with a red to blue excursion (log_10_[Teff/K] from <3.6 to >4.0) and transition back to red, during the core helium-burning phase. Models with overshoot outside this range of f values kept the star in the red supergiant state throughout the post-helium-ignition phases. The surface CNO abundance shows enrichment post-main-sequence and again around the time when helium is exhausted in the core. These evolutionary changes in surface CNO abundance are indistinguishable in the currently available observations due to large observational uncertainties. However, these observations may distinguish between the ratio of surface nitrogen to oxygen at different evolutionary stages of the star. We also compare the effects of convective overshoot on various parameters related to likelihood of explosion of a star as opposed to collapse to a black hole. These parameters are the compactness parameter, M_4_, and {mu}_4_. The combination {mu}_4_xM_4_, and {mu}_4_ have similar variations with f and both peak at f=0.032. We find that all of our 13M_{sun}_ models are likely to explode.
- ID:
- ivo://CDS.VizieR/J/ApJ/738/162
- Title:
- SN Ia candidates from the SDSS-II SN Survey
- Short Name:
- J/ApJ/738/162
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We analyze the three-year Sloan Digital Sky Survey II (SDSS-II) Supernova (SN) Survey data and identify a sample of 1070 photometric Type Ia supernova (SN Ia) candidates based on their multiband light curve data. This sample consists of SN candidates with no spectroscopic confirmation, with a subset of 210 candidates having spectroscopic redshifts of their host galaxies measured while the remaining 860 candidates are purely photometric in their identification. We describe a method for estimating the efficiency and purity of photometric SN Ia classification when spectroscopic confirmation of only a limited sample is available, and demonstrate that SN Ia candidates from SDSS-II can be identified photometrically with ~91% efficiency and with a contamination of ~6%. Although this is the largest uniform sample of SN candidates to date for studying photometric identification, we find that a larger spectroscopic sample of contaminating sources is required to obtain a better characterization of the background events. A Hubble diagram using SN candidates with no spectroscopic confirmation, but with host galaxy spectroscopic redshifts, yields a distance modulus dispersion that is only ~20%-40% larger than that of the spectroscopically confirmed SN Ia sample alone with no significant bias. A Hubble diagram with purely photometric classification and redshift-distance measurements, however, exhibits biases that require further investigation for precision cosmology.
- ID:
- ivo://CDS.VizieR/J/ApJ/713/871
- Title:
- Sources in star-forming region Cyg-OB2 from X-ray
- Short Name:
- J/ApJ/713/871
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Cygnus OB2 is the nearest example of a massive star-forming region (SFR), containing over 50 O-type stars and hundreds of B-type stars. We have analyzed the properties of young stars in two fields in Cyg OB2 using the recently published deep catalog of Chandra X-ray point sources with complementary optical and near-IR photometry. Our sample is complete to ~1M_{sun}_ (excluding A- and B-type stars that do not emit X-rays), making this the deepest study of the stellar properties and star formation history in Cyg OB2 to date. Combining our stellar mass function (MF) and an estimate of the radial density profile of the association suggests a total mass of Cyg OB2 of ~3x10^4^M_{sun}_, similar to that of many of our Galaxy's most massive SFRs.
- ID:
- ivo://CDS.VizieR/J/ApJ/691/672
- Title:
- Spectrocopy of YSOs in Serpens molecular cloud
- Short Name:
- J/ApJ/691/672
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report on the results of an optical spectroscopic survey designed to confirm the youth and determine the spectral types among a sample of young stellar object (YSO) candidates in the Serpens molecular cloud. We observed 150 infrared (IR) excess objects previously discovered by the Spitzer Legacy Program "From Molecular Cores to Planet-Forming Disks" (c2d), bright enough for subsequent Spitzer/InfraRed Spectrograph (IRS) spectroscopy. We obtained 78 optical spectra of sufficient signal-to-noise for analysis. Extinctions, effective temperatures, and luminosities are estimated for this sample, and used to construct Hertzsprung-Russell (H-R) diagrams for the population. We identified 20 background giants contaminating the sample, based on their relatively high extinction, position in the H-R diagram, the lack of H{alpha} emission and relatively low IR excess. Such strong background contamination (25%) is consistent with the location of Serpens being close to the Galactic plane (5{deg} Galactic latitude). The remaining 58 stars (75%) were all confirmed to be young, mostly K- and M-type stars that are presumed to belong to the cloud. Our analysis shows that the majority of the IR excess objects detected in Serpens are actively accreting, young T-Tauri stars.
- ID:
- ivo://CDS.VizieR/J/AJ/145/125
- Title:
- Spectroscopy and HST photometry in Westerlund 2
- Short Name:
- J/AJ/145/125
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a spectroscopic and photometric determination of the distance to the young Galactic open cluster Westerlund 2 using WFPC2 imaging from the Hubble Space Telescope (HST) and ground-based optical spectroscopy. HST imaging in the F336W, F439W, F555W, and F814W filters resolved many sources previously undetected in ground-based observations and yielded photometry for 1136 stars. We identified 15 new O-type stars, along with two probable binary systems, including MSP188 (O3+O5.5). We fit reddened spectral energy distributions based on the Padova isochrones to the photometric data to determine individual reddening parameters R_V_ and A_V_for O-type stars in Wd2. We find average values <R_V_>=3.77+/-0.09 and <A_V_>=6.51+/-0.38mag, which result in a smaller distance than most other spectroscopic and photometric studies. After a statistical distance correction accounting for close unresolved binaries (factor of 1.08), our spectroscopic and photometric data on 29 O-type stars yield that Westerlund 2 has a distance <d>=4.16+/-0.07(random)+0.26(systematic)kpc. The cluster's age remains poorly constrained, with an upper limit of 3Myr. Finally, we report evidence of a faint mid-IR polycyclic aromatic hydrocarbon ring surrounding the well-known binary candidate MSP 18, which appears to lie at the center of a secondary stellar grouping within Westerlund 2.
- ID:
- ivo://CDS.VizieR/J/ApJ/885/53
- Title:
- Spectroscopy & HST photometry of galaxy Leo V
- Short Name:
- J/ApJ/885/53
- Date:
- 16 Mar 2022 09:00:29
- Publisher:
- CDS
- Description:
- The ultra-faint dwarf galaxy Leo V has shown both photometric overdensities and kinematic members at large radii, along with a tentative kinematic gradient, suggesting that it may have undergone a close encounter with the Milky Way. We investigate these signs of disruption through a combination of (I) high precision photometry obtained with the Hubble Space Telescope (HST), (II) two epochs of stellar spectra obtained with the Hectochelle Spectrograph on the MMT, and (III) measurements from the Gaia mission. Using the HST data, we examine one of the reported stream-like overdensities at large radii, and conclude that it is not a true stellar stream, but instead a clump of foreground stars and background galaxies. Our spectroscopic analysis shows that one known member star is likely a binary, and challenges the membership status of three others, including two distant candidates that had formerly provided evidence for overall stellar mass loss. We also find evidence that the proposed kinematic gradient across Leo V might be due to small number statistics. We update the systemic proper motion of Leo V, finding ({mu}_{alpha}_/cos{delta},{mu}_{delta}_)= (0.009{+/-}0.560,-0.777{+/-}0 .314)mas/yr, which is consistent with its reported orbit that did not put Leo V at risk of being disturbed by the Milky Way. These findings remove most of the observational clues that suggested Leo V was disrupting; however, we also find new plausible member stars, two of which are located >5 half-light radii from the main body. These stars require further investigation. Therefore, the nature of Leo V still remains an open question.
- ID:
- ivo://CDS.VizieR/J/AJ/153/46
- Title:
- Spectroscopy of candidate members in Taurus
- Short Name:
- J/AJ/153/46
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Previous studies have found that ~1deg^2^ fields surrounding the stellar aggregates in the Taurus star-forming region exhibit a surplus of solar-mass stars relative to denser clusters like IC 348 and the Orion Nebula Cluster. To test whether this difference reflects mass segregation in Taurus or a variation in the initial mass function, we have performed a survey for members of Taurus across a large field (~40deg^2^) that was imaged by the Sloan Digital Sky Survey (SDSS). We obtained optical and near-infrared spectra of candidate members identified with those images and the Two Micron All Sky Survey, as well as miscellaneous candidates that were selected with several other diagnostics of membership. We have classified 22 of the candidates as new members of Taurus, which includes one of the coolest known members (M9.75). Our updated census of members within the SDSS field shows a surplus of solar-mass stars relative to clusters, although it is less pronounced than in the smaller fields toward the stellar aggregates that were surveyed for previously measured mass functions in Taurus. In addition to spectra of our new members, we include in our study near-IR spectra of roughly half of the known members of Taurus, which are used to refine their spectral types and extinctions. We also present an updated set of near-IR standard spectra for classifying young stars and brown dwarfs at M and L types.
