- ID:
- ivo://CDS.VizieR/VI/109
- Title:
- Population Synthesis Models at very low metallicities
- Short Name:
- VI/109
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Using new sets of stellar evolution models at very low metallicities (Z=10^-7^; 10^-5^) and previously published grids we examine spectral properties of the ionising continua, the Lyman-break, and the Ly{alpha} and He1640 recombination lines in starbursts. The metallicity dependence of these properties, especially the transition from primordial galaxies (Population III) to currently observed metallicities, is examined for various IMFs and star formation histories.
Number of results to display per page
Search Results
- ID:
- ivo://CDS.VizieR/J/A+A/641/A142
- Title:
- Post-AGB candidates in LMC and SMC SALT spectra
- Short Name:
- J/A+A/641/A142
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We selected a sample of post-AGB candidates in the Magellanic Clouds on the basis of their near- and mid-infrared colour characteristics. Fifteen of the most optically bright post-AGB candidates were observed with the South African Large Telescope in order to determine their stellar parameters and thus to validate or discriminate their nature as post-AGB objects in the Magellanic Clouds. The spectral types of absorption-line objects were estimated according to the MK classification, and effective temperatures were obtained by means of stellar atmosphere modelling. Emission-line objects were classified on the basis of the fluxes of the emission lines and the presence of the continuum. Out of 15 observed objects, only 4 appear to be genuine post-AGB stars (27%). In the SMC, 1 out of 4 is post-AGB, and in the LMC, 3 out 11 are post-AGB objects. Thus, we can conclude that the selected region in the colour-colour diagram, while selecting the genuine post-AGB objects, overlaps severely with other types of objects, in particular young stellar objects and planetary nebulae. Additional classification criteria are required to distinguish between post-AGB stars and other types of objects. In particular, photometry at far-IR wavelengths would greatly assist in distinguishing young stellar objects from evolved ones. On the other hand, we showed that the low-resolution optical spectra appear to be sufficient to determine whether the candidates are post-AGB objects.
- ID:
- ivo://CDS.VizieR/J/MNRAS/439/2211
- Title:
- Post-AGB/RGB and YSOs in SMC
- Short Name:
- J/MNRAS/439/2211
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have carried out a search for optically visible post-asymptotic giant branch (post-AGB) candidates in the Small Magellanic Cloud (SMC). First, we used mid-IR observations from the Spitzer Space Telescope to select optically visible candidates with excess mid-IR flux and then we obtained low-resolution optical spectra for 801 of the candidates. After removing poor-quality spectra and contaminants, such as M-stars, C-stars, planetary nebulae, quasi-stellar objects and background galaxies, we ended up with a final sample of 63 high-probability post-AGB/RGB candidates of A-F spectral type. From the spectral observations, we estimated the stellar parameters: effective temperature (T_eff_), surface gravity (logg) and metallicity ([Fe/H]). We also estimated the reddening and deduced the luminosity using the stellar parameters combined with photometry. For the post-AGB/RGB candidates, we found that the metallicity distribution peaks at [Fe/H]~-1.00dex. Based on a luminosity criterion, 42 of these 63 sources were classified as post-red giant branch (post-RGB) candidates and the remaining 21 as post-AGB candidates. From the spectral energy distributions, we were able to infer that 6 of the 63 post-AGB/RGB candidates have a surrounding circumstellar shell suggesting that they are single stars, while 27 of the post-AGB/RGB candidates have a surrounding disc, suggesting that they lie in binary systems. For the remaining 30 post-AGB/RGB candidates the nature of the circumstellar environment was unclear. Variability is displayed by 38 of the 63 post-AGB/RGB candidates with the most common variability types being the Population II Cepheids (including RV-Tauri stars) and semiregular variables. This study has also revealed a new RV Tauri star in the SMC, J005107.19-734133.3, which shows signs of s-process enrichment. From the numbers of post-AGB/RGB stars in the SMC, we were able to estimate evolutionary rates. We find that the number of post-AGB and post-RGB candidates that we have identified are in good agreement with the stellar evolution models with some mass-loss in the post-AGB phase and a small amount of re-accretion in the lower luminosity post-RGB phase. This study also resulted in a new sample of 40 young stellar objects (YSOs) of A-F spectral type. The 40 YSO candidates for which we could estimate stellar parameters are luminous and of high mass (~3-10M_{sun}_). They lie on the cool side of the usually adopted birthline in the HR-diagram. This line separates visually obscured protostars from optically visible pre-main-sequence stars, meaning that our YSO candidates have become optically visible in the region of the HR diagram usually reserved for obscured protostars. Additionally, we also identified a group of 63 objects whose spectra are dominated by emission lines and in some cases, a significant UV continuum. These objects are very likely to be either hot post-AGB/RGB candidates or luminous YSOs.
- ID:
- ivo://CDS.VizieR/J/ApJ/737/L7
- Title:
- Post-AGB star SAO 40039 equivalent widths
- Short Name:
- J/ApJ/737/L7
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have conducted an LTE abundance analysis for SAO 40039, a warm post-AGB star whose spectrum is known to show surprisingly strong HeI lines for its effective temperature and has been suspected of being H-deficient and He-rich. High-resolution optical spectra are analyzed using a family of model atmospheres with different He/H ratios. Atmospheric parameters are estimated from the ionization equilibrium set by neutral and singly ionized species of Fe and Mg, the excitation of FeI and FeII lines, and the wings of the Paschen lines. On the assumption that the HeI lines are of photospheric and not chromospheric origin, a He/H ratio of approximately unity is found by imposing the condition that the adopted He/H ratio of the model atmosphere must equal the ratio derived from the observed HeI triplet lines at 5876, 4471, and 4713{AA}, and singlet lines at 4922 and 5015{AA}. Using the model with the best-fitting atmospheric parameters for this He/H ratio, SAO 40039 is confirmed to exhibit mild dust-gas depletion, i.e., the star has an atmosphere deficient in elements of high condensation temperature. The star appears to be moderately metal-deficient with [Fe/H]=-0.4dex. But the star's intrinsic metallicity as estimated from Na, S, and Zn, elements of a low condensation temperature, is [Fe/H]_o_~-0.2 ([Fe/H]_o_ refers to the star's intrinsic metallicity). The star is enriched in N and perhaps O as well, changes reflecting the star's AGB past and the event that led to He enrichment.
- ID:
- ivo://CDS.VizieR/J/ApJ/862/2
- Title:
- Post-starburst galaxy ages from SDSS
- Short Name:
- J/ApJ/862/2
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Detailed modeling of the recent star formation histories (SFHs) of post-starburst (or "E+A") galaxies is impeded by the degeneracy between the time elapsed since the starburst ended (post-burst age), the fraction of stellar mass produced in the burst (burst strength), and the burst duration. To resolve this issue, we combine GALEX ultraviolet photometry, SDSS photometry and spectra, and new stellar population synthesis models to fit the SFHs of 532 post-starburst galaxies. In addition to an old stellar population and a recent starburst, 48% of the galaxies are best fit with a second recent burst. Lower stellar mass galaxies (logM*/M_{sun}_<10.5) are more likely to experience two recent bursts, and the fraction of their young stellar mass is more strongly anticorrelated with their total stellar mass. Applying our methodology to other, younger post-starburst samples, we identify likely progenitors to our sample and examine the evolutionary trends of molecular gas and dust content with post-burst age. We discover a significant (4{sigma}) decline, with a 117-230Myr characteristic depletion time, in the molecular gas to stellar mass fraction with the post-burst age. The implied rapid gas depletion rate of 2-150M_{sun}_/yr cannot be due to current star formation, given the upper limits on the current star formation rates in these post- starbursts. Nor are stellar winds or supernova feedback likely to explain this decline. Instead, the decline points to the expulsion or destruction of molecular gas in outflows, a possible smoking gun for active galactic nucleus feedback.
- ID:
- ivo://CDS.VizieR/J/AJ/142/153
- Title:
- Precision spectrophotometry at the level of 0.1%
- Short Name:
- J/AJ/142/153
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Accurate relative spectrophotometry is critical for many science applications. Small wavelength-scale residuals in the flux calibration can significantly impact the measurements of weak emission and absorption features in the spectra. Using Sloan Digital Sky Survey data, we demonstrate that the average spectra of carefully selected red-sequence galaxies can be used as a spectroscopic standard to improve the relative spectrophotometry precision to 0.1% on small wavelength scales (from a few to hundreds of Angstroms). We achieve this precision by comparing stacked spectra across tiny redshift intervals. The redshift intervals must be small enough that any systematic stellar population evolution is minimized and is less than the spectrophotometric uncertainty. This purely empirical technique does not require any theoretical knowledge of true galaxy spectra. It can be applied to all large spectroscopic galaxy redshift surveys that sample a large number of galaxies in a uniform population.
- ID:
- ivo://CDS.VizieR/J/A+A/605/A86
- Title:
- Pre-main sequence stars VLT/X-Shooter spectra
- Short Name:
- J/A+A/605/A86
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Studies of the formation and evolution of young stars and their disks rely on knowledge of the stellar parameters of the young stars. The derivation of these parameters is commonly based on comparison with photospheric template spectra. Furthermore, chromospheric emission in young active stars impacts the measurement of mass accretion rates, a key quantity for studying disk evolution. Here we derive stellar properties of low-mass (M*~<2M_{sun}_) pre-main sequence stars without disks, which represent ideal photospheric templates for studies of young stars. We also use these spectra to constrain the impact of chromospheric emission on the measurements of mass accretion rates. The spectra are reduced, flux-calibrated, and corrected for telluric absorption, and are made available to the community. We derive the spectral type for our targets by analyzing the photospheric molecular features present in their VLT/X-Shooter spectra by means of spectral indices and comparison of the relative strength of photospheric absorption features. We also measure effective temperature, gravity, projected rotational velocity, and radial velocity from our spectra by fitting them with synthetic spectra with the ROTFIT tool.The targets have negligible extinction (AV<0.5mag) and spectral type from G5 to K6, and from M6.5 to M8. They thus complement the library of photospheric templates presented in our previous paper. We perform synthetic photometry on the spectra to derive the typical colors of young stars in different filters. We measure the luminosity of the emission lines present in the spectra and estimate the noise due to chromospheric emission in the measurements of accretion luminosity in accreting stars. Results. We provide a calibration of the photospheric colors of young pre-main sequence stars as a function of their spectral type in a set of standard broad-band optical and near-infrared filters. The logarithm of the noise on the accretion luminosity normalized to the stellar luminosity is roughly constant and equal to -2.3 for targets with masses larger than 1 solar mass, and decreases with decreasing temperatures for lower-mass stars. For stars with masses of 1.5Msun and ages of 1-5Myr, the chromospheric noise converts to a limit of measurable mass accretion rates of 3x10^-10^M_{sun}_/yr. The limit on the mass accretion rate set by the chromospheric noise is of the order of the lowest measured values of mass accretion rates in Class II objects.
- ID:
- ivo://CDS.VizieR/J/ApJ/745/74
- Title:
- Pre-maximum spectra of type Ia SNe
- Short Name:
- J/ApJ/745/74
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The presence of unburned material in the ejecta of normal Type Ia supernovae (SNe Ia) is investigated using early-time spectroscopy obtained by the Carnegie Supernova Project. The tell-tale signature of pristine material from a C+O white dwarf progenitor star is the presence of carbon, as oxygen is also a product of carbon burning. The most prominent carbon lines in optical spectra of SNe Ia are expected to arise from CII. We find that at least 30% of the objects in the sample show an absorption at {approx}6300{AA} which is attributed to CII{lambda}6580. An alternative identification of this absorption as H{alpha} is considered to be unlikely. These findings imply a larger incidence of carbon in SNe Ia ejecta than previously noted.
- ID:
- ivo://CDS.VizieR/J/A+A/530/A50
- Title:
- Principal Component Analysis of QSO UV spectra
- Short Name:
- J/A+A/530/A50
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- From a principal component analysis (PCA) of 78 z~3 high-quality quasar spectra in the SDSS-DR7 we derive the principal components that characterize the QSO continuum over the full available wavelength range. The shape of the mean continuum is similar to that measured at low-z (z~1), but the equivalent width of the emission lines is larger at low redshift. We calculate the correlation between fluxes at different wavelengths and find that the emission line fluxes in the red part of the spectrum are correlated with those in the blue part. We construct a projection matrix to predict the continuum in the Lyman{alpha} forest from the red part of the spectrum.
2020. Probing the LHS Catalog
- ID:
- ivo://CDS.VizieR/J/PASP/109/849
- Title:
- Probing the LHS Catalog
- Short Name:
- J/PASP/109/849
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present moderate resolution spectroscopy of 111 cool dwarf stars to supplement the observations we have already presented in the Palomar/MSU Nearby-Star Spectroscopic Survey. The sample consists of 71 suspected nearby stars added to the Preliminary Third Catalog of Nearby Stars since 1991 as well as 40 faint red stars selected from the LHS catalog. The study was aimed at identifying interesting red dwarfs, particularly new nearby, ultracool dwarfs, and very metal-poor stars. The observations were made using the Palomar 60-inch, the Hale 200-inch and the Las Campanas 100-inch telescopes between June 1995 and January 1996. The spectral resolution is approximately 3 Angstroms per pixel with wavelength coverage from 6200 to 7500 Angstroms. Table 2 contains bandstrengths for TiO, CaH, and CaOH indices.
