- ID:
- ivo://CDS.VizieR/J/A+A/658/A167
- Title:
- (Al2O3)n, n=1-10, clusters data
- Short Name:
- J/A+A/658/A167
- Date:
- 22 Feb 2022
- Publisher:
- CDS
- Description:
- Aluminium oxide (alumina; Al_2_O_3_) is a promising candidate as a primary dust condensate in the atmospheres of oxygen-rich evolved stars. Therefore, alumina 'seed' particles might trigger the onset of stellar dust formation and of stellar mass loss in the wind. However, the formation of alumina dust grains is not well understood. Aims. We aim to shed light on the initial steps of cosmic dust formation (i.e. nucleation) in oxygen-rich environments via a quantum- chemical bottom-up approach. Starting with an elemental gas-phase composition, we construct a detailed chemical-kinetic network that describes the formation and destruction of aluminium-bearing molecules and dust- forming (Al_2_O_3_)_n_ clusters up to the size of dimers (n=2) coagulating to tetramers (n=4). Intermediary species include the prevalent gas- phase molecules AlO and AlOH as well as Al_x_O_y_ clusters with x=1-5, y=1-6. The resulting extensive network is applied to two model stars, which represent a semi-regular variable and a Mira type, and to different circumstellar gas trajectories, including a non-pulsating outflow and a pulsating model. The growth of larger-sized (Al_2_O_3_)_n_ clusters with n=4-10 is described by the temperature-dependent Gibbs free energies of the most favourable structures (i.e. the global minima clusters) as derived from global optimisation techniques and calculated via density functional theory. We provide energies, bond characteristics, electrostatic properties, and vibrational spectra of the clusters as a function of size, n, and compare these to corundum, which corresponds to the crystalline bulk limit (n to infinity). The circumstellar aluminium gas-phase chemistry in oxygen- rich giants is primarily controlled by AlOH and AlO, which are tightly coupled by the reactions AlO+H_2_, AlO+H_2_O, and their reverse. Models of semi-regular variables show comparatively higher AlO abundances, as well as a later onset and a lower efficiency of alumina cluster formation when compared to Mira-like models. The Mira-like models exhibit an efficient cluster production that accounts for more than 90% of the available aluminium content, which is in agreement with the most recent ALMA observations. Chemical equilibrium calculations fail to predict both the alumina cluster formation and the abundance trends of AlO and AlOH in the asymptotic giant branch dust formation zone. Furthermore, we report the discovery of hitherto unreported global minimum candidates and low-energy isomers for cluster sizes n=7, 9, and 10. A homogeneous nucleation scenario, where Al2O3 monomers are successively added, is energetically viable. However, the formation of the Al2O3 monomer itself represents an energetic bottleneck. Therefore, we provide a bottom-up interpolation of the cluster characteristics towards the bulk limit by excluding the monomer, approximately following an n^(-1/3)^ dependence.
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- ID:
- ivo://CDS.VizieR/J/A+A/580/A113
- Title:
- A low-luminosity type-1 QSO sample. III.
- Short Name:
- J/A+A/580/A113
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report on the optical spectroscopic analysis of a sample of 99 low-luminosity quasi-stellar objects (LLQSOs) at z<=0.06 base the Hamburg/ESO QSO Survey (HES). To better relate the low-redshift active galactic nucleus (AGN) to the QSO population it is important to study samples of the latter type at a level of detail similar to that of the low-redshift AGN. Powerful QSOs, however, are absent at low redshifts due to evolutionary effects and their small space density. Our understanding of the (distant) QSO population is, therefore, significantly limited by angular resolution and sensitivity. The LLQSOs presented here offer the possibility of studying the faint end of this population at smaller cosmological distances and, therefore, in greater detail. In comparing two spectroscopic methods, we aim to establish a reliable activity classification scheme of the LLQSOs sample. Our goal is to enrich our systematic multiwavelength analysis of the AGN/starburst relation in these systems and give a complementary information on this particular sample of LLQSOs from the Hamburg ESO survey. Here, we present results of the analysis of visible wavelength spectroscopy provided by the HES and the 6 Degree Field Galaxy Survey (6dFGS). These surveys use different spectroscopic techniques, long-slit and circular fiber, respectively. These allow us to assess the influence of different apertures on the activity of the LLQSOs using classical optical diagnostic diagrams. We perform a Gaussian fitting of strong optical emission lines and decompose narrow and broad Balmer components. A small number of our LLQSO present no broad component, which is likely to be present but buried in the noise. Two sources show double broad components, whereas six comply with the classic NLS1 requirements. As expected in NLR of broad line AGNs, the [SII]-based electron density values range between 100 and 1000N_e_/cm^3^. Using the optical characteristics of Populations A and B, we find that 50% of our sources with H{beta} broad emission are consistent with the radio-quiet sources definition. The remaining sources could be interpreted as low-luminosity radio-loud quasar. The BPT-based classification renders an AGN/Seyfert activity between 50 to 60%. For the remaining sources, the possible starburst contribution might control the LINER and HII classification. Finally, we discuss the aperture effect as responsible for the differences found between data sets, although variability in the BLR could play a significant role as well.
- ID:
- ivo://CDS.VizieR/J/A+A/615/A172
- Title:
- alpha Cen A and B chemical composition
- Short Name:
- J/A+A/615/A172
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The two solar-like stars alpha Cen A and B have long served as cornerstones for stellar physics in virtue of their immediate proximity, association in a visual binary, and masses that bracket the solar one. The recent detection of a terrestrial planet in the cool, suspected tertiary Proxima Cen now makes the system also of prime interest in the context of planetary studies. It is therefore of fundamental importance to tightly constrain the properties of the individual stellar components. We present a fully self-consistent, line-by-line differential abundance analysis of alpha Cen AB based on high-quality HARPS data. Various line lists are used and analysis strategies implemented to improve the reliability of the results. Abundances of 21 species with a typical precision of 0.02-0.03dex are reported. We find that the chemical composition of the two stars is not scaled solar (e.g., Na and Ni excess, depletion of neutron-capture elements), but that their patterns are strikingly similar, with a mean abundance difference (A-B) with respect to hydrogen of -0.01+/-0.04dex. Much of the scatter may be ascribed to physical effects that are not fully removed through a differential analysis because of the mismatch in parameters between the two components. We derive an age for the system from abundance indicators (e.g., [Y/Mg] and [Y/Al]) that is slightly larger than solar and in agreement with most asteroseismic results. Assuming coeval formation for the three components belonging to the system, this implies an age of about ~6Gyrs for the M dwarf hosting the terrestrial planet Proxima Cen b. After correction for Galactic chemical evolution effects, we find a trend between the abundance ratios and condensation temperature in alpha Cen A akin to that of the Sun. However, taking this finding as evidence for the sequestration of rocky material locked up in planets may be premature given that a clear link between the two phenomena remains to be established. The similarity between the abundance pattern of the binary components argues against the swallowing of a massive planet by one of the stars after the convective zones have shrunk to their present-day sizes.
- ID:
- ivo://CDS.VizieR/J/ApJ/711/619
- Title:
- {alpha}-element abundances in galactic PNe
- Short Name:
- J/ApJ/711/619
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In this paper, we present emission line strengths, abundances, and element ratios (X/O for Ne, S, Cl, and Ar) for a sample of 38 Galactic disk planetary nebulae (PNe) consisting primarily of Peimbert classification Type I. Spectrophotometry for these PNe incorporates an extended optical/near-IR range of 3600-9600{AA} including the [SIII] lines at 9069{AA} and 9532{AA}, setting this relatively large sample apart from typical spectral coverage. We have utilized Emission Line Spectrum Analyzer, a five-level atom abundance routine, to determine Te, Ne, ionization correction factors, and total element abundances, thereby continuing our work toward a uniformly processed set of data. With a compilation of data from >120 Milky Way PNe, we present results from our most recent analysis of abundance patterns in Galactic disk PNe. With a wide range of metallicities, galactocentric distances, and both Type I and non-Type I objects, we have examined the alpha elements against HII regions and blue compact galaxies (H2BCGs) to discern signatures of depletion or enhancement in PNe progenitor stars, particularly the destruction or production of O and Ne.
- ID:
- ivo://CDS.VizieR/J/A+A/580/A17
- Title:
- {alpha}-element abundances of Cepheid stars
- Short Name:
- J/A+A/580/A17
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present new homogeneous measurements of Na, Al, and three alpha-elements (Mg, Si, Ca) for 75 Galactic Cepheids, complemented with Cepheid abundances available in the literature, for a total of 439 stars. The abundances are based on high spectral resolution (R~38000) and high signal-to-noise ratio (S/N~50-300) spectra collected with UVES at ESO VLT. Special attention was given to providing a homogeneous abundance scale for these five elements plus iron. In addition, accurate Galactocentric distances (RG) based on near-infrared photometry are also available for all the Cepheids in the sample. We found that these five elements display well-defined linear radial gradients and modest standard deviations over the entire range of RG. The [element/Fe] abundance ratios are constant across the entire thin disk and over the entire period range; only the Ca radial distribution shows marginal evidence of slopes. These results indicate that the chemical enrichment history of iron and of the quoted elements has been quite similar across the four quadrants of the Galactic thin disk, and very homogenous within the range in age covered by the Cepheids (~10-300Myr). Finally, we also find that Cepheid abundances agree with similar abundances for thin and thick disk dwarf stars, and they follow the typical Mg-Al and Na-O correlations.
- ID:
- ivo://CDS.VizieR/J/ApJS/160/176
- Title:
- alpha-enhanced integrated Lick/IDS spectral indices
- Short Name:
- J/ApJS/160/176
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- All 25 Lick/IDS spectral indices have been computed for the integrated light of simple stellar populations over broad ranges of age and metallicity and with effects from horizontal-branch stars fully implemented. Our models employ {alpha}-enhanced isochrones at the sub-solar metallicity regime, but solar-scaled ones at solar and super-solar metallicity. We have also employed the updated response functions of Houdashelt et al. (2002AAS...201.1405H) at the solar and super-solar metallicity regime, so that we could assess the light-element enhancement phenomena seen from metal-rich early-type galaxies.
- ID:
- ivo://CDS.VizieR/J/ApJ/883/128
- Title:
- alpha & Fe abundances in the giant stellar stream
- Short Name:
- J/ApJ/883/128
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the first measurements of [Fe/H] and [{alpha}/Fe] abundances, obtained using spectral synthesis modeling, for red giant branch stars in M31's giant stellar stream (GSS). The spectroscopic observations, obtained at a projected distance of 17kpc from M31's center, yielded 61 stars with [Fe/H] measurements, including 21 stars with [{alpha}/Fe] measurements, from 112 targets identified as M31 stars. The [Fe/H] measurements confirm the expectation from photometric metallicity estimates that stars in this region of M31's halo are relatively metal rich compared to stars in the Milky Way's inner halo: more than half the stars in the field, including those not associated with kinematically identified substructure, have [Fe/H] abundances >-1.0. The stars in this field are {alpha}-enhanced at lower metallicities, while [{alpha}/Fe] decreases with increasing [Fe/H] above metallicities of [Fe/H]>~-0.9. Three kinematical components have been previously identified in this field: the GSS, a second kinematically cold feature of unknown origin, and M31's kinematically hot halo. We compare probabilistic [Fe/H] and [{alpha}/Fe] distribution functions for each of the components. The GSS and the second kinematically cold feature have very similar abundance distributions, while the halo component is more metal poor. Although the current sample sizes are small, a comparison of the abundances of stars in the GSS field with abundances of M31 halo and dSph stars from the literature indicate that the progenitor of the stream was likely more massive, and experienced a higher efficiency of star formation, than M31's existing dSphs or the dEs NGC147 and NGC185.
- ID:
- ivo://CDS.VizieR/J/other/RAA/16.G10
- Title:
- [alpha/Fe] ratios from LAMOST spectra
- Short Name:
- J/other/RAA/16.G
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The [{alpha}/Fe] ratios in stars are good tracers to probe the formation history of stellar populations and the chemical evolution of the Galaxy. The spectroscopic survey of LAMOST provides a good opportunity to determine [{alpha}/Fe] of millions of stars in the Galaxy. We present a method of measuring the [{alpha}/Fe] ratios from LAMOST spectra using the template-matching technique of the LSP3 pipeline. We use three test samples of stars selected from the ELODIE and MILES libraries, as well as the LEGUE survey to validate our method. Based on the test results, we conclude that our method is valid for measuring [{alpha}/Fe] from low-resolution spectra acquired by the LAMOST survey. Within the range of the stellar parameters Teff=[5000, 7500]K, logg=[1.0, 5.0]dex and [Fe/H]=[-1.5, +0.5]dex, our [{alpha}/Fe] measurements are consistent with values derived from high-resolution spectra, and the accuracy of our [{alpha}/Fe] measurements from LAMOST spectra is better than 0.1dex with spectral signal-to-noise higher than 20.
- ID:
- ivo://CDS.VizieR/J/MNRAS/511/4724
- Title:
- alpha Leo spectro-interferometric observation
- Short Name:
- J/MNRAS/511/4724
- Date:
- 15 Mar 2022 08:32:16
- Publisher:
- CDS
- Description:
- Differential Interferometry allows to obtain the differential visibility and phase, in addition to the spectrum. The differential phase contains important information about the structure and motion of stellar photosphere such as stellar spots and non-radial pulsations, and particularly the rotation. Thus, this interferometric observable strongly helps to constrain the stellar fundamental parameters of fast rotators. The spectroastrometry mainly uses the photocentre displacements, which is a first approximation of the differential phase, and is applicable only for unresolved or marginally objects. We study here the sensitivity of relevant stellar parameters to the simulated photocentres using the scirocco code: a semi-analytical algorithm dedicated to fast rotators, applied to two theoretical modelling stars based on Achernar and Regulus, in order to classify the importance of these parameters and their impact on the modelling. We compare our simulations with published VLTI/AMBER data. This work sets the limits of application of photocentre displacements to fast rotators, and under which conditions we can use the photocentres and/or the differential phase, through a pre-established physical criterion. To validate our theoretical study, we apply our method of analysis on observed data of the edge-on fast rotator Regulus. For unresolved targets, with a visibility V~1, the photocentre can constrain the main stellar fundamental parameters of fast rotators, whereas from marginally resolved objects (0.8<=V<1), mainly the rotation axis position angle (PA_rot_) can be directly deduced from the vectorial photocentre displacement, which is very important for young cluster studies.
- ID:
- ivo://CDS.VizieR/J/AN/319/201
- Title:
- Alpha Per cluster low-mass members. I.
- Short Name:
- J/AN/319/201
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Following the work of Randich et al. (1996A&A...305..785R) involving a ROSAT raster scan survey of the {alpha} Persei open cluster, we present here the results of a photometric/spectroscopic program examining the possible optical counterparts to a group of 73 X-ray sources in the raster survey which were not matched to catalogued stars. Of the 73 sources investigated, ~40 have an optical counterpart with photometry acceptable for cluster membership and ~20 of these also have radial velocities consistent with membership. We discuss the X-ray properties of these potential new members and why they may not have been identified in earlier membership surveys of this cluster.
