Context. Among intermediate-mass and massive stars, Be stars are the fastest rotators in the main sequence (MS) and, as such, these stars are a cornerstone to validate models of structure and ...evolution of rotating stars. Several phenomena, however, induce under- or overestimations either of their apparent V sini, or true velocity V. Aims. In the present contribution we aim at obtaining distributions of true rotational velocities corrected for systematic effects induced by the rapid rotation itself, macroturbulent velocities, and binarity. Methods. We study a set of 233 Be stars by assuming they have inclination angles distributed at random. We critically discuss the methods of Cranmer and Lucy-Richardson, which enable us to transform a distribution of projected velocities into another distribution of true rotational velocities, where the gravitational darkening effect on the V sini parameter is considered in different ways. We conclude that iterative algorithm by Lucy-Richardson responds at best to the purposes of the present work, but it requires a thorough determination of the stellar fundamental parameters. Results. We conclude that once the mode of ratios of the true velocities of Be stars attains the value V/V sub(c)Asymp totically = to 0.77 in the main-sequence (MS) evolutionary phase, it remains unchanged up to the end of the MS lifespan. The statistical corrections found on the distribution of ratios V/V sub(c) for overestimations of V sini, due to macroturbulent motions and binarity, produce a shift of this distribution toward lower values of V/V sub(c) when Be stars in all MS evolutionary stages are considered together. The mode of the final distribution obtained is at V/V sub(c)Asymp totically = to 0.65. This distribution has a nearly symmetric distribution and shows that the Be phenomenon is characterized by a wide range of true velocity ratios 0.3 <, ~ V/V sub(c)<, ~ 0.95. It thus suggests that the probability that Be stars are critical rotators is extremely low. Conclusions. The corrections attempted in the present work represent an initial step to infer indications about the nature of the Be-star surface rotation that will be studied in the second paper of this series.
Context. The launches of the MOST, CoRoT, and Kepler missions opened up a new era in asteroseismology, the study of stellar interiors via interpretation of pulsation patterns observed at the surfaces ...of large groups of stars. These space missions deliver a huge amount of high-quality photometric data suitable to study numerous pulsating stars. Aims. Our ultimate goal is a detection and analysis of an extended sample of γ Dor-type pulsating stars with the aim to search for observational evidence of non-uniform period spacings and rotational splittings of gravity modes in main-sequence stars typically twice as massive as the Sun. This kind of diagnostic can be used to deduce the internal rotation law and to estimate the amount of rotational mixing in the near core regions. Methods. We applied an automated supervised photometric classification method to select a sample of 69 Gamma Doradus (γ Dor) candidate stars. We used an advanced method to extract the Kepler light curves from the pixel data information using custom masks. For 36 of the stars, we obtained high-resolution spectroscopy with the HERMES spectrograph installed at the Mercator telescope. The spectroscopic data are analysed to determine the fundamental parameters like Teff, log g, vsini, and M/H. Results. We find that all stars for which spectroscopic estimates of Teff and log g are available fall into the region of the HR diagram, where the γ Dor and δ Sct instability strips overlap. The stars cluster in a 700 K window in effective temperature; log g measurements suggest luminosity class IV-V, i.e. sub-giant or main-sequence stars. From the Kepler photometry, we identify 45 γ Dor-type pulsators, 14 γ Dor/δ Sct hybrids, and 10 stars, which are classified as “possibly γ Dor/δ Sct hybrid pulsators”. We find a clear correlation between the spectroscopically derived vsini and the frequencies of independent pulsation modes. Conclusions. We have shown that our photometric classification based on the light curve morphology and colour information is very robust. The results of spectroscopic classification perfectly agree with the photometric classification. We show that the detected correlation between vsini and frequencies has nothing to do with rotational modulation of the stars but is related to their stellar pulsations. Our sample and frequency determinations offer a good starting point for seismic modelling of slow to moderately rotating γ Dor stars.
The HERMES high-resolution spectrograph project aims at exploiting the specific potential of small but flexible telescopes in observational astrophysics. The optimised optical design of the ...spectrograph is based on the well-proven concept of white-pupil beam folding for high-resolution spectroscopy. In this contribution we present the complete project, including the spectrograph design and procurement details, the telescope adaptor and calibration unit, the detector system, as well as the optimised data-reduction pipeline. We present a detailed performance analysis to show that the spectrograph performs as specified both in optical quality and in total efficiency. With a spectral resolution of 85 000 (63 000 for the low-resolution fibre), a spectral coverage from 377 to 900 nm in a single exposure and a peak efficiency of 28%, HERMES proves to be an ideal instrument for building up time series of high-quality data of variable (stellar) phenomena.
We present a study of spectral diagnostics available from optical spectra with R = 17 000 obtained with the VLT/Giraffe HR15n setup, using observations from the Gaia-ESO Survey, on the γ Vel young ...cluster, with the purpose of classifying these stars and finding their fundamental parameters. We define several spectroscopic indices, sampling the amplitude of TiO bands, the Hα line core and wings, and temperature- and gravity-sensitive sets of lines, each useful as a Teff or log g indicator over a limited range of stellar spectral types. Hα line indices are also useful as chromospheric activity or accretion indicators. Furthermore, we use all indices to define additional global Teff- and log g-sensitive indices τ and γ, valid for the entire range of types in the observed sample. We find a clear difference between gravity indices of main-sequence and pre-main-sequence stars, as well as a much larger difference between these and giant stars. The potentially great usefulness of the (γ,τ) diagram as a distance-independent age measurement tool for young clusters is discussed. We discuss the effect on the defined indices of classical T Tauri star veiling, which is however detected in only a few stars in the present sample. Then, we present tests and calibrations of these indices, on the basis of both photometry and literature reference spectra, from the UVES Paranal Observatory Projectand the ELODIE 3.1 Library. The known properties of these stars, spanning a wide range of stellar parameters, enable us to obtain a good understanding of the performances of our new spectral indices. For non-peculiar stars with known temperature, gravity, and metallicity, we are able to calibrate quantitatively our indices, and derive stellar parameters for a wide range of stellar types. To this aim, a new composite index is defined, providing a good metallicity indicator. The ability of our indices to select peculiar, or otherwise rare classes of stars is also established. For pre-main-sequence stars outside the parameter range of the ELODIE dataset, index calibration relies on model isochrones. We check our calibrations against current Gaia-ESO UVES results, plus a number of Survey benchmark stars, and also against Gaia-ESO observations of young clusters, which contribute to establishing the good performance of our method across a wide range of stellar parameters. Our gravity determination for late-type PMS stars is found to be accurate enough to let us obtain gravity-based age estimates for PMS clusters. Finally, our gravity determinations support the existence of an older pre-main-sequence population in the γ Vel sky region, in agreement with evidence obtained from the lithium depletion pattern of the same stars.
A sample of 97 galactic field Be stars were studied by taking into account the effects induced by the fast rotation on their fundamental parameters. All program stars were observed in the BCD ...spectrophotometric system in order to minimize the perturbations produced by the circumstellar environment on the spectral photospheric signatures. This is one of the first attempts at determining stellar masses and ages by simultaneously using model atmospheres and evolutionary tracks, both calculated for rotating objects. The stellar ages (t) normalized to the respective inferred time that each rotating star can spend in the main sequence phase (tMS) reveal a mass-dependent trend. This trend shows that: a) there are Be stars spread over the whole interval 0 QQQ ? t/tMS QQQ ? 1 of the main sequence evolutionary phase; b) the distribution of points in the (t/tMS, M/M QQQ ?) diagram indicates that in massive stars (M QQQ ? 12 M QQQ ?) the Be phenomenon is present at smaller t/tMS age ratios than for less massive stars (M QQQ ? 12 M QQQ ?). This distribution can be due to: i) higher mass-loss rates in massive objects, which can act to reduce the surface fast rotation; ii) circulation time scales to transport angular momentum from the core to the surface, which are longer the lower the stellar mass.
Gaia Data Release 3 Frémat, Y.; Royer, F.; Marchal, O. ...
Astronomy and astrophysics (Berlin),
06/2023, Letnik:
674
Journal Article, Web Resource
Recenzirano
Odprti dostop
Context.
The third release of the
Gaia
catalogue contains radial velocities for 33 812 183 stars with effective temperatures ranging from 3100 K to 14 500 K. The measurements are based on the ...comparison of the spectra observed with the Radial Velocity Spectrometer (RVS; wavelength coverage: 846–870 nm, median resolving power: 11 500) to synthetic data broadened to the adequate along-scan line spread function. The additional line-broadening, fitted as it would only be due to axial rotation, is also produced by the pipeline and is available in the catalogue (field name
vbroad
).
Aims.
We describe the properties of the line-broadening information extracted from the RVS and published in the catalogue, and analyse the limitations imposed by the adopted method, wavelength range, and instrument.
Methods.
We used simulations to express the link between the line-broadening measurement provided in
Gaia
Data Release 3 and
V
sin
i
. We then compared the observed values to the measurements published by various catalogues and surveys (GALAH, APOGEE, LAMOST, etc.).
Results.
While we recommend caution in the interpretation of the
vbroad
measurement, we also find a reasonable general agreement of the
Gaia
Data Release 3 line-broadening values and values in other catalogues. We discuss and establish the validity domain of the published
vbroad
values. The estimate tends to be overestimated at the lower
V
sin
i
end, and at
T
eff
> 7500 K its quality and significance degrade rapidly when
G
RVS
> 10. Despite all the known and reported limitations, the
Gaia
Data Release 3 line-broadening catalogue contains measurements obtained for 3 524 677 stars with
T
eff
ranging from 3500 to 14 500 K, and
G
RVS
< 12. It gathers the largest stellar sample ever considered for the purpose, and allows a first mapping of the
Gaia
line-broadening parameter across the Hertzsprung-Russel diagram.
Gaia Data Release 3 Blomme, R.; Frémat, Y.; Sartoretti, P. ...
Astronomy and astrophysics (Berlin),
06/2023, Letnik:
674
Journal Article, Web Resource
Recenzirano
Odprti dostop
Context.
The second
Gaia
data release, DR2, contained radial velocities of stars with effective temperatures up to
T
eff
= 6900 K. The third data release,
Gaia
DR3, extends this up to
T
eff
= 14 ...500 K.
Aims.
We derive the radial velocities for hot stars (i.e., in the
T
eff
= 6900 − 14 500 K range) from data obtained with the Radial Velocity Spectrometer (RVS) on board
Gaia
.
Methods.
The radial velocities were determined by the standard technique of measuring the Doppler shift of a template spectrum that was compared to the observed spectrum. The RVS wavelength range is very limited. The proximity to and systematic blueward offset of the calcium infrared triplet to the hydrogen Paschen lines in hot stars can result in a systematic offset in radial velocity. For the hot stars, we developed a specific code to improve the selection of the template spectrum, thereby avoiding this systematic offset.
Results.
With the improved code, and with the correction we propose to the DR3 archive radial velocities, we obtain values that agree with reference values to within 3 km s
−1
(in median). Because of the required S/N for applying the improved code, the hot star radial velocities in DR3 are mostly limited to stars with a magnitude in the RVS wavelength band ≤12 mag.
Context.Gaia Data Release 3 (DR3) contains the first release of magnitudes estimated from the integration of Radial Velocity Spectrometer (RVS) spectra for a sample of about 32.2 million stars ...brighter than GRVS ∼ 14 mag (or G ∼ 15 mag). Aims: In this paper, we describe the data used and the approach adopted to derive and validate the GRVS magnitudes published in DR3. We also provide estimates of the GRVS passband and associated GRVS zero-point. Methods: We derived GRVS photometry from the integration of RVS spectra over the wavelength range from 846 to 870 nm. We processed these spectra following a procedure similar to that used for DR2, but incorporating several improvements that allow a better estimation of GRVS. These improvements pertain to the stray-light background estimation, the line spread function calibration, and the detection of spectra contaminated by nearby relatively bright sources. We calibrated the GRVS zero-point every 30 h based on the reference magnitudes of constant stars from the HIPPARCOS catalogue, and used them to transform the integrated flux of the cleaned and calibrated spectra into epoch magnitudes. The GRVS magnitude of a star published in DR3 is the median of the epoch magnitudes for that star. We estimated the GRVS passband by comparing the RVS spectra of 108 bright stars with their flux-calibrated spectra from external spectrophotometric libraries. Results: The GRVS magnitude provides information that is complementary to that obtained from the G, GBP, and GRP magnitudes, which is useful for constraining stellar metallicity and interstellar extinction. The median precision of GRVS measurements ranges from about 0.006 mag for the brighter stars (i.e. with 3.5≲ GRVS ≲6.5 mag) to 0.125 mag at the faint end. The derived GRVS passband shows that the effective transmittance of the RVS is approximately 1.23 times better than the pre-launch estimate.
We present a spectroscopic analysis of the GIRAFFE and UVES data collected by the
Gaia
-ESO survey for the young open cluster NGC 3293. Archive spectra from the same instruments obtained in the ...framework of the ‘VLT-FLAMES survey of massive stars’ are also analysed. Atmospheric parameters, non-local thermodynamic equilibrium (LTE) chemical abundances for six elements (He, C, N, Ne, Mg, and Si), or variability information are reported for a total of about 160 B stars spanning a wide range in terms of spectral types (B1 to B9.5) and rotation rate (up to 350 km s
−1
). Our analysis leads to about a five-fold increase in the number of cluster members with an abundance determination and it characterises the late B-star population in detail for the first time. We take advantage of the multi-epoch observations on various timescales and a temporal baseline, sometimes spanning ∼15 years, to detect several binary systems or intrinsically line-profile variables. A deconvolution algorithm is used to infer the current, true (deprojected) rotational velocity distribution. We find a broad, Gaussian-like distribution peaking around 200–250 km s
−1
. Although some stars populate the high-velocity tail, most stars in the cluster appear to rotate far from critical. We discuss the chemical properties of the cluster, including the low occurrence of abundance peculiarities in the late B stars and the paucity of objects showing CN-cycle burning products at their surface. We argue that the former result can largely be explained by the inhibition of diffusion effects because of fast rotation, while the latter is generally in accord with the predictions of single-star evolutionary models under the assumption of a wide range of initial spin rates at the onset of main-sequence evolution. However, we find some evidence for a less efficient mixing in two quite rapidly rotating stars that are among the most massive objects in our sample. Finally, we obtain a cluster age of ∼20 Myr through a detailed, star-to-star correction of our results for the effect of stellar rotation (e.g., gravity darkening). This is significantly older than previous estimates from turn-off fitting that fully relied on classical, non-rotating isochrones.
Context.
Studies of the correlation between different diffuse interstellar bands (DIBs) are important for exploring their origins. However, the
Gaia
–RVS spectral window between 846 and 870 nm ...contains few DIBs, the strong DIB at 862 nm being the only convincingly confirmed one.
Aims.
Here we attempt to confirm the existence of a broad DIB around 864.8 nm and estimate its characteristics using the stacked
Gaia
–RVS spectra of a large number of stars. We study the correlations between the two DIBs at 862 nm (
λ
862) and 864.8 nm (
λ
864.8), as well as the interstellar extinction.
Methods.
We obtained spectra of the interstellar medium (ISM) absorption by subtracting the stellar components using templates constructed from real spectra at high Galactic latitudes with low extinctions. We then stacked the ISM spectra in Galactic coordinates (ℓ,
b
) – pixelized by the HEALPix scheme – to measure the DIBs. The stacked spectrum is modeled by the profiles of the two DIBs, Gaussian for
λ
862 and Lorentzian for
λ
864.8, and a linear continuum. We report the fitted central depth (CD), central wavelength, equivalent width (EW), and their uncertainties for the two DIBs.
Results.
We obtain 8458 stacked spectra in total, of which 1103 (13%) have reliable fitting results after applying numerous conservative filters. This work is the first of its kind to fit and measure
λ
862 and
λ
864.8 simultaneously in cool-star spectra. Based on these measurements, we find that the EWs and CDs of
λ
862 and
λ
864.8 are well correlated with each other, with Pearson coefficients (
r
p
) of 0.78 and 0.87, respectively. The full width at half maximum (FWHM) of
λ
864.8 is estimated as 1.62 ± 0.33 nm which compares to 0.55 ± 0.06 nm for
λ
862. We also measure the vacuum rest-frame wavelength of
λ
864.8 to be
λ
0
= 864.53 ± 0.14 nm, smaller than previous estimates.
Conclusions.
We find solid confirmation of the existence of the DIB around 864.8 nm based on an exploration of its correlation with
λ
862 and estimation of its FWHM. The DIB
λ
864.8 is very broad and shallow. That at
λ
862 correlates better with
E
(
BP
−
RP
) than
λ
864.8. The profiles of the two DIBs could strongly overlap with each other, which contributes to the skew of the
λ
862 profile.
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