Abstract
We study the rotation–activity correlations (RACs) in a sample of stars from spectral type dK4 to dM4. We study RACs using chromospheric data and coronal data. We study the Ca
ii
line ...surface fluxes-
RACs. We fit the RACs with linear homoscedastic and heteroscedastic regression models. We find that these RACs differ substantially from one spectral sub-type to another. For dM3 and dM4 stars, we find that the RACs cannot be described by a simple model, but instead that there may exist two distinct RAC behaviors for the low-activity and the high-activity stellar sub-samples, respectively. Although these results are preliminary and will need confirmation, the data suggest that these distinct RACs may be associated with different dynamo regimes. We also study
as a function of the Rossby number
R
0
. We find (i) for dK4 stars,
as a function of
R
0
agrees well with previous results for F-G-K stars and (ii) in dK6, dM2, dM3, and dM4 stars, at a given
R
0
, the values of
lie at a factor of 3, 10, 20, and 90, respectively, below the F-G-K RAC. Our results suggest a significant decrease in the efficiency of the dynamo mechanism(s) as regards chromospheric heating before and at dM3, i.e., before and at the transition to complete convection. We also show that the ratio of coronal heating to chromospheric heating
L
X
/
L
HK
increases by a factor of 100 between dK4 and dM4 stars.
The reliable determination of rotation-activity correlations (RACs) depends on precise measurements of the following stellar parameters: T sub(eff), parallax, radius, metallicity, and rotational ...speed v sin i. In this paper, our goal is to focus on the determination of these parameters for a sample of K and M dwarfs. In a future paper (Paper II), we will combine our rotational data with activity data in order to construct RACs. Here, we report on a determination of effective temperatures based on the (R-I)C color from the calibrations of Mann et al. and Kenyon & Hartmann for four samples of late-K, dM2, dM3, and dM4 stars. We also determine stellar parameters (T sub(eff), log(g), and M/H) using the principal component analysis-based inversion technique for a sample of 105 late-K dwarfs. We compile all effective temperatures from the literature for this sample. We determine empirical radius-M/H correlations in our stellar samples. This allows us to propose new effective temperatures, stellar radii, and metallicities for a large sample of 612 late-K and M dwarfs. Our mean radii agree well with those of Boyajian et al. We analyze HARPS and SOPHIE spectra of 105 late-K dwarfs, and we have detected v sin i in 92 stars. In combination with our previous v sin i measurements in M and K dwarfs, we now derive P/sin i measures for a sample of 418 K and M dwarfs. We investigate the distributions of P/sin i, and we show that they are different from one spectral subtype to another at a 99.9% confidence level.
Abstract
We report on a detailed abundance study of six bright, mostly southern, slowly rotating late B-stars: HD 1279 (B8III), HD 99803 (B9V), HD 123445 (B9V), HD 147550 (B9V), HD 171961 (B8III), ...and HD 202671 (B5II/III), hitherto reported as normal stars. We compare them to the two classical HgMn stars
μ
Lep and
β
Scl, and to the superficially normal star
ν
Cap. In the spectra of the six stars, the Hg
ii
line at 3984 Å line is clearly seen and numerous lines of P, Ti, Mn, Fe, Ga, Sr, Y, and Zr appear to be strong absorbers. A comparison of newly acquired and archival spectra of these objects with a grid of synthetic spectra for selected unblended lines reveals large overabundances of P, Ti, Cr, Mn, Sr, Y, Zr, Ba, Pt, and Hg, and underabundances of He, Mg, Sc, and Ni. The effective temperatures, surface gravities, low projected rotational velocities, and the peculiar abundance patterns of the six investigated stars show that they are new chemically peculiar stars, mostly new HgMn stars, and are reclassified as such. The evolutionary status of these stars has been inferred, and their ages and masses estimated. The two most massive objects, HD 1279 and HD 202671, might have evolved away from the main sequence recently, the other stars are main-sequence objects. HD 99803A is a sharp lined HgMn star with grazing eclipses. From TESS and MASCARA photometry, we determine an orbital period of
P
orb
= 26.12022 ± 0.00004 day.
Context. We present an automated procedure that simultaneously derives the effective temperature Teff, surface gravity log g, metallicity Fe/H, and equatorial projected rotational velocity vsini for ...“normal” A and Am stars. The procedure is based on the principal component analysis (PCA) inversion method, which we published in a recent paper . Aims. A sample of 322 high-resolution spectra of F0-B9 stars, retrieved from the Polarbase, SOPHIE, and ELODIE databases, were used to test this technique with real data. We selected the spectral region from 4400−5000 Å as it contains many metallic lines and the Balmer Hβ line. Methods. Using three data sets at resolving powers of R = 42 000, 65 000 and 76 000, about ~6.6 × 106 synthetic spectra were calculated to build a large learning database. The online power iteration algorithm was applied to these learning data sets to estimate the principal components (PC). The projection of spectra onto the few PCs offered an efficient comparison metric in a low-dimensional space. The spectra of the well-known A0- and A1-type stars, Vega and Sirius A, were used as control spectra in the three databases. Spectra of other well-known A-type stars were also employed to characterize the accuracy of the inversion technique. Results. We inverted all of the observational spectra and derived the atmospheric parameters. After removal of a few outliers, the PCA-inversion method appeared to be very efficient in determining Teff, Fe/H, and vsini for A/Am stars. The derived parameters agree very well with previous determinations. Using a statistical approach, deviations of around 150 K, 0.35 dex, 0.15 dex, and 2 km s-1 were found for Teff, log g, Fe/H, and vsini with respect to literature values for A-type stars. Conclusions. The PCA inversion proves to be a very fast, practical, and reliable tool for estimating stellar parameters of FGK and A stars and for deriving effective temperatures of M stars.
Aims. Abundances of 15 chemical elements have been derived for 28 F and 16 A stars members of the Hyades open cluster in order to set constraints on self-consistent evolutionary models that include ...radiative and turbulent diffusion. Methods. A spectral synthesis, iterative procedure was applied to derive the abundances from selected high-quality lines in high-resolution, high-signal-to-noise spectra obtained with SOPHIE and AURELIE at the Observatoire de Haute Provence. Results. The abundance patterns found for A and F stars in the Hyades resemble those observed in Coma Berenices and Pleiades clusters. In graphs representing the abundances versus the effective temperature, A stars often display much more scattered abundances around their mean values than the coolest F stars do. Large star-to-star variations are detected in the Hyades A dwarfs in their abundances of C, Na, Sc, Fe, Ni, Sr, Y, and Zr, which we interpret as evidence of transport processes competing with radiative diffusion. In A and Am stars, the abundances of Cr, Ni, Sr, Y, and Zr are found to be correlated with that of Fe as in the Pleiades and in Coma Berenices. The ratios C/Fe and O/Fe are found to be anticorrelated with Fe/H as in Coma Berenices. All Am stars in the Hyades are deficient in C and O and overabundant in elements heavier than Fe but not all are deficient in Ca and/or Sc. The F stars have solar abundances for almost all elements except for Si. The overall shape of the abundance pattern of the slow rotator HD 30210 cannot be entirely reproduced by models including radiative diffusion and different amounts of turbulent diffusion. Conclusions. While part of the discrepancies between derived and predicted abundances could come from non-LTE effects, including competing processes such as rotational mixing and/or mass loss seems necessary in order to improve the agreement between the observed and predicted abundance patterns.
Context. We have discovered four new HgMn stars while monitoring a sample of apparently slowly rotating superficially normal bright late-B and early-A stars in the northern hemisphere. Aims. ...Important classification lines of Hg ii and Mn ii are found as conspicuous features in the high resolution SOPHIE spectra of these stars (R = 75 000). Methods. Several lines of Hg ii, Mn ii and Fe ii were synthesized using model atmospheres and the spectrum synthesis code SYNSPEC48, including hyperfine structure of various isotopes when relevant. These synthetic spectra were compared to high-resolution observations of these stars that have a high signal-to-noise ratio to derive abundances of these key elements. Results. The four stars are found to have distinct enhancements of Hg and Mn, which shows that they are not superficially normal B and A stars, but are new HgMn stars and need to be reclassified as such.
We report on a detailed abundance study of the fairly bright slow rotators HD 30085 (A0 IV), HD 30963 (B9 III), and HD 174567 (A0 V), hitherto reported as normal stars and the sharp-lined χ Lupi A ...(B9 IV HgMn). In the spectra of HD 30085 and HD 30963, the Hg ii line at 3984 Å is conspicuous and numerous lines of silicon, manganese, chromium, titanium, iron, strontium, yttrium, and zirconium appear to be strong absorbers. A comparison of the mean spectra of HD 30085 and HD 30963 with a grid of synthetic spectra for selected unblended lines having reliable updated atomic data reveals large overabundances of phosphorus, titanium, chromium, manganese, strontium, yttrium, zirconium, barium, platinum, and mercury, and underabundances of helium, magnesium, scandium, and nickel. The surface abundances of χ Lupi A have been rederived on the same effective temperature scale and using the same atomic data for consistency and comparison for HD 30085 and HD 30963. For HD 174567, milder deficiencies and excesses are found. The abundances of sodium, magnesium, and calcium have been corrected for non-LTE (NLTE) effects. The effective temperatures, surface gravities, low projected rotational velocities, and the peculiar abundance patterns of HD 30085 and HD 30963 show that these stars are two new HgMn stars and should be reclassified as such. HD 174567 is most likely a new marginally chemically peculiar star. A list of the identifications of lines absorbing more than 2% in the spectrum of HD 30085 is also provided.
Gaia Data Release 1 Carrasco, J M; Evans, D W; Montegriffo, P ...
Astronomy and astrophysics (Berlin),
11/2016, Letnik:
595
Journal Article
Recenzirano
Odprti dostop
Context. Gaia is an ESA cornerstone mission launched on 19 December 2013 aiming to obtain the most complete and precise 3D map of our Galaxy by observing more than one billion sources. This paper is ...part of a series of documents explaining the data processing and its results for Gaia Data Release 1, focussing on the G band photometry. Aims. This paper describes the calibration model of the Gaia photometric passband for Gaia Data Release 1. Methods. The overall principle of splitting the process into internal and external calibrations is outlined. In the internal calibration, a self-consistent photometric system is generated. Then, the external calibration provides the link to the absolute photometric flux scales. Results. The Gaia photometric calibration pipeline explained here was applied to the first data release with good results. Details are given of the various calibration elements including the mathematical formulation of the models used and of the extraction and preparation of the required input parameters (e.g. colour terms). The external calibration in this first release provides the absolute zero point and photometric transformations from the GaiaG passband to other common photometric systems. Conclusions. This paper describes the photometric calibration implemented for the first Gaia data release and the instrumental effects taken into account. For this first release no aperture losses, radiation damage, and other second-order effects have not yet been implemented in the calibration.
Aims. We derive the abundances of 18 chemical elements for 16 A-dwarf, both normal and chemically-peculiar, and 5 F-dwarf members of the Pleiades open cluster to place constraints on evolutionary ...models. Methods. Abundances and rotational and microturbulent velocities were derived by fitting synthetic spectra to high-resolution (R ~ 42 000 and R ~ 75 000) observations of high signal-to-noise ratio (S/N). Results. The abundances exhibit correlation with neither the effective temperature nor the projected rotational velocity. Interestingly, A stars exhibit larger star-to-star variations in C, Sc, Ti, V, Cr, Mn, Sr, Y, Zr and Ba, than F stars. F stars have solar abundances of almost all elements. In A stars, the abundances of Si, Ti and Cr are correlated with that of Fe, and the X/Fe ratios are solar for these three elements. The derived abundances are compared with the predictions of evolutionary models for the age of Pleiades (100 Myr). For F stars, small predicted underabundances of light elements and overabundances of Cr, Fe and Ni are confirmed by our findings. For A stars, the predicted overabundances in iron-peak elements are confirmed for a few stars only. Conclusions. The large scatter in the abundances of A stars, discovered previously in the Hyades, Coma Berenices, UMa group, and in field stars, appears to be a characteristic property of dwarf A stars. Hydrodynamical processes competing with radiative diffusion in the radiative zone of A dwarfs, could account for the scatter in abundances that we determine.
While monitoring a sample of apparently slowly rotating superficially normal early-A stars, we have discovered that HR 8844 (A0 V) is actually a new chemically peculiar star. We first compared the ...high-resolution spectrum of HR 8844 with that of four slow rotators near A0V ( Cap, Cnc, Sirius A, and HD 72660) to highlight similarities and differences. The lines of Ti ii, Cr ii, Sr ii, and Ba ii are conspicuous features in the high-resolution high signal-to-noise SOPHIE spectra of HR 8844 and much stronger than in the spectra of the normal star Cap. The Hg ii line at 3983.93 is also present in a 3.5% blend. Selected unblended lines of 31 chemical elements from He up to Hg have been synthesized using model atmospheres computed with ATLAS9 and the spectrum synthesis code SYNSPEC48 including hyperfine structure of various isotopes when relevant. These synthetic spectra have been adjusted to the mean SOPHIE spectrum of HR 8844, and high-resolution spectra of the comparison stars. Chi-squares were minimized to derive abundances or upper limits to the abundances of these elements for HR 8844 and the comparison stars. HR 8844 is found to have underabundances of He, C, O, Mg, Ca, and Sc, mild enhancements of Ti, V, Cr, Mn, and distinct enhancements of the heavy elements Sr, Y, Zr, Ba, La, Pr, Sm, Eu, and Hg, the overabundances increasing steadily with atomic number. This chemical pattern suggests that HR 8844 may actually be a new transition object between the coolest HgMn stars and the Am stars.
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