The effective temperature scale of giant stars (F0–K5)

Nordström

et al. 2016

A&A

Context. An increasing fraction of carbon-enhanced metal-poor (CEMP) stars is found as their iron abundance, [Fe/H], decreases below [Fe/H] = −2.0. The CEMP-s stars have the highest absolute carbon abundances, [C/H], and are thought to owe their enrichment in carbon and the slow neutron-capture (s-process) elements to mass transfer from a former asymptotic giant branch (AGB) binary companion. The most Fe-poor CEMP stars are normally single, exhibit somewhat lower [C/H] than CEMP-s stars, but show no s-process element enhancement (CEMP-no stars). Abundance determinations of CNO offer clues to their formation sites. Aims. Our aim is to use the medium-resolution spectrograph X-Shooter/VLT to determine stellar parameters and abundances for C, N, Sr, and Ba in several classes of CEMP stars in order to further classify and constrain the astrophysical formation sites of these stars. Methods. Atmospheric parameters for our programme stars were estimated from a combination of V − K photometry, model isochrone fits, and estimates from a modified version of the SDSS/SEGUE spectroscopic pipeline. We then used X-Shooter spectra in conjunction with the 1D local thermodynamic equilibrium spectrum synthesis code MOOG, 1D ATLAS9 atmosphere models to derive stellar abundances, and, where possible, isotopic 12 C/ 13 C ratios. Results. Abundances (or limits) of C, N, Sr, and Ba are derived for a sample of 27 faint metal-poor stars for which the X-Shooter spectra have sufficient signal-to-noise ratios (S/N). These moderate resolution, low S/N (∼10−40) spectra prove sufficient to perform limited chemical tagging and enable assignment of these stars into the CEMP subclasses (CEMP-s and CEMP-no). According to the derived abundances, 17 of our sample stars are CEMP-s and 3 are CEMP-no, while the remaining 7 are carbon-normal. For four CEMP stars, the subclassification remains uncertain, and two of them may be pulsating AGB stars. Conclusions. The derived stellar abundances trace the formation processes and sites of our sample stars. The [C/N] abundance ratio is useful for identifying stars with chemical compositions unaffected by internal mixing, and the [Sr/Ba] abundance ratio allows us to distinguish between CEMP-s stars with AGB progenitors and the CEMP-no stars. Suggested formation sites for the latter include faint supernovae with mixing and fallback and/or primordial, rapidly-rotating, massive stars (spinstars). X-Shooter spectra have thus proved to be valuable tools in the continued search for their origin.

Section: Stellar Atmospheric Parameter Estimatesmentioning

confidence: 99%

Abundances of carbon-enhanced metal-poor stars as constraints on their formation

Nordström

et al. 2016

A&A

“…Using the calibrations of Alonso et al (1999) for HD 27536 and the Alonso et al (1996) calibrations for HD 216803 with the Strömgren indices found by Olsen (1993) in both cases assuming Solar metallicity. The uncertainties are around 120−140 K for all values.…”

Section: Temperature Gravity Microturbulencementioning

confidence: 99%

Binarity, activity and metallicity among late-type stars

Dall¹,

Bruntt²,

Strassméier

2005

A&A

We present the first in a series of papers that attempt to investigate the relation between binarity, magnetic activity, and chemical surface abundances of cool stars. In the current paper, we lay out and test two abundance analysis methods and apply them to two well-known, active, single stars, HD 27536 (G8IV-III) and HD 216803 (K5V), presenting photospheric fundamental parameters and abundances of Li, Al, Ca, Si, Sc, Ti, V, Cr, Fe, Co and Ni. The abundances from the two methods agree within the errors for all elements except calcium in HD 216803, which means that either method yields the same fundamental model parameters and the same abundances. Activity is described by the radiative loss in the Ca ii H and K lines with respect to the bolometric luminosity, through the activity index R HK . Binarity is established by very precise radial velocity (RV) measurements using HARPS spectra. The spectral line bisectors are examined for correlations between RV and bisector shape to distinguish between the effects of stellar activity and unseen companions. We show that HD 27536 exhibit RV variations mimicking the effect of a low-mass (m ∼ 4 M J ) companion in a relatively close (a ∼ 1 AU) orbit. The variation is strongly correlated with the activity, and consistent with the known photometric period P = 306.9 d, demonstrating a remarkable coherence between R HK and the bisector shape, i.e. between the photosphere and the chromosphere. We discuss the complications involved in distinguishing between companion and activity induced RV variations.

“…To take into account the effect of metallicity on the zero-point F (CI 0 ) st , we estimated the gradient dF (CI 0 ) st /d[F e/H] ≈ +0.24 from the calibrations by Alonso et al 1999, Sekiguchi & Fukugita 2000, Gonzalez Hernandez & Bonifacio 2009. In some cases (particularly for large-amplitude color variations) the "free" calibration, i.e.…”

Section: Resultsmentioning

confidence: 99%

“…The key point of our approach is that the values of function F are computed from the already available calibrations of the bolometric correction BC(CI 0 ) and the effective temperature logT (CI 0 ) (Flower 1996, Bessel, Castelli and Plez 1998, Alonso, Arribas and Martinez-Roger 1999, Sekiguchi and Fukugita 2000, Ramirez and Melendez 2005, Biazzo et al 2007, Gonzalez Hernandez and Bonifacio 2009. These calibrations are expressed as high-order power series in the intrinsic colour:…”

Section: Theoretical Backgroundmentioning

confidence: 99%

The Baade–Becker–Wesselink technique and the fundamental astrophysical parameters of Cepheids

Rastorguev¹,

Дамбис²,

Zabolotskikh³

et al. 2012

Proc. IAU

Abstract. The BBW method remains one of most demanded tool to derive full set of Cepheid astrophysical parameters. Surface brightness version of the BBW technique was preferentially used during last decades to calculate Cepheid radii and to improve PLC relations. Its implementation requires a priory knowledge of Cepheid reddening value. We propose a new version of the Baade-Becker-Wesselink technique, which allows one to independently determine the colour excess and the intrinsic colour of a radially pulsating star, in addition to its radius, luminosity, and distance. It is considered to be a generalization of the Balona light curve modelling approach. The method also allows calibration of the function F (CI0) = BC(CI0) + 10 × log(T ef f (CI0)) for the class of pulsating stars considered. We apply this technique to a number of classical Cepheids with very accurate light and radial-velocity curves. The new technique can also be applied to other pulsating variables, e.g. RR Lyraes. We also discuss the possible dependence of the projection factor on the pulsation phase.