The extremely low-metallicity star SDSS J102915+172927: a subgiant scenario
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Abstract:
Spectroscopic analysis of the Galactic halo star SDSS J102915+172927 has shown it to have a very low heavy element abundance, Z < 7.4 10-7, with [Fe/H] = -4.89 plus/minus 0.10 and an upper limit on the C abundance of [C/H] < -4.5. The low C/Fe ratio distinguishes this object from most other extremely metal poor stars. The effective temperature and surface gravity have been determined to be Teff = 5811 plus/minus 150 K and log g = 4.0 plus/minus 0.5. The surface gravity estimate is problematical in that it places the star between the main sequence and the subgiants in the Hertzsprung-Russell diagram. If it is assumed that the star is on the main sequence, its mass and are estimated to be M = 0.72 plus/minus 0.06 Msun and L = 0.45 plus/minus 0.10 Lsun, placing it at a distance of 1.35 plus/minus 0.16 kpc. The upper limit on the lithium abundance, A(Li) < 0.9, is inconsistent with the star being a dwarf, assuming that mixing is due only to convection. In this paper, we propose that SJ102915 is a sub-giant that formed with significantly higher Z than currently observed, in agreement with theoretical predictions for the minimum C and/or O abundances needed for low mass star formation. In this scenario, extremely low Z and low Li abundance result from gravitational settling on the main sequence followed by incomplete convective dredge-up during subgiant evolution. The observed Fe abundance requires the initial Fe abundance to be enhanced compared to C and O, which we interpret as formation of SJ102915 occurring in the vicinity of a type Ia supernova.Keywords:
Subgiant
Surface gravity
Effective temperature
Low Mass
Giant star
Star (game theory)
Stellar light curves are well known to encode physical stellar properties. Precise, automated and computationally inexpensive methods to derive physical parameters from light curves are needed to cope with the large influx of these data from space-based missions such as Kepler and TESS. Here we present a new methodology which we call The Swan, a fast, generalizable and effective approach for deriving stellar surface gravity ($\log g$) for main sequence, subgiant and red giant stars from Kepler light curves using local linear regression on the full frequency content of Kepler long cadence power spectra. With this inexpensive data-driven approach, we recover $\log g$ to a precision of $\sim$0.02 dex for 13,822 stars with seismic $\log g$ values between 0.2-4.4 dex, and $\sim$0.11 dex for 4,646 stars with Gaia derived $\log g$ values between 2.3-4.6 dex. We further develop a signal-to-noise metric and find that granulation is difficult to detect in many cool main sequence stars ($T_{\text{eff}}$ $\lesssim$ 5500 K), in particular K dwarfs. By combining our $\log g$ measurements with Gaia radii, we derive empirical masses for 4,646 subgiant and main sequence stars with a median precision of $\sim$7%. Finally, we demonstrate that our method can be used to recover $\log g$ to a similar mean absolute deviation precision for TESS-baseline of 27 days. Our methodology can be readily applied to photometric time-series observations to infer stellar surface gravities to high precision across evolutionary states.
Subgiant
Surface gravity
Effective temperature
Starspot
Red-giant branch
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Doppler-based planet surveys have shown that, besides metallicity, the planet occurrence is also correlated with stellar mass, increasing from M to F-A spectral types. However, it has recently been argued that the subgiants (which represent A stars after they evolve off the main sequence) may not be as massive as suggested initially, which would significantly change the correlation found. To start investigating this claim, we have studied the subgiant star HD 185351, which has precisely measured physical properties based on asteroseismology and interferometry. An independent spectroscopic differential analysis based on excitation and ionization balance of iron lines yielded the atmospheric parameters $T_{\rm eff}$ = 5035 $\pm$ 29 K, $\log$ g = 3.30 $\pm$ 0.08 and [Fe/H] = 0.10 $\pm$ 0.04. These were used in conjunction with the PARSEC stellar evolutionary tracks to infer a mass M = 1.77 $\pm$ 0.04 M$_{\odot}$, which agrees well with the previous estimates. Lithium abundance was also estimated from spectral synthesis (A(Li) = 0.77 $\pm$ 0.07) and, together with $T_{\rm eff}$ and [Fe/H], allowed to determine a mass M = 1.64 $\pm$ 0.06 M$_{\odot}$, which is independent of the star's parallax and surface gravity. Our new measurements of the stellar mass support the notion that HD185351 is a Retired A Star with a mass in excess of 1.6 M$_{\odot}$.
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Surface gravity
Effective temperature
Star (game theory)
Asteroseismology
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Context: A radial velocity survey of about 380 G and K giant stars is ongoing at Lick observatory. For each star we have a high signal to noise ratio template spectrum, which we use to determine spectroscopic stellar parameters. Aim: The aim of this paper is to present spectroscopic stellar parameters, i.e. effective temperature, surface gravity, metallicity and rotational velocity for our sample of G and K giant stars. Methods: Effective temperatures, surface gravities and metallicities are determined from the equivalent width of iron lines. Rotational velocities are determined from the full width at half maximum (FWHM) of moderate spectral lines. A calibration between the FWHM and total broadening (rotational velocity and macro turbulence) is obtained from stars in common between our sample and the sample from Gray (1989). Results: The metallicity we derive is essentially equal to the literature values, while the effective temperature and surface gravity are slightly higher by 56 K and 0.15 dex, respectively. Our rotational velocities are comparable with the ones obtained by Gray (1989), but somewhat higher than the ones obtained by de Medeiros & Mayor (1999), consistent with the different diagnostics used. Conclusions: We are able to determine spectroscopic stellar parameters for about 380 G and K giant stars in a uniform way (112 stars are being analysed spectroscopically for the first time). For stars available in the literature, we find reasonable agreement between literature values and values determined in the present work. In addition, we show that the metallicity enhancement of companion hosting stars might also be valid for giant stars, with the planet-hosting giants being 0.13 +/- 0.03 dex (i.e. 35 +/- 10%) more metal-rich than our total sample of stars.
Surface gravity
Effective temperature
Giant star
Radial velocity
Equivalent width
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Spectroscopic analysis of the Galactic halo star SDSS J102915+172927 has shown it to have a very low heavy element abundance, Z < 7.4 10-7, with [Fe/H] = -4.89 plus/minus 0.10 and an upper limit on the C abundance of [C/H] < -4.5. The low C/Fe ratio distinguishes this object from most other extremely metal poor stars. The effective temperature and surface gravity have been determined to be Teff = 5811 plus/minus 150 K and log g = 4.0 plus/minus 0.5. The surface gravity estimate is problematical in that it places the star between the main sequence and the subgiants in the Hertzsprung-Russell diagram. If it is assumed that the star is on the main sequence, its mass and are estimated to be M = 0.72 plus/minus 0.06 Msun and L = 0.45 plus/minus 0.10 Lsun, placing it at a distance of 1.35 plus/minus 0.16 kpc. The upper limit on the lithium abundance, A(Li) < 0.9, is inconsistent with the star being a dwarf, assuming that mixing is due only to convection. In this paper, we propose that SJ102915 is a sub-giant that formed with significantly higher Z than currently observed, in agreement with theoretical predictions for the minimum C and/or O abundances needed for low mass star formation. In this scenario, extremely low Z and low Li abundance result from gravitational settling on the main sequence followed by incomplete convective dredge-up during subgiant evolution. The observed Fe abundance requires the initial Fe abundance to be enhanced compared to C and O, which we interpret as formation of SJ102915 occurring in the vicinity of a type Ia supernova.
Subgiant
Surface gravity
Effective temperature
Low Mass
Giant star
Star (game theory)
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Citations (11)
A grid of ATLAS9 model atmospheres has been computed, spanning 3500 K <= T_eff <= 8000 K, 0.0 <= log g <= 5.0, -4.0 <= [M/H] <= 0.0, and -0.8 < [alpha/Fe] <= +1.2. These parameters are appropriate for stars in the red giant branch, subgiant branch, and the lower main sequence. The main difference from a previous, similar grid (Castelli & Kurucz 2003) is the range of [alpha/Fe] values. A grid of synthetic spectra, calculated from the model atmospheres, is also presented. The fluxes are computed every 0.02 Angstrom from 6300 Angstrom to 9100 Angstrom. The microturbulent velocity is given by a relation to the surface gravity. This relation is appropriate for red giants, but not for subgiants or dwarfs. Therefore, caution is urged for the synthetic spectra with log g > 3.5 or for any star that is not a red giant. Both the model atmosphere and synthetic spectrum grids are available online through VizieR. Applications of these grids include abundance analysis for large samples of stellar spectra and constructing composite spectra for stellar populations.
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Red giant
Giant star
Surface gravity
Effective temperature
Red-giant branch
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Aims. The differences between the neutron-capture element abundances of halo stars are important to our understanding of the nucleosynthesis of elements heavier than the iron group. We present a detailed abundance analysis of carbon and twelve neutron-capture elements from Sr up to Pb for a peculiar halo star G 24-25 with [Fe/H] = −1.4 in order to probe its origin. Methods. The equivalent widths of unblended lines are measured from high resolution NOT/FIES spectra and used to derive abundances based on Kurucz model atmospheres. In the case of CH, Pr, Eu, Gd, and Pb lines, the abundances are derived by fitting synthetic profiles to the observed spectra. Abundance analyses are performed both relative to the Sun and to a normal halo star G 16-20 that has similar stellar parameters as G 24-25. Results. We find that G 24-25 is a halo subgiant star with an unseen component. It has large overabundances of carbon and heavy s-process elements and mild overabundances of Eu and light s-process elements. This abundance distribution is consistent with that of a typical CH giant. The abundance pattern can be explained by mass transfer from a former asymptotic giant branch component, which is now a white dwarf.
Subgiant
Asymptotic giant branch
Giant star
Red giant
s-process
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Doppler-based planet surveys have shown that, besides metallicity, the planet occurrence is also correlated with stellar mass, increasing from M to F-A spectral types. However, it has recently been argued that the subgiants (which represent A stars after they evolve off the main sequence) may not be as massive as suggested initially, which would significantly change the correlation found. To start investigating this claim, we have studied the subgiant star HD 185351, which has precisely measured physical properties based on asteroseismology and interferometry. An independent spectroscopic differential analysis based on excitation and ionization balance of iron lines yielded the atmospheric parameters $T_{\rm eff}$ = 5035 $\pm$ 29 K, $\log$ g = 3.30 $\pm$ 0.08 and [Fe/H] = 0.10 $\pm$ 0.04. These were used in conjunction with the PARSEC stellar evolutionary tracks to infer a mass M = 1.77 $\pm$ 0.04 M$_{\odot}$, which agrees well with the previous estimates. Lithium abundance was also estimated from spectral synthesis (A(Li) = 0.77 $\pm$ 0.07) and, together with $T_{\rm eff}$ and [Fe/H], allowed to determine a mass M = 1.64 $\pm$ 0.06 M$_{\odot}$, which is independent of the star's parallax and surface gravity. Our new measurements of the stellar mass support the notion that HD185351 is a Retired A Star with a mass in excess of 1.6 M$_{\odot}$.
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Surface gravity
Effective temperature
Asteroseismology
Star (game theory)
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High resolution, high -- ratio optical spectra have been obtained for a sample of 6 K-type dwarf and subgiant stars, and have been analysed with three different LTE methods in order to derive detailed photospheric parameters and abundances and to compare the characteristics of analysis techniques. The results have been compared with the aim of determining the most robust method to perform complete spectroscopic analyses of K-type stars, and in this perspective the present work must be considered as a pilot study. In this context we have determined the abundance ratios with respect to iron of several elements. In the first method the photospheric parameters (Teff, , and ξ) and metal abundances are derived using measured equivalent widths and Kurucz LTE model atmospheres as input for the MOOG software code. The analysis proceeds in an iterative way, and relies on the excitation equilibrium of the lines for determining the effective temperature and microturbulence, and on the ionization equilibrium of the and lines for determining the surface gravity and the metallicity. The second method follows a similar approach, but discards the low excitation potential transitions (which are potentially affected by non-LTE effects) from the initial line list, and relies on the colour index to determine the temperature. The third method relies on the detailed fitting of the 6162 Å line to derive the surface gravity, using the same restricted line list as the second method. Methods 1 and 3 give consistent results for the program stars; in particular the comparison between the results obtained shows that the low-excitation potential transitions do not appear significantly affected by non-LTE effects (at least for the subgiant stars), as suggested by the good agreement of the atmospheric parameters and chemical abundances derived. The second method leads to systematically lower Teff and values with respect to the first one, and a similar trend is shown by the chemical abundances (with the exception of the oxygen abundance). These differences, apart from residual non-LTE effects, may be a consequence of the colour-Teff scale used. The α-elements have abundance ratios consistent with the solar values for all the program stars, as expected for “normal” disk stars. The first method appears to be the most reliable one, as it is self-consistent, it always leads to convergent solutions and the results obtained are in good agreement with previous determinations in the literature.
Microturbulence
Subgiant
Effective temperature
Surface gravity
Line (geometry)
Equivalent width
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Doppler-based planet surveys have shown that, besides metallicity, the planet occurrence is also correlated with stellar mass, increasing from M to F-A spectral types. However, it has recently been argued that the subgiants (which represent A stars after they evolve off the main sequence) may not be as massive as suggested initially, which would significantly change the correlation found. To start investigating this claim, we have studied the subgiant star HD 185351, which has precisely measured physical properties based on asteroseismology and interferometry. An independent spectroscopic differential analysis based on excitation and ionization balance of iron lines yielded the atmospheric parameters $T_{\rm eff}$ = 5035 $\pm$ 29 K, $\log$ g = 3.30 $\pm$ 0.08 and [Fe/H] = 0.10 $\pm$ 0.04. These were used in conjunction with the PARSEC stellar evolutionary tracks to infer a mass M = 1.77 $\pm$ 0.04 M$_{\odot}$, which agrees well with the previous estimates. Lithium abundance was also estimated from spectral synthesis (A(Li) = 0.77 $\pm$ 0.07) and, together with $T_{\rm eff}$ and [Fe/H], allowed to determine a mass M = 1.64 $\pm$ 0.06 M$_{\odot}$, which is independent of the star's parallax and surface gravity. Our new measurements of the stellar mass support the notion that HD185351 is a Retired A Star with a mass in excess of 1.6 M$_{\odot}$.
Subgiant
Surface gravity
Effective temperature
Asteroseismology
Star (game theory)
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Citations (0)
Stellar light curves are well known to encode physical stellar properties. Precise, automated and computationally inexpensive methods to derive physical parameters from light curves are needed to cope with the large influx of these data from space-based missions such as Kepler and TESS. Here we present a new methodology which we call The Swan, a fast, generalizable and effective approach for deriving stellar surface gravity ($\log g$) for main sequence, subgiant and red giant stars from Kepler light curves using local linear regression on the full frequency content of Kepler long cadence power spectra. With this inexpensive data-driven approach, we recover $\log g$ to a precision of $\sim$0.02 dex for 13,822 stars with seismic $\log g$ values between 0.2-4.4 dex, and $\sim$0.11 dex for 4,646 stars with Gaia derived $\log g$ values between 2.3-4.6 dex. We further develop a signal-to-noise metric and find that granulation is difficult to detect in many cool main sequence stars ($T_{\text{eff}}$ $\lesssim$ 5500 K), in particular K dwarfs. By combining our $\log g$ measurements with Gaia radii, we derive empirical masses for 4,646 subgiant and main sequence stars with a median precision of $\sim$7%. Finally, we demonstrate that our method can be used to recover $\log g$ to a similar mean absolute deviation precision for TESS-baseline of 27 days. Our methodology can be readily applied to photometric time-series observations to infer stellar surface gravities to high precision across evolutionary states.
Subgiant
Surface gravity
Effective temperature
Starspot
Red-giant branch
Cite
Citations (0)