Line-depth Ratios Research Articles

A new Böhm-Vitense gap in the temperature range 5560 to 5610 K in the main sequence

Highly precise temperatures ( K) have been determined from line depth ratios for a set of 248 F-K field dwarfs of about solar metallicity (), based on high resolution (), high echelle spectra. A new gap has been discovered in the distribution of stars on the Main Sequence in the temperature range 5560 to 5610 K. This gap coincides with a jump in the microturbulent velocity and the well-known Li depression near 5600 K in field dwarfs and open clusters. As the principal cause of the observed discontinuities in stellar properties we propose the penetration of the convective zone into the inner layers of stars slightly less massive than the Sun and related to it, a change in the temperature gradient.

Spectroscopic line-depth ratios and solar variability

Line-depth ratios, i.e., the ratios of the central depths of suitably-chosen spectral lines, have proved to be an effective thermometer for stars on the cool half of the HR diagram. Several individual stars show line-depth-ratios changing in concert with Ca II H&K emission over decade time scales; some have amplitudes of many degrees. The disk-integrated solar spectrum shows variations in line-depth ratios that are quite weak, amounting to a degree or so. Although we have observations for only one solar cycle, there can be little doubt that the variation is part of the cycle process. The physical cause of the variation is likely the changing fraction of the surface infused with magnetic field (plage), where the run of temperature with optical depth differs from the non-magnetic regions. Line-depth ratios may prove useful in understanding the variation in photospheric structure with changing magnetic activity, and for monitoring secular changes over many solar cycles.

High precision effective temperatures for 181 F–K dwarfs from line-depth ratios

Line depth ratios measured on high resolution (), high echelle spectra are used for the determination of precise effective temperatures of 181 F, G, K Main Sequence stars with about solar metallicity (). A set of 105 relations is obtained which rely on ratios of the strengths of lines with high and low excitation potentials, calibrated against previously published precise (one per cent) temperature estimates. The application range of the calibrations is 4000–6150 K (F8V–K7V). The internal error of a single calibration is less than 100 K, while the combination of all calibrations for a spectrum of reduces uncertainty to only 5–10 K, and for or higher – to better than 5 K. The zero point of the temperature scale is directly defined from reflection spectra of the Sun with an uncertainty about 1 K. The application of this method to investigation of the planet host stars properties is discussed.

Measuring starspot temperature from line depth ratios

Gray and collaborators have recently demonstrated that line-depth ratios are a powerful tool for temperature discrim- ination, able to resolve dierences10 K. The method has been applied to detect temperature variations in the 5-15 K range due to activity cycles (e.g. Gray et al. 1996a, 1996b) or to rotation modulation produced by large surface features, called star-patches, like that detected in Boo A by Toner & Gray (1988). Cool starspots of a few tenths of the stellar surface produce bumps in a line profile, which migrate through the line profile allowing Doppler-imaging in fast rotating stars. In the hypothesis that in slowly-rotating stars the passage of dark spots pro- duces modulation of the center line depth of dierent amount in lines of dierent sensitivity to temperature, we have made test observations on three active binaries of the RS CVn type. Based on observations made at the Catania Astrophysical Observatory at a resolution R= 14 000, we show that line-depth ratios can be eectively used to determine spot temperatures of active binary systems. Using, on average, ten line pairs, selected in the 6100-6300 A wavelength range, with the help of observations of 30 main sequence and giant stars, we have derived a calibration relation of line-depth ratios (LDR) in an absolute temperature scale, taking into account the gravity eect in the calibration relation. Single LDRs converted to temperature through the calibration relations have led to clear rotational modulation of the average surface temperature with amplitudes of 177 K, 119 K, and 127 K for VY Ari, IM Peg and HK Lac, with average estimated errors of about 10 K. We show that the observed temperature variation amplitude allows us to define a minimum fractional spotted area coverage as a function of spot-photosphere temperature ratio. Adopting the maximum value of average temperature, determined from the LDRs, as that of the unspotted photosphere, we computed the average spot temperature corresponding to the minimum spot coverage. Although not univocally constrained, the temperature dierence (T = Tph Tsp) obtained for the three systems,T= 890 K for VY Ari,T= 750 K for IM Peg, andT= 810 K for HK Lac, are in good agreement with values derived with other methods.

Temperature and size of starspots from line depth ratios

It has been recently proved that line-depth ratios (LDR) are a powerful tool for temperature discrimination, able to resolve differences K. With the help of such diagnostics it has been possible to detect long-term variations with time scales of several years, ascribed to activity cycles (e.g. Gray et al. 1996a, 1996b). In very few cases, rotational modulation due to surface inhomogeneities has been observed (Toner & Gray 1988). In this work, we have made a test of this method on three slowly-rotating active binaries of RS CVn type by means of observations accomplished at Catania Astrophysical Observatory at a resolution R = 14 000 with the aim to search for and analyze rotational modulation of photospheric temperature. We suppose that in these stars the passage of dark spots produces modulation of the center-line depth of different amount in lines of different sensitivity to temperature, that cannot be analyzed by means of the Doppler-imaging technique. It is instead useful for fast-rotating active stars where cool spots of a few tenths of the stellar surface give rise to bumps in the line profiles. In order to determine spot temperatures of active binary systems, we have converted single LDRs to temperature through calibration relations. A sharp rotational modulation of the average surface temperature with amplitudes of 177 K, 119 K, and 127 K for VY Ari, IM Peg and HK Lac, respectively, has been detected with errors of about 10 K. Such variations are almost perfectly correlated to the simultaneous light curves, lower temperature corresponding to lower luminosity. We show that, on the basis of the amplitudes and maximum observed values of temperature and brightness, it is possible to define, in an univocal way, the temperature and the fractional spotted area by means of an analytical approach.

A temperature calibration for MK-class III giants from high-resolution spectral line-depth ratios

A temperature calibration for MK-class III giants from high-resolution spectral line-depth ratios

A temperature calibration for MK-class III giants from high-resolution spectral line-depth ratios

A temperature calibration for MK-class III giants from high-resolution spectral line-depth ratios

Star-to-Star Abundance Variations among Bright Giants in the Mildly Metal-poor Globular Cluster M4

We present a chemical composition analysis of 36 giants in the nearby mildly metal-poor ( = -1.18) CN-bimodal globular cluster M4. The stars were observed at the Lick and McDonald Observatories using high-resolution echelle spectrographs and at the Cerro Tololo Inter-American Observatory using the multiobject spectrometer. Confronted with a cluster having interstellar extinction that is large and variable across the cluster face, we combined traditional spectroscopic abundance methods with modifications to the line depth ratio technique pioneered by Gray to determine the atmospheric parameters of our stars. We derive a total-to-selective extinction ratio of 3.4 ± 0.4 and an average E(B-V) reddening of 0.33 ± 0.01, which is significantly lower than that estimated by using the dust maps made by Schlegel and coworkers. We determine abundance ratios typical of halo field and cluster stars for scandium, titanium, vanadium, nickel, and europium with star-to-star variations in these elements of less than ±0.1. Silicon, aluminum, barium, and lanthanum are overabundant with respect to what is seen in other globular clusters of similar metallicity. These overabundances confirm the results of an earlier study by Brown & Wallerstein based on a much smaller sample of M4 giants. Superposed on the primordial abundance distribution is evidence for the existence of proton capture synthesis of carbon, oxygen, neon, and magnesium. We recover some of the C, N, O, Na, Mg, and Al abundance swings and correlations found in other more metal-poor globular clusters, but the range of variation is muted. In the case of Mg and Al, this is compatible with the idea that the Al enhancements are derived from the destruction of 25,26Mg, not 24Mg. We determine that the C + N + O abundance sum is constant to within the observational errors and agrees with the C + N + O total that might be expected for M4 stars at birth. The asymptotic giant branch (AGB) stars in M4 have C, N, and O abundances that show less evidence for proton capture nucleosynthesis than is found in the less evolved stars of the red giant branch (RGB). Deeply mixed stars of the RGB, subsequent to the helium core flash, might take up residence on the blue end of the horizontal branch and thus fail to evolve back to the AGB, but reasons for skepticism concerning this scenario are noted.

The Lack of Spectral Variability in 51 Pegasi: Confirmation of the Planet Hypothesis

We present-high resolution (R > 200,000), high signal-to-noise spectral data for 51 Pegasi spanning a complete orbital period of the purported planetary companion. A detailed examination of the line bisectors from eight spectral lines as well as the ratio of the line depth of the V I 6251 A line to the Fe I 6253 A line were performed. The changes in the mean bisector velocity span and curvature are about 1 m s-1 and 4 m s-1, respectively. These variations are an order of magnitude less than the variations reported by Gray and are small enough to exclude all nonradial sectoral pulsation modes. The line-depth ratio of V I/Fe I also shows no significant variations, and a least-squares sine fit using the planet period of 4.23 days yields an amplitude of 0.00038 for any line-depth ratio variations. This corresponds to a disk-integrated temperature variation (peak-to-peak) of less than 1.7 K. We conclude that the radial velocity variations are not due to a dynamical stellar atmosphere and that a planetary companion remains as the only viable explanation for the observed radial velocity variations. The lack of bisector variability in our data indicates that the contribution to the observed spectra due to reflected light from the planet is at least 2000 times less than that of the primary light. Assuming a planetary albedo of unity, this places an upper limit of 0.2 R☉ for the radius of the planet.

Magnetic, Photometric, Temperature, and Granulation Variations of XI Bootis A 1984--1993

view Abstract Citations (48) References (47) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Magnetic, Photometric, Temperature, and Granulation Variations of XI Bootis A 1984--1993 Gray, David F. ; Baliunas, Sallie L. ; Lockwood, G. W. ; Skiff, Brian A. Abstract The magnetically active G8 dwarf star, ξ Boo A = HR 5544 = HD 13 1156 is studied for magnetic- cycle type variations over the 1984-1993 interval. We present measurements of Ca II H and K emission as an indirect indicator of magnetic activity, blue and visual magnitudes as an indication of the power output and temperature, line-depth ratios of V I λ6251.83 to Fe I λ6252.57 as a measure of temperature, and line bisectors as a measure of the star's granulation. The season means of all these parameters show the same pattern of variation with several irregular rises and falls, rather different from the relatively smooth variations seen for the Sun. As found for several other stars in previous studies, the magnetic signal leads the others in time. Time lags relative to the H and K index variation are 1.4±0.4 yr for the photometric brightness, 1.5±0.5 yr for the b -y color index, 1.8±0.3 yr for the line-depth ratio, and 2.1±0.4 yr for the line bisectors. The ≍1.7 year temperature lag for ξ Boo A is close to the linear relation between lag and effective temperature found for the other stars that have been measured. Publication: The Astrophysical Journal Pub Date: July 1996 DOI: 10.1086/177478 Bibcode: 1996ApJ...465..945G Keywords: STARS: INDIVIDUAL CONSTELLATION NAME: XI BOOTIS; STARS: LATE-TYPE; STARS: MAGNETIC FIELDS full text sources ADS | data products SIMBAD (8)

SPECTRAL LINE-DEPTH RATIOS AS TEMPERATURE INDICATORS FOR COOL STARS

ABSTRACT The use of spectral line-depth ratios as a stellar thermometer in G and K dwarfs is developed and refined beyond an earlier study (Gray and Johnson 1991). Ratios incorporating a line with any degree of saturation, as with the lambda-6252 V I to lambda-6253 Fe I ratio used in the 1991 work, produce metallicity dependent results. This dependence is investigated here, and a correction derived. Ten line-depth ratios using only weak lines are shown to have negligible metallicty dependence and resolve temperature differences as small as 6 K for early K dwarfs from a single exposure having a signal-to-noise ratio of 500. Precision deteriorates badly toward G0 for these particular spectral lines. Smaller temperature differences can be resolved by combining exposures. Relative temperatures of 65 dwarfs are given, a few having errors near 1 K. Inconsistencies ~50 K between temperatures derived from color indices and from spectral lines are most likely a result of interstellar reddening affecting the photometry.

ANALYSES OF ARCHIVAL DATA FOR COOL DWARFS. II. A CATALOG OF TEMPERATURES

ABSTRACT A calibration presented in a previous paper is used in this paper to derive temperatures for FGK stars near the main sequence. The calibration is checked against published counterparts, and it is found that previous calibrations have not established K-dwarf temperatures in particular beyond reasonable doubt. The data base assembled to derive the temperatures is described, and the problems posed by close binaries are evaluated. The newly derived temperatures are used to check a line-depth ratio proposed as a thermometer by Gray and Johanson (1991), and it is found that the ratio is metallicity-sensitive. Temperatures are given for a total of 417 stars.

The activity cycle of Sigma Draconis

The temperature and granulation of several dwarf stars have been monitored since 1984. The results for σ Dra (HR 7462, HD 185144, K0 V) are discussed in this paper. Temperature is monitored using a line-depth ratio, and temperature variations: 5 K are seen. Intermediate-band photometry and Ca II H and K-line emission mimic the temperature changes: a monotonic decline from the 1984 season, a smooth minimum around 1988, followed by a rise back to the 1984 values at the current time. These types of correlated changes are similar to the solar behavior

THE INFERRED COLOR INDEX OF THE SUN

ABSTRACT The high temperature sensitivity of the line-depth ratio V I lambda-6251.83 to Fe I lambda-6252.57, measured in the daytime sky, is used to infer the color indices of the sun: B-V = 0.656 ± 0.005, b-y = 0.414 ± 0.003, and B2-V1 = 0.393 ± 0.004.