Spectral line-depth ratios as temperature indicators for cool stars

Abstract

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 Johanson 1991). Ratios incorporating a line with any degree of saturation, as with the X6252 Vi to X6253 Fel 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 metallicity 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 GO 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 of ^50 K between temperatures derived from color indices and from spectral lines are most likely a result of interstellar reddening affecting the photometry. BACKGROUNDThe use of stellar spectral lines as temperature indicators has a venerable history. The technique of establishing the spectral type of a star is based on low-resolution (^1 A) spectral data, and therefore necessarily employs the equivalent widths of the lines. High-resolution data allow us to gain precision by replacing equivalent widths with line depths. The gain comes primarily from avoiding the distortions of blending among lines, which is a universal problem in cool stars. A reliable line depth can be had if only the continuum and the core of the line are unblended. A lesser gain accrues from not having to deal with the uncertainty of the contribution of the line wings to the equivalent width. Some common sense should be exercised here. In cases where significant variation occurs in a line profile, for example, when a starspot moves across the face of the star, line depth may well show variations from physical variables other than temperature.Taking the ratio of two line depths is the parallel of comparing two line strengths in spectral classification. This step can be crucial if the stars being compared show a range in rotational broadening or metallicity, or if the spectra are composite. Even within a single exposure, the precision is increased by using ratios rather than absolute line depths. Small differences in spectrograph focus are accommodated, but more importantly, differences in line strength arising from differences in chemical abundances largely cancel out (for weak lines).Several investigations of temperature variations during magnetic cycles of dwarf stars have used the ratio of depths of X6251.83 Vi to X6252.57 Fel. The calibration of this ratio is described in Gray and Johanson (1991), and involved measuring the depth ratio for stars covering a wide range in effective temperatures. Resolution of temperature differences as small as 1 K are obtained in favorable cases where large numbers of observations were available (e.g., Gray and Bahúnas 1994). The absolute calibration of the effec...

Analyses of Archival Data for Cool Dwarfs. VI. The 1993 Versions of the Temperature and [Fe/H] Catalogs

“…The Paper II algorithm for obtaining these errors is retained. Users should note, however, that a problem is posed by these errors and those quoted by Gray (1994) for his spectroscopic temperature indices. The two errors together cannot explain the scatter around the mean relation between the spectroscopic and photometric temperatures (see Sec.…”

“…The entry for this star in Paper II does not make it clear that it applies only for star A. A corresponding problem exists in Paper V. Gray (1994) has brought the problem to the author's attention by correcting the Paper II f Boo temperature on the assumption that it applies for combined light. The entries for £ Boo in Tables 1 and 2 show that they apply for star A specifically.…”

“…This section explores the problem (noted above) posed by comparison of the Paper II and Gray (1994) temperatures. Four possible explanations for the excess scatter-including the explanation which Gray favors-will be considered here.…”

“…In Sec. 3, a question raised by Gray (1994) about the Paper II temperatures is discussed at some length. In Sec.…”

Statistical cataloging of archival data for luminosity class IV–V stars

“…Proceeding to the third line of Table 3, one encounters results for a second pertinent data set. That data set has been published by Gray (1994) and was discussed in Sect. 3 of Paper II.…”