J/AJ/110/2319 Abundances in RR Lyr variables (Clementini+ 1995)
The composition of HB stars: RR Lyrae variables
Clementini G., Carretta E., Gratton R., Merighi R., Mould J.R.
McCarthy J.K.
<Astron. J. 110, 2319 (1995)>
=1995AJ....110.2319C 1995AJ....110.2319C
ADC_Keywords: Stars, horizontal branch ; Equivalent widths ; Abundances
Abstract:
We have used moderately high-resolution, high S/N spectra to study the
chemical composition of ten field ab-type RR Lyrae stars. Variables
having accurate photometric and radial-velocity data were selected, in
order to derive a precise estimate of the atmospheric parameters
independently of excitation and ionization equilibria. A new
temperature scale was determined from literature "Infrared Flux
Method" measures of subdwarfs and the Kurucz (1992) (priv. com.) model
atmospheres, and used to calibrate colors for both dwarfs and
RR Lyraes. Photometric reddening estimates for the program stars were
carefully examined, and compared with other determinations. The
applicability of Kurucz (1992) (priv. com.) model atmospheres in
the analysis of RR Lyraes at minimum light was analyzed: we found that
they are able to reproduce colors, excitation, and ionization
equilibria as well as the wings of Halpha. The comparison solar
abundances were carefully determined. From a new analysis of weak Fe I
lines with accurate gfs [Bard & Kock, A&A, 282, 1014 (1994)] we
derived log epsilon(Fe)_Sun=7.52, in agreement with the Fe abundances
determined from meteorites and Fe II lines. We derived abundances for
21 species. Main results are: The metal abundances of the program
stars span the range -2.50<[Fe/H]<+0.17. Lines of most elements are
found to form in LTE conditions. Fe lines satisfy very well the
excitation and ionization equilibria. A comparison with statistical
equilibrium computations shows that rather large collisional cross
sections are required to reproduce observations. If these cross
sections are then used in the analysis of the formation of Fe lines in
subdwarfs and RGB stars, no significant departures from LTE are found
for these stars, thus validating the very numerous LTE analyses. RR
Lyraes share the typical abundance pattern of other stars of similar
[Fe/H]: alpha-elements are overabundant by ∼0.4dex and Mn is
underabundant by ∼0.6dex in stars with [Fe/H]←1. Solar scaled
abundances are found for most of the other species, except for the low
Ba abundance in the extremely metal-poor star X Ari ([Fe/H]~-2.5).
Significant departures from LTE are found for a few species: Nd II, Ce
II, Y II, and Sc II are severely underabundant (∼0.5dex) in metal-rich
variables; Ti I and Cr I are slightly (∼0.1-0.2dex) underabundant in
metal-poor stars. These effects are attributed to overionization. We
suggest that the photoionization of the alkaline earth-like ions is
due to Lyman lines emission produced by the shock waves that propagate
in the atmosphere of these variables [Fokin (1992MNRAS.256...26F 1992MNRAS.256...26F)].
Departures from LTE were considered in detail in the derivation of
abundances for the light elements (O and Na). Significant corrections
were required for the O I IR triplet and the Na D lines. The resulting
pattern reproduces that observed in less evolved field stars. We did
not find any evidence for an O-Na anticorrelation among these field HB
stars, suggesting that the environment is likely to be responsible for
the anticorrelation found in metal-poor globular cluster stars
[Sneden et al. =1992AJ....104.2121S 1992AJ....104.2121S]. We used our new [Fe/H]
abundances, as well as values from Butler and co-workers (corrected to
our system), and from high- resolution spectroscopy of globular
clusters giants, to obtain a revised calibration of the low-resolution
metallicity index Delta(S) [Preston =1959ApJ...130..507P 1959ApJ...130..507P]:
[Fe/H]=-0.194(±0.011)Delta(S)-0.08(±0.18). Our new metallicity
scale is stretched on both low and high metallicity ends with respect
to Butler's [1975ApJ...200...68B 1975ApJ...200...68B]. The error in [Fe/H] by Delta(S)
observations is 0.16dex, well of the same order of high-resolution
metallicity determinations. The slope of the calibration obtained
considering only stars with 4<Delta(S)<10 is slightly smaller than
that obtained using all stars. While this difference is only barely
significant, it might point out the presence of a nonlinearity of the
Delta(S) vs [Fe/H] relation, as suggested by
Manduca [ApJ, 245, 258 (1981)]. The new [Fe/H] values were used to
update the metallicity calibration of the Ca II K line index
[Clementini et al. =1991AJ....101.2168C 1991AJ....101.2168C]. Using the present new
metallicities, and W'(K) values and relative errors from Clementini et
al. (1991), a least-squares fit weighted both in W'(K) and [Fe/H]
gives [Fe/H]=0.65(±0.17)W'(K)-3.49(±0.39). Finally, our new
metallicity scale was used to revise the metallicity dependence of the
absolute magnitude of RR Lyrae stars, M_V. Using M_V values from
Fernley [1994A&A...284L..16F 1994A&A...284L..16F] for the field stars, and estimates from
Liu & Janes [1990ApJ...360..561L 1990ApJ...360..561L] and Storm et al.
[1994A&A...290..443S 1994A&A...290..443S] for the cluster variables, we found
M_V=0.20(±0.03) [Fe/H]+1.06(±0.04) and
M_V=0.19(±0.03)[Fe/H]+0.96(±0.04), the last being obtained by
using M_V estimates derived for a value of the conversion factor
between observed and true pulsation velocity p=1.38 (Fernley 1994).
The adoption of the new metallicity scale does not yield significant
changes in the slope or zero point of the M_V vs [Fe/H] relation.
Observations do not rule out the possibility that the slope of the M_V
vs [Fe/H] relation might be different for metal-poor and metal-rich
variables. However, a larger sample of Baade-Wesselink M_V
determinations is required to definitely settle this question.
Objects:
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RA (2000) DE Designations
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01 32 08.1 +01 20 32 RR Cet = HD 9356
19 25 28.3 +42 47 14 RR Lyr = HD 182989
15 30 39 +35 47 00 ST Boo
00 23 43.0 +29 24 04 SW And
00 04 04 -16 59 48 UU Cet
19 32 18.9 -23 51 30 V440 Sgr
16 24 41 -06 32 06 V445 Oph
16 30 41 +18 21 42 VX Her
15 31 02.1 +01 41 02 VY Ser = HD 138279
03 08 30.6 +10 26 50 X Ari = HD 19510
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table3.dat 158 290 List of lines and adopted gf values and their
equivalent widths
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Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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1- 6 A6 --- Element Element
8- 14 F7.2 0.1nm Lambda Wavelength
16- 21 F6.2 eV EP Energy potential
23- 28 F6.2 --- loggf Oscillator strength
30- 35 F6.1 0.1nm EWRRCet Equivalent width for RR Cet
37- 41 F5.2 [Sun] lognRRCet Abundance for RR Cet
43- 48 F6.1 0.1nm EWRRLyr Equivalent width for RR Lyr
50- 54 F5.2 [Sun] lognRRLyr Abundance for RR Lyr
56- 61 F6.1 0.1nm EWSTBoo Equivalent width for ST Boo
63- 67 F5.2 [Sun] lognSTBoo Abundance for ST Boo
69- 74 F6.1 0.1nm EWSWAnd Equivalent width for SW And
76- 80 F5.2 [Sun] lognSWAnd Abundance for SW And
82- 87 F6.1 0.1nm EWUUCet Equivalent width for UU Cet
89- 93 F5.2 [Sun] lognUUCet Abundance for UU Cet
95-100 F6.1 0.1nm EWV440Sgr Equivalent width for V440 Sgr
102-106 F5.2 [Sun] lognV440Sgr Abundance for V440 Sgr
108-113 F6.1 0.1nm EWV445Oph Equivalent width for V445 Oph
115-119 F5.2 [Sun] lognV445Oph Abundance for V445 Oph
120-126 F7.1 0.1nm EWVXHer Equivalent width for VX Her
127-132 F6.2 [Sun] lognVXHer Abundance for VX Her
133-139 F7.1 0.1nm EWVYSer Equivalent width for VY Ser
140-145 F6.2 [Sun] lognVYSer Abundance for VY Ser
146-152 F7.1 0.1nm EWXAri Equivalent width for X Ari
153-158 F6.2 [Sun] lognXAri Abundance for X Ari
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Origin: AAS CD-ROM series, Volume 6, 1996 Lee Brotzman [ADS] 19-Feb-1996
(End) [CDS] 05-Sep-1996