HIGH RESOLUTION OPTICAL SPECTROSCOPY of the Hg-Mn STAR HR7143

D. J. Bord (UMich-Dearborn)

C. R. Cowley (UMich-Ann Arbor)

S. Hubrig (ESO)

W. P. Bidelman (CWRU)

ABSTRACT

HR7143 is a sharp-lined Hg-Mn star that, despite much study, still appears to harbor some interesting spectroscopic characteristics. Based on high resolution, high signal-to-noise data covering the range 3050-6750 Å, we have carried out wavelength coincidence statistics (WCS) analyses based on 3134 measured lines. In addition to the common element identifications expected in a star of this type, we find very strong statistical evidence to support the presence O I, S II, Pd I, and Xe II. Our examination of individual spectral features has corroborated these identifications, each of which presents a fresh challenge to our understanding of this object. For example, although singly ionized palladium has been identified in several chemically peculiar stars, HR7143 appears to be the only star of this type, save HR465, in which Pd I has been reported. The strong presence of sulfur is also intriguing insofar as most Hg-Mn stars, while over abundant in phosphorus, are deficient in S relative to the Sun. We present here the full details of our WCS study and elaborate the implications of the presence of the species noted above and a few others. For Pd, we report new abundance data based on spectrum synthesis techniques.

THE STAR

· HR7143 = HD175640

- Spectral Type: B9p; Hg-Mn (CP3¹)

- Parallax: 6.21 mas

- V Magnitude: 6.2

- Strömgren Photometry: b-y = +0.001

m₁ = 0.103

c₁ = 0.747

beta = 2.771

- Color Excess: E(b-y) = 0.044; E(B-V) = 0.061

- Numerous Low Resolution Optical & IUE Spectrum Studies; Cr II and Mn II Emission Lines Observed²

- No Significant Magnetic Field³

THE DATA

· HIGH RESOLUTION ECHELLE SPECTRA

- ESO VLT UV-VIS Echelle Spectrograph

- Wavelength Range: 3050-6750 Å

- Resolution: R = 80,000 in blue = 0.050 Å. @ 4000 Å ;

R = 110,000 in red = 0.055 Å @ 6000 Å

- S/N > 300

- Four Data Segments: 3050-3916 Å, 3756-4978 Å, 4782-5659 Å, and 5835-6750 Å

WAVELENGTH MEASUREMENTS & LINE IDENTIFICATIONS

· 3134 STELLAR FEATURES MEASURED

· TRADITIONAL IDENTIFICATION STUDY CARRIED OUT BY WPB

- Over 90% of Features below 4800 Å Identified

- <50% of Features between 4800 and 6750 Å Identified

- Complete Listing Available at:

http://www.astro.lsa.umich.edu/users/cowley/AAS0503Don/

· WAVELENGTH COINCIDENCE STATISTICS (WCS) STUDIES⁴: 1000 Random Trials with a Tolerance of 60 mÅ

· MAJOR RESULTS from WCS INVESTIGATIONS:

1. O I: This ion is strongly represented in the spectrum of HR7143. WCS data yield 11 coincidences out of 14 lab lines, for a nearly 10% confidence level. Line-by-line surveys confirm the identification, including the presence of the primary probes of O abundance in Hg-Mn stars used by Roby and Lambert⁵ at 6158.2, 6156.8 and 6156.0 Å. Most Hg-Mn stars show mild deficiencies of this element (of the order of a few tenths of a dex) relative to the Sun. The strong presence of subordinate O I lines in HR7143 may suggest a more nearly normal abundance of this element. We plan to determine the oxygen abundance in this star using the method of spectral synthesis.

2. P II/S II: Most Hg-Mn stars have phosphorus over abundances of ~1 dex relative to the Sun, while sulfur is commonly down by ~0.7 dex or more⁶. Although surely present, WCS analyses yield marginal identification of P II (85% confidence interval). Guthrie⁷ finds P up by ~0.8 dex in this star. By contrast, S II is identified with WCS at nearly the 11 sigma level, with 12 of 13 of the strongest lines found within ± 0.01 Å. Measured equivalent widths for these lines are in the 4-7 mÅ range, and suggest a sulfur abundance comparable to that in Upsilon Her (which shares a similar temperature and gravity). If so, HR7143 would be deficient in sulfur by only 0.5 dex⁸.

3. Sr II/Y II/Zr II: WCS studies reveal only marginal support for the identification of Sr II, but this ion is surely present as the resonance lines at 4077.71 and 4215.53 Å are both found with measured wavelengths within 0.02 Å of the laboratory positions. By contrast, Y II is richly represented at >18σ confidence level with 40 of 41 coincidences on lines drawn from Monograph 145⁹ and from stellar features found in Gamma Equ. Interestingly, WCS produces only marginal evidence (97.7% confidence) for Zr II. The two strongest lines in Monograph 145, 3391.8 and 3438.23 Å are present, both within 0.01 Å of the laboratory wavelengths, but their central intensities are only about 0.90 (10% deep). WCS data thus strongly corroborate the violation of the even-odd effect in this triplet of elements discovered by Guthrie⁷, who found Y (Z=39) up by a factor of 40 over its adjacent even-Z neighbors Sr and Zr.

4. Yb III: WCS gives 98.9% confidence to this identification on the basis of 8 coincidences out of 21 wavelengths sought. Based on their strengths and appearance, several of the lines are clearly dominated by other species, but at least one line (3325.51 Å) is an unblended Yb III feature. Preliminary abundance analysis (F. Castelli, private communication) yields a value for this ion nearly 4500 times solar. To our knowledge, Yb III has been found in only one other CP star, Alpha² CVn, in IUE spectra obtained by Hensberge et al.¹⁰

5. Xe II: Singly-ionized xenon has been previously identified in Kappa Cnc and 112 Her¹¹, and in HR7361⁸, which is extremely Xe-rich (~4.6 dex over solar). WCS reveals the presence of this ion at the 6σ level, and all the strongest features in Monograph 145 can be accounted for, with the measured lines all within 0.01 Å of the lab wavelengths. Measured equivalent widths for Xe II lines at 4414.84 Å and 4603.03 Å yield values of ~5 mÅ in each case, smaller than those found in κ Cnc or HR7361 but still likely to lead to a substantial over abundance of this element in HR7143. Spectrum synthesis calculations for this species are now underway.

· FOR ADDITIONAL WCS DETAILS, SEE THE OUR WEBPAGE.

· THE CASE FOR PALLADIUM (Z=46):

-Pd I/Pd II: Pd I has been identified in HR7143 by WCS at greater than the 5 sigma; level, while Pd II is observed at about the 3 sigma; level (99.9% confidence). Line-by-line searches have revealed at least 6 weak (central intensities <0.90), unblended features attributable to Pd I, and 4 others of similar character that are due to Pd II. Although singly-ionized palladium has been found in other CP stars (e.g., HR465¹¹ and chi Lupi¹², to our knowledge HR7143 is the only other CP star, save HR465, known to display lines of neutral Pd.

- Spectrum synthesis calculations have been carried out for most of the unblended lines using the suite of programs developed at Michigan¹³.

- Wavelengths and oscillator strengths for the Pd I lines were taken from the VALD site¹⁴, while comparable information for the Pd II lines was derived from the work of Quinet¹⁵.

- A Kurucz model atmosphere with T_eff = 12,000 K, log g = 3.95 and V_t = 1.0 km/s was adopted following Hubrig and Castelli³. A value of v sin i = 2.5 km/s and a Gaussian broadening of 0.02 Å was also employed.

- The initial abundance file was prepared specially for this star based on prior abundance analyses in the literature (see e.g., References 5-7, 16-22). Some of the starting abundances were adjusted on the basis of subsequent spectrum synthesis analyses to produce better agreement with the observations. For example, the Y abundance was increased by 0.5 dex over the value initially taken from Guthrie⁷, and the Mn abundance, originally from Smith & Dworetsky¹⁶, was raised by 0.4 dex. Similar changes were made for Fe and Ti.

- Figures 1-4 show representative plots of 2.5 Å segments of the synthetic spectrum (solid line) of HR7143 in the vicinity of several Pd I and Pd II lines compared to the observations (dotted line). Other prominent features in each panel are also identified. The adopted log(Pd) abundance in each figure is 5.69 on a scale where log(H) = 12.00. This is 4.00 dex greater than the solar abundance of this element²³, and nearly five times the Pd abundance found by Lundberg et al.¹² in chi Lupi.

- It is noteworthy that both Pd I and Pd II appear to be well fit by the same abundance. This suggests that the excitation/ionization conditions are similar for both species and that stratification effects, increasingly found in many CP stars^24,25, are not present, at least for this element, in HR7143.

ACKNOWLEDGEMENTS

The authors would like to thank Dr. Fiorella Castelli for helpful correspondence during the course of this investigation and for providing us with the model atmosphere used in the spectrum synthesis calculations. We also appreciate Dr. Castelli’s willingness to share the results of her abundance analysis for Yb III in advance of publication.

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