A comprehensive ultraviolet and optical region abundance analysis of the field horizontal branch Population 2 A-type star HD 109995 is described. Coaddition of IUE high dispersion images and DAO 6.5 A/mm IIaO spectrograms improved the signal-to-noise ratio of the data. We have identified ultraviolet lines whose analysis will provide more complete and accurate elemental abundances than those obtained from optical region spectra alone. A preliminary elemental abundance analysis of the optical region shows that log Z/Z (solar) approx. = -2. A first attempt to synthesize two Fe 2 ultraviolet resonance lines yields an iron abundance a few tenths of a deg higher than the average obtained from optical region Fe 2 lines

Adelman, S. J.

Leckrone, D. S.

English

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https://ntrs.nasa.gov/search.jsp?R=19860003804 2020-03-20T15:57:21+00:00Z
THE F1ELD HORIZONTAL-BRANCH STAR HD 109995: NEW RESULTS WITH COADDED 
ULTRAVIOLET AND OPTICAL REGION SPECTRA 
Saul J. Adelman1 ,2,3,4 and David S. Leckrone2 
NASA Goddard Space FH-ght Center 
ABSTRACT. A compr.ehensive ultraviolet and optica.l region abundance 
analysis of the field horizontal branch Population II A-type star HD 
109995 is described. Coaddition of IU~ high dispersion images and DAO 
6.5 A/mm IIaO spectrograms itnpI'oved the signal-to--noise ratio of the 
data. We have ~.dentified ultraviolet lines whose analysis will provide 
more complete and accurate elemental abundances than those obtained from 
optical region spectra alone. A preliminary elemental abundance 
analysis of the optical region shows that log Z/Ze = -2. A first 
attempt to synthesize two Fe II ultraviolet resonance lines yields an 
iron abundance a few tenths of a dex higher than the average obtained 
from optical region Fe II lines. 
1. INTRODUCTION 
HD 109995 is one of the pro\:otype field horizontal-branch 
Population II A-type stars. FrDm a study of low dispersion IUE 
exposures, Bohm-Vitense (1980) found its mass is about 0.5 Me and its 
temperature 8100 K, with these values somewhat dependent on the exact 
metallicity. Abundance analyses (e.g., Wallerstein and Hunziker 1964, 
Kodaira et ale 1969, Kodaira 1973) show it is metal-poor (log Z/Ze _ 
-1.5) with the anomalies generally increasing with atomic number. In 
the optical only lines of Na I, Mg I, Mg II, Al I, S1 I, Si II, Ca I, 
1 NRC-NASA Research Associate 
2 Guest Investigator with the International Ultraviolet Explorer 
Satellite which is sponsored and operated by the National Aeronautics 
and Space Administration, by the Space Research Council of the United 
Kingdom, and by the European Space Agency; Guest Invesitgator at the 
Jegiona1 Dat,~\ Anal.ysis Facility at NASA Goddard Space Flight Center 
4 Visiting Observer at Dominion Astrophyical ObsbL'vatory 
Visiting Astronomer, Kitt Peak National Observatory 
(NASA-CB-176327) IHE FIELD 
HORIZONTAL-BRANCH STAB HD 109995: NEW 
RESULTS WITH COADDED ULTRAVIOLET AND OPXICAL 
REGION SPECTRA (Citadel colI.) 8 p 
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ea II, Sc II, Ti II, V II, Cr I, Cr II, Mn I, Fe I, Fe II, Sr II, Y II, 
and Ba II have been studied. But the derived abundances except for 
those from Ti II, Fe I, and Pe II lines, depend on at most two lines per 
at,omic species. 
To improve our knowledge of the abundances of lID 109995, we have 
undertaken the fi~st high dispersion ultraviolet analysis with tUE 
satellite data of a Population II field horizontal-branch A star and a 
new photographic region analysis. Further Adelman with D. S. Hayes and 
A. G. Davis Philip have observ~d C I, N I, and 0 I lines in the infrared 
with ceo detl~ctors in the coude spectrograph of Kitt Peak National 
Observatory_ The 0 I triplet ~~7772-7775 is definitely present. From 
these obset"vations C, N, and 0 abundances (or upper limits) will be 
del'ived consistent with our forthcoming ultraviolet and optical study" 
An earlier study by Kodaira and Tanaka (1972) found that C and 0 are at 
least deficient by a factor of ten. 
2. ULTRAVIOL.ET LINE IDENTIFICATIONS 
We have obtained eight SWP and six LWP high dispersion IUE images 
of lID 109995. Two exposure times were used with each camera to increase 
the region of good expo9ure. The images for each camera were added as 
described by Adelman and Leckrone (1985) to 'increase the signal-to-noise 
ratio. 
Longward of ).1500, the ultraviolet energy distribution of lID 
109995 is fairly flat in FA" But shortuard of thi,; wavelength it 
rapidly decreases in brightness. As the Geasitivity of the IUE camera-
detector combinations increase towards the longward portions of their 
spectral ranges, the primary regions for our line identification study 
are AA1400-2100 and AA2450-3050. Some, especially strong lines can be 
distinguished from noise shortward of these regions. 
In the ultraViolet, the line density is much greater than the 
optical region. But it is not as great as nor are the lines as strong 
as those in e Leo (A2V), the coolest Population I star for which we have 
coadded many IUE high dispersion spectra. Figures 1 through 3 show 
selected regions of the uv spectrum. Figure 1 shows AA2590-2602 where 
there are strong Fe II lines, Figure 2 AA1740-1750 with two strong N I 
lines, and Figure 3 AA1655-1660 with C I multiplet 2. 
Our primary atomic line ;ldentification sources were The 
Ultraviolet Multiplet Ta.bl~ (Moo~e 1950, 1952, 1962), the finding list 
for Moore's newer multiplet tables (Adelman et ale 1985), A Multiplet" 
T~bles of Astrophysical Interest (Moore 19/.5) for lonr:ward of >.3000, 
N'SRDS-NBS 68. part I (Reader and Corliss 1980) t and studies of 
particu~ '~ atomic species: Ti II (Huldt ~t ale 1982), Cr II (Johansson, 
private couununication), and Fe II (Jnhanason 1978). There are still 
many lines without good identificatJ.ons. The majority of identified 
lines are those of Fe II. 'rhe foUorlng atomic species have lines which 
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WAVELENGTH IAI 
Fig. 1. The AA2590-2602 region of HD 109995. The strongest identified 
lines are those of Fe II. The interstellar (IS) component of Fe II. (1) 
2599.40 is shifted longward by 1.14 A as the stellar radial velocity is 
-132 km s-1 This spectrum is the coaddition of three LWP images • 
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WAVELENGTH IAI 
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Fig. 2. The AA1740-1750 region of HD 109995. The two lines of N I 
multiplet 9 are blended with weaker lines of other elements. To deduce 
the nitrogen abundance, the abundances of thes~ other elements must also 
be determined. Also seen are two lines of Ni II multiplet 5. The Fe It 
lines with a J for their multiplet numbers are from Johansson (1978). 
This spectra is the coaddition of five SWP images. 
we are certain are pre!3cllt in our IUE spectra: C I, C II, N I, 0 I, Mg 
I, Mg II, Al II, Al III, Si I, Si II, P II, S 1, Cl I, Ti II, V II, Cr 
II, Mn II, Fe I, Fe IJ., Co II, Ni II, Zn II, and Ge II. We expect both 
to improve the accuracy of abundances based on only a few optical lines 
and to derive abundances of additional elements. For a few atomic 
species, comparison of resonance lines and those arising from higher 
lying lower levels, might yield information on possible non-LTE effects. 
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Fig. 3. The AA1655-1660 region of HD 109995. The lines of C I 
multiplet 2 and Fe II lines are identified. The positions of the 
interstellar (IS) components of the C I lines are indicated. 
3. OPTICAL REGION SPECTRA 
There are considerable uncertainties in the published equivalent 
widths of the metal lines of HD 109995 in the literature (e.g., 
Wallerstein and Hunziker 1964 vs. Kodaira et ale 1969). To resolve 
these differences and to produce an improved set of equivalent widths, 
15 6.5 Almm IIaO spectrograms of HD 109995 were obtained at the Dominion 
Astrophysical Observatory in March 1985. Using the procedures discussed 
in Hill and Adelman (1985), they were added together to increase the 
signa1-to-noise ratio. The SiN of the coadded spectrum is estimated 
conservatively to be 30. The equivalent widths were measured by fitting 
Gaus~tan profiles thro~lgh the metal lines. The radial velocity of -132 
km s 1.s the same ~r that of K~daira et al.!l (1969). However, we find v 
sin i = 17 ± 1 km s compared with 27 km s found by Peterson et al. 
(1983). The rUE data ~L~ ~onsistent with the former$ but not the latter 
value. 
The optical lines were identified primarily by use of A Multiplet 
Table of Astrophysical Interest (Moore 1945). Certain weak lines 
included in previous studies were not positively identified, in 
particular the V II and Ba I! lines identified by Wallerstein and 
Hunziker (1964) and the Mn I, Y. II, and Ba II lines identifj.ed by 
Kodaira ~~. (1969). The equivalent widths of the former authors (WH) 
tend to be smaller than those of this study while those of the latter 
authors (KGO) tend to be larger. This study's values are close to the 
average of these two previous studies. The least squares relations for 
WA in rnA are as follows: WA(DAO) a 1.25 WA(WH) + 2.73 
and WA(DAO)- 0.81 W>.(KGO) - 6.08. 
The Hy profile published by Kodaira ~t, ~l. (1969) is in good 
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agreement with that derived from the coadded DAO spectrograms. The line 
shoulders are well-matched with the cOlldded Hy profile being slightly 
shallower in the core and deeper in the wings. The latter .is an effect 
of a better signal-to-noise ratio. 
4/, CHOICE OF MODEL AND PRELIMINARY RESULTS 
The criteria to be used in the selection Qf a final model 
atmosphere for HD 109995 are the match of the w)del's predictions with 
the ultraviolet and optical region fluxes, especially the Balmer jump 
region, and Hy profile; equal abundances from Fe I and Fe It lines; and 
the model having the same metal1icty as the star. Bohm-Vitense (1980) 
showed that the ultraviolet fluxes would be reasonably fit by an 8000 K, 
log g - 3.0 model with log Z/ZQ • -1.5. 
For an intia1 estimate of the iron abundance and microtubu1ent 
velocity, we selected 8000 K and 8500 K, log g = 3.0 and 3.5 models with 
log z/ze .. -1 and -2 from Kuruc2! (1979). '£he Fe I and Fe II gf,wvalues 
are from the critical comp:i.1ation by Martin, Fuhr, and Wiese (1986). 
The Fe II damping constants were calculated from the sums given by 
Kurucz (1981) and those for Fe I are semi-classical approximations. 
Microturbu1ent velocities are found by requiring that there be no 
dependence of the deduced Fe abundance on equivalent Hidth. Both 
optical and ultraviolet lines indicated that [Fe/Hi ~ -2 in accord with 
the results of Kodaira (1973). 
Comparison of the predictions of log z/ze = -1.5 and -2.0 model 
atmospheres calculated with the ATLAS6 code (Kurucz 1979) with optical 
spectrophotometry over ;\A3400-6790 of Philip and Hayes (1983) indicates 
8250 K, log g = 3.40. However, the predicted fluxes do not agree well 
with those derived from rUE low dispersion exposures. The uv fluxes and 
Hy profile suggest a cooler effective temperature of order 300 K. The 
log z/ze = -1.5 model fluxes agree slightly better with the uv fluxes 
than those of the log z/z(j .,. -2 ftO model. This indicates that the model 
atmospheres are underblanketed compared to the star. 
As the log z/Ze = -1.5 and - 2.0 model atmospheres which fit the 
optical fluxes are very similar, we decided to use our It)g z/z. = -2.0 
model for the abundance analysis. A joicroturbulence of 1.4 ± ~.1 km s-l 
was found from analysis of the Fe I and Fe II lines. In Talble I we give 
the values of log NIH and [NIH] = lCJ NIH - (log N/H)o for ~ID 109995, 
the values of log NIH for the Sun, and the average log NIH for ten 
Population I B and A stat's analyzed with the same gf values (Adelman 
1985) • The differences between the lGa I and ea II and the Cr I and Cr 
II results are also seen in the Population I stars. the latter 
difference may well be due to the poor quality of the Cr II gf-values. 
The abundance anomalies for the most part agree quite well with those of 
Kodaira (1973) who found HD 109995 more metal-poor than most other 
researchers. The major differences are that both calciu.m and chromium 
are more deficient in the present analysis despite the differences 
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Table t: Comparison of Abundances 
Atomic lID 109995 Sun Pop I Stars 
Speci-as log NIH [NIH) log NIH log N/lt 
Mg I -5.90 -1.52 -4.38 -4.23 
MgII -5.44 -0.96 -4.38 -4.29 
Al I -8.00 -2.49 -5.51 -5.81 
Si II -5.84 -1.47 -4.37 -4.40 
Ca I -7.48 -1.82 -5.66 -5.71 
Ca II -7.74 -2.18 -5.66 -5.96 
sc II -10.87 -1.91 -8.96 -9.16 
'I'i II -8.32 -1.30 -7.02 -6.97 
Cr I -8.44 -2.56 -5.88 -6.05 
Cr II -1.57 -1.69 -5.88 -5.'18 
Fe 1 -6.51 -2.14 -4.37 -4.30 
Fe II -6.24 -1.87 -4.37 -4.37 
Sr II -11.66 -2.46 -9.10 --8.88 
S. ULTRf.VIOLET LINE SYNTHESIS 
We have undertaken fully selt-consistent abundance analyses of 
nine stars, including lID 109995, six normal late '13 and early A-type 
dwarfs, and two Bp HgMn stars, which are based on the combination of 
high signal-to-noise optical region and ruE spectra& The rationale and 
methodology are briefly described in Leckrone and Adelman (1985). The 
~echniques and problems associated with the detailed synthesis of the 
complex line blends found in IUE sectra are discussed in Leckrone 
(1984). Our initial efforts have focused on the synthesis of two 
resonance features of Fe II, A2607.09 and A2625.7. The log gf values of 
these lines are particularly well known (to about 10%). Moreover, since 
the optical region spectrum of iron is rich and well understood, we can 
immediately establish whether the ruE data y:l.eld quantitative abundances 
consistent with ground-based observations. Preliminary results for the 
normal stars indicate agreement between A2607 and the average optical 
iron abundance to within about ± 0.1 dex, while A2626 typically yields 
values higher than the optical average by about 0.25 dex. Since 
abundances deri-ved from optical region spectra alone are usually 
uncertain by at best 0.2 dex, the agreement between IUE and optical 
region data can be viewed as reasonably good, giving co'nfidence that the 
rich ultraviolet spectra will yield reliable and unique ~bundance 
information. 
Figure 4 :l.llustrates the preliminary synthesis of Fe II A2607 in 
HD 109995. The calculations utilized the model atmosphere, micro-
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WAVELENGTH (AI 
Fig. 4. the profile of the ultraviolet resonance 1in~ of Fe II at 
),2607.09 in lID 109995. The soU.d curve portI;'sys the observations, while 
the do~t··d curve represents the computed profile with log Fe/R ,.. - 6.0. 
The computations include approximately 55 relatively weak contaminant 
blending lines, but is dominated by ),2607. 
turbulent velocity and rotation described in Sections 3 and 4. 
Abundances of contaminant blending species (principally Ti, V, Cr, Mn, 
and Co) were either adopted from Kodairn (1973) or were taken as 2 dex 
below solar. The observational data is based on a coadditon of only 3 
of our 6 LWP high dispersion images. When we incorporate our recently 
obtained data, the signa1-to-noise ratio of the observations will be 
somewhat improved. The best fit to ),2607 was obtainLd with log FelH = 
-6.0, while log FelH = -5.6 was obtained for ),2626. These are somewhat 
higher than the average value, - 6.2, obtained from optical Fe II 
lines. No great signficance should yet be attached to these 
differences, however, pending further reHnement of the model 
atmosphere, development of the final coadded IUE spectra, and analysis 
of many more ultraviolet lines. 
It is important to note that the problems of signal-to-noise and 
the resolution of blends which make analyses of the rUE spectra very 
difficult will be greatly ameliorated by the Hubble Space Telescope High 
Resolution Spectrograph. It is only with the 1atta~ instrument that 
moderately weak ultaviolet lines will be observed with sufficient 
accuracy to allow their reliable analysis. 
ACKNOWLEUGMENTS. SJA thanks Dr. Graham Hill and Mr. Wesley Fisher for 
their help with the coaddition of the DAD spectra of lID 109995. This 
research ws supported in pa'rt by NASA Grant NAG 5-218 to The Citadel. 
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The field horizontal-branch star HD 109995: New results with coadded ultraviolet and optical region spectra

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