1996MNRAS.279...25F
Mon. Not. R. Astron. Soc. 279, 25-31 (1996)
Higher Paschen lines in the spectra of early-type stars
Y. Fremat, 1,3 *:J: Leo Houziaux 1, 3t:J: and Y. Andrillat2
I/nstitut d'Astrophysique, Universite de Liege, 5, avenue de Cointe, B4000 Liege, Belgium
1281 (OMP) and Laboratoire d'Astronomie, Universite de Montpellier II, F34000 Montpellier, France
J Departement d'Astrophysique, Universite de Mons-Hainaut, 15 rue de la Halle, B7000 Mons, Belgium
2 URA
Accepted 1995 September 8. Received 1995 September 6; in original form 1995 June 14
ABSTRACT
We present observed and computed line profiles of Paschen lines in two nearly
atmospheric absorption-free spectral regions: 8350 to 8790 A and around the P7
line. We show that the Edmonds, Schluter & Wells semi-empirical theory of line
broadening is suitable for the computation of higher members of the Paschen series.
We compare the variations of the equivalent widths of P7 and P14 with CCD
observations of a sample of 09.5 to AO stars. It is shown that such spectral characteristics are well suited to the determination of atmospheric parameters of stars.
Key words: line: profiles - stars: early-type - stars: fundamental parameters.
1
INTRODUCTION
Hydrogen line profiles have ·long been considered as an
efficient tool for the determination of fundamental parameters of stellar atmospheres. Early Balmer lines have been
almost exclusively used for evaluating spectral types (Petrie
1965), effective temperatures and principally surface
gravities of stars in the 09-A9 range. The observation of
Paschen line profiles has been much rarer in these objects,
for several reasons. First of all, detectors available in the
0.7-1.1 /Lm range were until very recently of rather poor
sensitivity and difficult to use. Secondly, this spectral region
is badly obscured by numerous strong telluric absorption
bands owing to O 2 and water vapour. In fact only the P7 line
and the higher members of the series from P 11 to P20 fall in
an almost clear region; the profiles of P8, P9 and PI0 are
heavily distorted by atmospheric molecular lines. Earlier
members of the recently developed CCD receivers allow us
to nowadays observe properly calibrated spectra of earlytype stars from Pl1 up to P20, as well as clean spectra in the
region around 1 /Lm where P6 and P7 are located.
In this paper, we make use of spectra obtained at Observatoire de Haute-Provence and we propose to:
( 1) check the validity of the line-broadening theories
presently available;
(2) find out whether suitable diagnostic criteria may be
determined from measurable features in the spectrum.
* Fellow of the FRIA (Belgium).
Hnstitut de Mathematique, 15 Avenue des Tilleuls, B4000 Liege.
~E-mail: [email protected](YF);[email protected]
(LH)
2
THE PROGRAMME STARS
Table 1 lists the programme stars identified by their HD and
HR numbers. Most of them are MK standards; spectral types
are taken from the Bright Star Catalogue (Hoffieit 1982). We
searched in the literature for determinations of the effective
temperature and surface gravity for each object. The
adopted values are listed in Table 1; the numbers in brackets
correspond to the sources of information, which are listed in
the references beneath Table 1.
3
OBSERVATIONS
Spectra have been obtained between 1990 and 1993 with
the 1.93-m telescope at Observatoire de Haute-Provence.
Table 1. Programme stars identified by HD and HR numbers.
HR
1029
1552
1735
1790
1855
5291
6175
7001
Sp. Typ.
B7V
B2m
B5m
B2m
BOV
AOm
09.5 V
AOV
T.
VSio(i)
(KmIs)
('IX)
43
35 (1)
65 (1)
59
15 (1)
15 (1)
379
15
13500
20460 (2)
13510 (2)
21040 (2)
31500 (6)
9164 (4)
32500(3)
9550 (5)
LoN)
4.2
3.25 (2)
3.32(2)
3.41 (2)
4.13 (6)
3.3 (4)
3.5 (3)
3.95 (5)
References. (1) Slettebak et al. (1975), (2) Moon & Dworetsky
(1985), (3) Herrero et al. (1992), (4) Cayrel de Strogel et al.
( 1992), (5) Castelli & Kurucz ( 1994), (6) Killian ( 1992).
©1996 RAS
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1996MNRAS.279...25F
26
Y. Fremat, L. Houziaux and Y. Andrillat
..,on
on
on
e=
I
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
"I,' t..
,
++
t",11 II
1IIIII II
II I ,
I
I
I
I
.....
J
J++
3000
4000
5000
6000
7000
8000
9000
10000
11000
A
Figure 1. Computed spectrum (dashes) of Vega between 3000 and 11 000 A using the Castelli & Kurucz (1994) Vega model. Ordinates are
relative magnitudes m. - m S553 ' Crosses denote the observations by Hayes & Latham (1975) and the solid line gives the monochromatic
magnitudes by Cochran & Barnes (1981 ).
The Carelec spectrograph (Lemaitre et al.1990), mounted at
the Cassegrain focus, was equipped with different receivers:
in 1989, an RCA 512 x 323 30-/Lm2 pixel CCO, from 1990
January to 1992 April (resolution 1.6 A), a Thomson CCO
of 576 x 384 23-/Lm2 pixel (resolution 1.2 A) and from 1992
November, a 512 x 512 27-/Lm2 pixel (resolution 1.5 A).
The slit corresponds to 2 arcsec on the sky. Both spectral
regions were observed, covering the 8350-8760 A and
9840-10 200 A wavelength intervals. A neon lamp provided
wavelength calibration spectra in the region of the higher
Paschen lines, while a helium source was used in the P7
region. The spectra have been reduced at Observatoire de
Haute-Provence with the IHAP software; flat-field corrections
were made with a tungsten lamp. Equivalent widths of the P7
line were measured by linearly interpolating the continuum
between two wavelengths as close as possible to the line
centre, in order to avoid the inclusion of noisy regions, which
arise mainly from atmospheric absorption on the shortwavelength side. The line surface has been corrected for the
contribution of the He I ,t 10028 line. Also, a pseUdo-equivalent width has been measured for the Pl4line, which blends
with neighbouring lines of the series. The pseudo-continuum
has been defined by taking the nearest highest flux points on
either side of the line centre.
4
L TE SPECTRUM SYNTHESIS
Model atmospheres have been interpolated in the grid of
Kurucz (1979). Continuum opacities are taken from several
ATLAS6 subroutines (Kurucz 1979). Hydrogen line opacities
have been computed with a semi-empirical method used by
Edmonds, Schluter & Wells (1967) (ESW hereafter). In this
approximation both ions and electrons are treated as in the
quasi-static approximation, but ESW suggest that the
electron density should vary over the line profile. These
authors propose tables of Stark broadening functions which
include the effects of correlation and shielding for lines of the
Lyman, Balmer, Paschen and Brackett series up to n = 18.
Rather than interpolating the data in these tables, we established new subroutines under the same assumptions and
computed line absorption coefficients from these subroutines, which are now freely available on request. 1 Axial
rotation has been taken into account using a code provided
by Oelcroix (1974), improving the method proposed by
Underhill (1968).
5
NL TE EFFECTS
As it was likely that NLTE effects could affect the line
profiles especially for our hottest stars, we used the update
TLUSIY code version 178 (Hubeny 1988; Hubeny & Lanz
1992; Hubeny, Hummer & Lanz 1994; Hubeny & Lanz
1995; Lanz & Hubeny 1995) to compute a model atmosphere with the fundamental parameters of our programme
star HO 149757. NLTE popUlations of the hydrogen levels
are used to compute (ESW) line profiles for the same star. A
comparison indicates that the central intensities are somewhat lower in the NLTE case, but the highest difference,
occurring in the centre of P7, amounts, at most, to 1 per
cent. In view of the uncertainties in the measured profile (see
Section 8), we decided to neglect the NLTE effects.
6 TESTING THE SPECTRUM SYNTHESIS
PROCEDURE
In order to check the validity of our spectrum synthesis code,
we computed the spectrum of Vega between 3000 and
11 000 A, using an atmospheric model proposed by Castelli
& Kurucz (1994 ), with the following parameters:
Teff =9550 K, logg=3.95 cgs and ~lurb=2 km S-I. Fig. 1
gives the monochromatic magnitudes mv - m5 55 3' compared
to observations by Hayes & Latham (1975) and Cochran &
Barnes (1981). The computed fluxes have been adapted to .
the 42-A resolution of the observations of Cochran &
IFrom e-mail address for YF, given on title page.
©1996 RAS, MNRAS 279,25-31
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1996MNRAS.279...25F
Higher Paschen line profiles
atmospheric parameters of 11 Hydrae (HD 74280), as provided by Moon & Dworetsky (1985), and a rotational
velocity of 128 km s - I. These computations are in fair agreement with the monochromatic magnitudes measured by
Cochran (1981).
( i
=
~
I
7
VEGA (ESW)
O~--r---+---'---~--.---~---r---r--~--~
VEGA
o L -______
8350
~
8440
______
~
27
(ves)
_______ L_ _ _ _ _ __ L_ _ _ _ _ _
8530
8620
8710
~
8800
Figure 2. Above, relative fluxes of Vega (normalized at 8380 A) in
the Paschen lines computed ( + ) with the ESW broadening scheme
and the observed spectrum (solid line) where the Ca II infrared
triplet components blend with P13, PIS and P16. The 01 line at
,1,8446 blends with the PI8 line. In comparison with the lower part
of the figure, one can see that the VCS broadening theory as used in
the SYNTHE code is less adequate for computing the spectrum for
lines higher than P12.
Barnes. Our results seem to fit better the observed fluxes
than those of Castelli & Kurucz (1994). These authors used
their SYNTHE code based on the Vidal, Cooper & Smith (1973)
code (hereafter referred to as VCS). Our computations
show that for lines earlier than P12, ESW and VCS theories
lead to very similar results. However for lines higher than
P12 in the AA8360-8780 region, it can be seen (Fig. 2) that
the ESW broadening calculations lead to a better agreement
with observations than the VCS theory as applied by the
SYNTHE code. Fig. 3 displays the spectrum between 8200 and
9400 A for several temperatures and gravities around
Teft = 18 000 K and log g = 3.5 cgs. Ordinates are monochromatic magnitudes mv relative to msooo' Fig. 3(a) shows
the relative increase of the flux towards the longer wavelength when the temperature lowers, while Fig. 3(b) shows
that an increase in gravity has only a mild but non-negligible
effect on the continuum level in the Paschen and Brackett
continua. In Fig. 3(c), we computed the flux distribution with
RESULTS
As the above tests provided enough confidence in our
spectrum synthesis code, we computed theoretical spectra in
the 8400-8775 A range and around the Paschen 7 line at
10049 A with the parameters found in the literature as
mentioned in Table 1. Figs 4 and 5 show, for seven stars of
various types, the close fit for the P7 line profile. For some
stars a slight discrepancy appeared in the slope of the continuum, the observed continuum showing a more negative
slope which would lead to a temperature too low for the
star's spectral type. We attribute this discrepancy to guiding
problems. During some runs, the star was guided manually
on the slit, then more and more sophisticated guiding
manually on the slit, then more and more sophisticated
guiding systems were installed. Guiding defaults resulted in a
loss of longer wavelength radiation on the slit. Our flux
measurements of Vega mentioned above, taken in 1994
June, are in good agreement both with Cochran & Barnes
(1981) observations and with theoretical calculations. For
comparison of the computed and observed spectra, normalzed fluxes are taken at 8880 A (higher Paschen line region)
and at 10 200 A. However, for those spectra for which a
discrepancy is observed in the slope of the continuum, we
constrain the observed fluxes to be equal to the computed
values at two wavelengths, the second one being chosen at
the 'red' end of the spectral region. The very good agreement
between the observed and computed spectra for Vega leads
us to assume that the computed continuum is correct. We
then ascertain that the fit in the lines is indeed quite good. On
the other hand, the He I AA 10 028 and 10 138 lines are quite
conspicuous in the hotter objects. (See the spectrum of the
B2 III star HD 30836 in Fig. 4). In the range of the higher
Paschen lines (Figs 4 and 5), the agreement between synthetic and observed spectra is also quite satisfactory, the
obvious discrepancies being caused by the Ca II infrared
triplet and the 0 I A8446 line. In the hottest and most
luminous stars of the sample, He I lines at AA8779 and 8584
may influence the profiles of the P 12 and P 15 lines, respectively (Jaschek et al. 1994). The agreement with the observations is quite satisfactory as far as both the line shape and
continuum slope are concerned. However for some objects,
observed at about the same epoch, we note the same continuum discrepancy as in the case of P7. For the first test star
of Table 1 (HD 21071) (Fig. 4), we propose as atmospheric
parameters on the basis of our spectra, Teft = 13 500 K, and
log g = 4.2 cgs.
7.1 Variation of the P7 equivalent width for 0- to F-type
stars
P7 is located in a region free from atmospheric absorption
and its equivalent width can thus be easily measured. 84
Kurucz models have been used to compute the P7 line
profiles for temperatures ranging from 6000 to 40000 K
and for gravities ranging from logg=2 to 4.5. The general
© 1996 RAS, MNRAS 279, 25-31
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
9
[IJ
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.....
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= 3.5
-fSE.'\
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1
8800
A
,
.'t
K • Log g
K , Log g
K , Log g
K , Log g
K , Log g
17000 K , Log g
18000 K , Log g
= 20000
,-\
= 3.75
~
9100
c
b
9400
1;"'-1- .. -+=
'~+
K'-A
~~i<~~+JI\~jjr--i\}. - , '~-_'
~\ ~,'
+'\\
v.
+\\J/J
..
- , - -, - , '±" ..L'+>p,
a
Figure 3. (a) Variation of the relative magnitude ms"",,- m" with the temperature from 17000 to
20000 K. The crosses denote the observed magnitude of the star HD 74280 by Cochran (1981). (b)
Variations of brightness with gravity at Tefl = 18 000 K. (c) Best fit for the star HD 74280; rotational
velocity has been taken as 128 km s - ,.
0.08200
0.1
0.2
0.3
0.4
0.5
0.0
0.1
0.2
I
Q.,
e
0.3
Q.,
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0.5
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0.1
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~950
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He 1 (1002S)
I
He 1 (1013S)
HD 30836
B2 III (20460.3.25)
loJSO
HD 35468
B2 III (21040.3.41)
HD 34503
B5 III (13510.3.32)
p:
HD 35468
B2 III (21040,3.41)
101S0' =M::cS::::o--'-----'---'----'--s:::s::::oo
\:
HD 34503
B5 III (13510,3.32)
====l==+==1=~=
L ,
I I
He 1 (S779)
HD 30836
III (20460.3.25)
I ====l=====i'===='P'==4====\
--+----+---1--"
LI
I
~
HD 21071
B7 V (13500,4.2)
Figure 4. Computed (+) and (solid line) observed profiles of the P7 and higher
Paschen lines for HD 21071, 30836, 34503 and 35468 together with their spectral
class and atmospheric parameters. Ordinates are fluxes normalized at 10 200 A in
the P7 section. Note the presence of the He I AA 10 028 and 10 138 lines around P7
for HD 30836 and 35468. Ordinates are normalized at 8800 A in the section of the
higher lines. The presence of the He I AA 8584 and 8779 lines in the same two stars.
0.5
0.5
1
0.5
0.5
1
HD 21071
B7 V (13500,4.2)
~
~
.....
~
::::.:
~
;::s
~
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;::s
~
~.
~
~
r-
~
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00
tv
1996MNRAS.279...25F
1996MNRAS.279...25F
Higher Paschen line profiles
HD 36512
BO V (31500,4.13)
29
HD 36512
BO V (31500,4.13)
0.5 1 - - - - ! - - - ! - - - - i I - - - - - - 1 ';===*==!r==*==*=~
HD 123299
AO III (9164,3.3)
HD 123299
AO III (9164,3.3)
1----+---+---+-----1
F===i==~==;::==='r====;
HD 149757
09.5 V (32500,3.5)
HD 149757
09.5 V (32500,3.5)
1
0.5
1
0.5 '::-:-:::-:c---'---~~=----'----,-c~ '::-:-:::::---'-_ _-'--_---1_ _-'-~
9950
10050
10150 8350
8800
Figure 5. Normalized fluxes as in Fig. 4 for the stars HD 36512, 123299 and 149757.
shape of the results (Fig. 6) indicates a rapid increase with
temperature up to a maxima ranging from 9 to 19 A depending on the gravity; then the equivalent width slowly decreases
as in the case of other hydrogen lines. The temperature, at
maximum equivalent width, shifts towards higher temperatures as gravity increases. This is a combined effect of Stark
broadening, excitation and ionization. We plotted also in
Fig. 6 the mean curves observed for the equivalent width of
P7 as observed by Andrillat, Jaschek & Jaschek (1994) for
types I, III and V stars. Their fig. 3 has been converted from
spectral types to temperatures according to a scale given by
Gray & Corbally (1994). Those curves agree fairly well with
the computed values. However, it seems that for high-gravity
objects the observed values are lower than the computed
ones. For such stars, the wings may extend rather far and
noise may prevent a correct measurement of the surface of
the line (Cowley 1987).
7.2
Variation of the P 14 pseudo-equivalent width
The pseudo-equivalent width of the 'purely hydrogenic' P14
line, as defined in Section 3, has been computed for 68
Kurucz models, with temperatures ranging from 6000 to
30000 K and log g ranging from 2 to 4.5. Results are given
in Fig. 7. It can be seen that this pseudo-equivalent width
decreases with increasing gravity. This is of course an effect
of the very high blending of the components of the Paschen
series. It should be noticed in Fig. 7 that the superposition of
the lines around P 14 tend to vanish when log g - 2 cgs, as the
equivalent width decreases for the curve labelled '2' in Fig. 7.
We plotted, on the same graph, the observed variation of this
pseUdo-equivalent width, measured for a large sample of
stars by Andrillat, Jaschek & Jaschek (1995). Their fig. 14
has been converted from spectral types to effective temperatures using the same scale as above. The agreement is satisfactory for main-sequence and giant stars, but the supergiants do not seem to reach the high values of the equivalent
width predicted. For such lines, however, noise is observed in
the wings and they tend to merge with the 'continuum'. It is
also quite likely that in the 8000 to 12000 K temperature
range, the values of gravity in B8-A5 supergiants (e.g.
a Cygni) are lower than log g = 2. However, the shift of the
maximum equivalent width towards decreasing temperatures, as predicted by theory, fits with the observations. We
also explored the effect of high rotational velocities on the
P 14 pseudo-equivalent width. The effect varies with log g,
but remains quite modest. At v sin i = 300 km s - 1, the
equivalent width is 2 per cent lower at log g = 2.5, while at
log g = 4 the effect is at most 4 per cent.
©1996 RAS, MNRAS 279, 25-31
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1996MNRAS.279...25F
30
Y. Fremal, L. Houziaux and Y. Andrillal
20
o
5000
11000
17000
23000
29000
35000
41000
Effective Temperature (K)
Figure 6. The computed equivalent widths of the P7 line as a function of temperature for six values of the gravity [logg = 2 (0.5) 4.5J. Long
dashes represent the mean observed curve for class I stars, while medium and short dashes represent class III and class V stars, respectively.
Crosses' +. represent individual observed equivalent widths for class V stars, while' X' relates to class III stars. We also give for each object the
log g taken from the literature.
7
6
Ul
------
E5
e
en0'
!4
.r:
'D
~3
.....c
3.41
Q)
o
.2: 2
:::l
0"
W
o
6000
12000
18000
24000
30000
Effective Temperature (K)
Figure 7. The computed pseudo-equivalent widths of the P 14 line as a function of temperature for six values of the gravity as in Fig. 6. Short
dashes represent the mean observed curve for bright main-sequence stars, while medium and long dashes refer to III and I class stars,
respectively. Crosses and log g have the same meaning as in Fig. 6.
8
CONCLUSIONS
The semi-empirical method proposed by Edmonds et al.
(1967) for the calculation of the hydrogen line profiles, easy
to use, is not only suitable for computing Balmer line pro-
files, as shown by Peterson (1969), but leads to satisfactory
results also for Paschen lines. It is better suited to the computation of the higher members of the Paschen series than the
ves theory as used in the SYNTHE code. NLTE effects have
been found to have a negligible impact on the equivalent
©1996 RAS, MNRAS 279, 25-31
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1996MNRAS.279...25F
Higher Paschen line profiles
1.15
1.2
1.05
1.1
0.95
1.0
31
.•
.
09.5 V (32500.3.5)
0.85
9900
10000
10100
I
HD 149757
HD 149757
09.5 V (32500.3.5)
0.9
10200 8350
8440
8530
8620
8710
8800
Figure 8. Comparison of the observed (+) and computed relative fluxes for the 09.5 V star HD 149757. Comparison of the solid line (LTE)
to the dashed spectrum shows that at such a temperature (32 500 K) NLTE effects remain very limited.
widths. Equivalent widths of the P7 and P14lines, as well as
profiles of the Paschen lines observed by current CCD techniques, provide good tools for determining basic parameters
of stellar photospheres.
ACKNOWLEDGMENTS
We thank Drs I. Hubeny and F. Castelli fOJ: providing us with
the TLUSTY and SYNTHE computer codes, respl:!ctively. LH
would also like to express his thanks to the Fonds National
de la Recherche Scientifique (Belgium) for the grant
2.4502.94 (FRFC) and to the Commissariat General aux
Relations Internationales de la Communaute Fran~aise
(Belgium) for travel assistance. This workis based on observations obtained at Observatoire de Haute-Provence.
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©1996 RAS, MNRAS 279, 25-31
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System