Issue |
A&A
Volume 520, September-October 2010
|
|
---|---|---|
Article Number | A88 | |
Number of page(s) | 12 | |
Section | Stellar atmospheres | |
DOI | https://doi.org/10.1051/0004-6361/200913750 | |
Published online | 08 October 2010 |
Realistic model atmosphere and revised
abundances of the coolest Ap star HD 101065
,![[*]](/icons/foot_motif.png)
D. Shulyak1,3 - T. Ryabchikova2,3 - R. Kildiyarova4 - O. Kochukhov5
1 - Institute of Astrophysics, Georg-August-University,
Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
2 - Institute of Astronomy, Russian Academy of Science, Pyatnitskaya
48, 119017 Moscow, Russia
3 - Institute of Astronomy, Vienna University, Türkenschanzstrasse 17,
1180 Vienna, Austria
4 - Institute of Spectroscopy, Russian Academy of Science,
Physicheskaya 5, 142190 Troitsk, Russia
5 - Department of Physics and Astronomy, Uppsala University, Box 515,
751 20 Uppsala, Sweden
Received 26 November 2009 / Accepted 31 March 2010
Abstract
Aims. Among the known Ap stars,
HD 101065 is probably one of the most interesting
objects, demonstrating very rich spectra of rare-earth elements (REE).
Strongly peculiar photometric parameters of this star can not be fully
reproduced by any modern theoretical calculations, even those
accounting for realistic chemistry of its atmosphere. In this study we
investigate a role of missing REE line opacity and construct a
self-consistent atmospheric model based on accurate abundance and
chemical stratification analysis.
Methods.
We employed the LL MODELS stellar model
atmosphere code together with DDAFIT and SYNTHMAG software packages to derive
homogeneous and stratified abundances for 52 chemical elements
and to construct a self-consistent model of
HD 101065 atmosphere. The opacity in
REE lines is accounted for in details, by using
up-to-date extensive theoretical calculations.
Results. We show that REE elements play a key role
in the radiative energy balance in the atmosphere of
HD 101065, leading to the strong suppression of the Balmer
jump and energy redistribution very different from that of normal
stars. Introducing new line lists of REEs allowed us to reproduce, for
the first time, spectral energy distribution of HD 101065 and
achieve a better agreement between the unusually small observed
Strömgren index
and the model predictions. Using combined photometric and spectroscopic
approaches and based on the iterative procedure of abundance and
stratification analysis we find effective temperature of
HD 101065 to be
K.
Key words: stars: chemically peculiar - stars: atmospheres - stars: individual: HD 101065
1 Introduction
Since its discovery by Przybylski in 1961 (Przybylski 1961) and named later after him, the Przybylski's star (HD 101065, hereafter PS) remains one of the most intriguing objects among chemically peculiar (CP) stars. These objects are usually characterized by abundance anomalies in their atmospheres and by non-uniform horizontal and vertical distributions of chemical elements and/or surface magnetic fields of different intensities.
From many observed characteristics PS is a typical
representative of the coolest part of rapidly oscillating A-type (roAp)
stars. It pulsates with the typical roAp period, 12 min
(Kurtz & Wegner 1979),
and it was the first roAp star discovered. It possesses a
2.3 kG surface magnetic field (Cowley
et al. 2000) and its low Fe abundance (an
order of magnitude below the solar value)
follows the trend of Fe abundance versus effective temperature for
Ap stars in 6300-14 000 K range (Ryabchikova
et al. 2004; Kochukhov 2003). Przybylski was the
first to note a low Fe abundance and a strong overabundance of
some rare-earth (REE) elements (Przybylski
1966). These features were confirmed in many subsequent
spectroscopic studies of this star (Cowley et al. 1977,2000; Wegner &
Petford 1974; Cowley & Mathys 1998).
For instance, in the latter study Cowley
et al. (2000) reported an overabundance by up to
4 dex (compared to the Sun) for elements heavier than Ni and
enormously strong lines of the second ions of REEs compared to those of
the first ions. This effect is now commonly known as REE
anomaly (Ryabchikova
et al. 2004,2001).
As new data for REE transitions becomes available via laboratory measurements, more lines of REEs are identified and measured in spectra of cool CP stars, thus improving abundance results. Later on it was discovered that PS is not even a champion in REE overabundances. At least a couple of hotter Ap stars: HD 170973 - Kato (2003); HD 144897 - Ryabchikova et al. (2006), with accurately derived REE abundances from numerous lines of the first and second ions have similar or even higher REE atmospheric overabundances. But a combination off a low temperature, low iron-peak abundances and large REE overabundances result in the extremely unusual observed spectrum of PS: weak lines of the iron-peak elements are lost in a forest of strong and numerous REE lines. For example, the typical density of only classified spectral features contributing to the theoretical spectra at 5375 Å is 6-10 lines per Å, and still not enough for proper description of the PS spectrum since every second feature can not be accounted for in the modern spectrum synthesis.
Accurate spectroscopic analysis of PS requires dedicated model
atmospheres. Indeed, abnormally strong REE absorption indicates that
the atmospheric structure of the star may deviate significantly from
that of normal, solar abundance stars. This is clearly seen, for
instance, for the Strömgren
photometric index that is close to zero, which is very different than
for normal stars.
Previous attempt by Piskunov
& Kupka (2001) to account for peculiar absorption in
the spectrum of PS and to fit color-indices based on the model
atmosphere techniques faced a difficulty of the absence of complete
line lists of REE and the impossibility to fit simultaneously
photometric and spectroscopic features (like hydrogen lines)
with the same model atmosphere. Piskunov
& Kupka (2001) used ATLAS9 (Kurucz 1992,1993) atmospheres with opacity
distribution functions (ODF) recalculated for individual abundances and
artificially enhanced line (but not continuum) opacity of iron to
simulate the missing REE opacity. They finally adopted K,
model
as a preferable choice for spectroscopic analysis. However,
this model failed to explain peculiar photometric
parameters of the star. Enhanced metallicity model by Piskunov & Kupka (2001)
lead to the decrease of
index but its
theoretical magnitude is still too high compared with the observed
value. This model was used in extensive abundance study of PS
by Cowley et al. (2000).
Another striking evidence of the abnormality of the atmospheric structure of PS is the presence of the so-called core-wing anomaly in the hydrogen Balmer lines (Cowley et al. 2001). This abrupt transition between the Doppler core of the hydrogen lines and their Stark wings has been modeled empirically by Kochukhov et al. (2002) in terms of a temperature increase by 500-1000 K at intermediate atmospheric heights but so far has not been explained theoretically.
Thus, HD 101065 remains an extremely challenging and very intriguing target for the application of advanced model atmosphere analysis and high-resolution spectroscopy. In this paper we present a new attempt to combine both of these approaches and to construct a self-consistent model atmosphere of PS taking into account realistic chemistry of its atmosphere and employing up-to-date theoretical calculations of the spectra of selected REEs.
Starting with a brief description of observations in Sect. 2, we then present methods of analysis in Sect. 3 and describe the new lists of REE lines employed in the model atmosphere calculations. Results of our analysis are presented in Sect. 4 and are followed by general conclusions in Sect. 5. Our paper concludes with a brief discussion in Sect. 6.
2 Observations
In our study we employed several existing spectra of PS:
NTT spectrum with the resolving power
in the 3950-6630 Å
wavelength region, (Cowley
et al. 2000), UVES spectrum in the
3100-10 000 Å region with the resolving power
(Kochukhov
et al. 2002; Ryabchikova et al. 2008),
and an averaged UVES spectrum from the time-series observations
obtained in the 4960-6990 Å spectral region with the resolving
power
(Kochukhov
et al. 2007; Ryabchikova et al. 2007).
Details of the spectroscopic observations and full description
of the data reduction are given in the corresponding papers. Direct
comparison of the normalized spectra reveals a remarkable agreement
between all observations. This allows us to use the equivalent width
measurements from Cowley
et al. (2000), supplementing them with the
measurements in the spectral region Å
using the other two spectra.
3 Methods
3.1 Abundance analysis
The main goal of the present paper is to calculate a model atmosphere of PS accounting for its anomalous chemical composition. In magnetic chemically peculiar star an accurate abundance analysis may be carried out by fitting the magnetic synthetic spectrum to the observed line profiles. This procedure is very time consuming. Therefore, to simplify calculations, we analysed the equivalent widths replacing magnetic effects by a pseudo-microturbulence. It is sufficient for line opacity calculations, although the abundances derived using the common value of microturbulent velocity may be slightly inaccurate (see discussion in Cowley et al. 2000). In present analysis we used the Kurucz (1993) WIDTH9 code modified by Tsymbal (see Ryabchikova et al. 1997).
The VALD database (Piskunov et al. 1995; Kupka et al. 1999) and the DREAM REE line database (Biémont et al. 1999, and references therein), which is made accessible via the VALD extraction procedures, are our main sources for atomic parameters. For the REE, which provide the majority of lines in the PS spectrum, accurate laboratory transition probabilities measured by Wisconsin group are used: Lawler et al. (2001a) - La II; Lawler et al. (2001a) - La II; Lawler et al. (2009) - Ce II; Den Hartog et al. (2003) - Nd II; Lawler et al. (2006) - Sm II; Lawler et al. (2001b) - Eu II; Den Hartog et al. (2006) - Gd II; Lawler et al. (2001c) - Tb II; Wickliffe et al. (2000) - Dy I/Dy II; Lawler et al. (2008) - Er II; Wickliffe & Lawler (1997) - Tm II; Lawler et al. (2007) - Hf II. Among heavier elements Th and U are particularly interesting. For these species we used laboratory and calculated transition probabilities from the following papers: Nilsson et al. (2002a) - Th II; Biémont et al. (2002) - Th III; Nilsson et al. (2002b) - U II. Transition probabilities for REEs in the second ionization stage were taken from the DREAM database except Pr III Mashonkina et al. (2009), Nd III Ryabchikova et al. (2006), Eu III Wyart et al. (2008), Tb III and Dy III (Ryabtsev, private communication).
Extended measurements and theoretical calculations for the
REEs, Th and U justify revision of some of the
partition functions (PF), which until now mainly came from Kurucz (1993) ATLAS9 code.
Our recalculated PFs are made available online.
We did not take into account the hyperfine structure (hfs) in our abundance analysis because the hfs constants are available only for a small subset of spectral lines.
3.2 Stratification analysis
In the majority of cool Ap stars the atmospheres are not chemically homogeneous. Element stratification built up by atomic diffusion influences the atmospheric structure (Leblanc et al. 2009; Shulyak et al. 2009; Kochukhov et al. 2009). As it follows from Khan & Shulyak (2007), Fe, Si, and Cr are the elements that have the major effect on the atmospheric T-P structure in the case of homogenous abundances. The cumulative effect of element stratification on the model structure was demostrated by Wade et al. (2003) and later by Monin & LeBlanc (2007) who used diffusion calculations for 39 chemical elements simultaneously. Thus, the stratification analysis of PS was performed for four elements: Fe, Ba, Ca, and Si. It was not possible to carry out stratification analysis for Cr and other iron-peak elements due to weakness of their absorption features and heavy blending by REE lines. The lower energy levels of practically all Cr lines used in the abundance determination belong to a narrow energy range, which makes the Cr lines insensitive to abundance gradients. Hence, we restricted stratification analysis to the four elements since they are represented in HD 101065 by a sufficient number of atomic lines, probing different atmospheric layers and enabling line profile fitting.
We applied a step-function approximation of chemical stratification as implemented in DDAF IT - an automatic procedure for determination of vertical abundance gradients (Kochukhov 2007). In this routine, the vertical abundance distribution of an element is described by four parameters: chemical abundance in the upper atmosphere, abundance in deep layers, the position of the abundance jump and its width. All four parameters can be optimized simultaneously with the least-squares fitting procedure.
Most unblended lines in the PS spectrum are located in the red spectral region, where line profiles are also substantially distorted by the Zeeman splitting due to wavelength dependence of the Zeeman effect. DDAF IT enables an accurate stratification analysis of such magnetically-splitted lines. In the search for optimal vertical distribution of chemical elements we use magnetic spectrum synthesis with the S YNTHMAG code (Kochukhov 2007). This software represents an improved version of the program developed by Piskunov (1999).
A list of the lines used in stratification analysis is given
in Table 3
(Online material). To get a better sensitivity of the fitted
Ca distribution to the upper atmospheric layers, we used the
IR-triplet Ca II line at 8498 Å.
As shown by Ryabchikova
et al. (2008) and Cowley
et al. (2009), its core is represented
entirely by the absorption of the heavy isotope 48Ca.
Therefore wavelength of the line of this isotope is given in
Table 3.
3.3 Calculation of model atmospheres
To perform the model atmosphere calculations we used the recent version of the LL MODELS (Shulyak et al. 2004) stellar model atmosphere code. The code accounts for the effects of individual and stratified abundances. The stratification of chemical elements is an input parameter and thus does not change during the model atmosphere calculation process. This allows us to explore the changes in the model structure due to stratification that was inferred directly from observations, without modeling poorly understood processes that could be responsible for the observed inhomogeneities.
Note that such an empirical analysis of chemical element
stratification is based on the model atmosphere technique and thus the
temperature-pressure structure of the model atmosphere itself depends
upon the stratification which we want to determine. Therefore,
the calculation of the model atmosphere and the stratification
(abundances) analysis are linked together and an iterative procedure
should be used in this case. It consists of repeated steps of
stratification and abundance analysis that provide an input for the
calculations of model atmosphere until atmospheric
parameters (
,
, etc.)
converge and theoretical observables (photometric colors, profiles of
hydrogen lines, etc.) fit observations. We refer the reader to
the recent papers (Shulyak
et al. 2009; Kochukhov et al. 2009) for a
more detailed description of this technique.
The VALD database (Piskunov
et al. 1995; Kupka et al. 1999) was used
as a main source of the atomic line data for computation of the line
absorption coefficient. The recent VALD compilation contains
information on about 66 106 atomic
transitions. Most of them come from the latest theoretical calculations
performed by Kurucz
.
Since a 2.3 kG surface magnetic field of PS is too weak to affect noticeably the atmospheric structure and energy distribution (see Kochukhov et al. 2005; Khan & Shulyak 2006, for more details) we adopted a 1 km s-1microturbulent velocity to roughly account for the magnetic broadening and to avoid time-consuming model calculations with detailed polarized radiative transfer.
3.4 REE line lists for model atmosphere calculation
The main sources for the REEs transition probabilities data between experimentally known energy levels are VALD and DREAM databases, hereafter referred to ``VALD data''. These data were supplemented by the transition probabilities for the observed and predicted lines of Pr II-Pr III (Mashonkina et al. 2009), Nd II-Nd III (Mashonkina et al. 2005; Ryabchikova et al. 2006), Eu III (Wyart et al. 2008), Tb III and Dy III (Ryabtsev, private communication), hereafter referred to as ``ISAN data''. For the latter two ions transition probability calculations were based on the extended term analysis.
Table 2 demonstrates a dramatic difference between the number of observed (VALD) and predicted (ISAN) transitions for Pr and Nd, in particular for the lines of the first ions. We are missing more than 90% of the potential line absorbers. The same situation is expected for lines of the other REEs. Taking this situation into account, we performed calculations for Sm II - the second-abundant element after Nd in PS. The energy levels of Sm II were calculated by the Hartree-Fock method implemented in the Cowan (1981) code. The ground state of Sm II is the 4f66s configuration. In addition, our calculations include even configurations 4f6nd (n=5,6), 4f67s, 4f55d6p, 4f56s6p, and odd configurations 4f6np (n=6-8), 4f45d26p, 4f46s26p, 4f55d2, 4f55d6s. Calculations are based on the wave functions obtained by the fittings the energy levels. As in the case of Pr II and Nd II, all Hartree-Fock transition integrals are scaled by a factor 0.85. In total, transition probabilities for more than one million lines were calculated and added to the ISAN list of the REE lines.
The recent studies by Mashonkina et al. (2009,2005) demonstrated that the line formation of Pr and Nd can strongly deviate from the local thermodynamic equilibrium. For instance, the doubly ionized lines of these elements are unusually strong due to combined effects of stratification of these elements and departures from LTE. However, a precise NLTE analysis is beyond the scope of this paper and currently could not be coupled to a model calculation. Therefore, we followed the approach outlined in Shulyak et al. (2009) where authors used a simplified treatment of the REE NLTE opacity.
Taking into account a systematic difference in abundances
derived for the first and second REE ions, it is
essential to reduce the oscillator strengths for the singly ionized
REE lines by this difference in the model line list while
using the abundances derived from second ions as an input for model
atmosphere calculations (or vice versa). Obviously,
the adopted reduction factors depend upon the assumed
abundances of the first and second ions and thus change slightly if the
model parameters (
,
, etc.)
are modified in the course of the iterative procedure of abundance
analysis. The logarithmic scaling factors,
,
adopted for our final model are (in dex): -1.6 (Ce),
-2.4 (Pr), -1 (Nd), -2.66 (Tb),
-2.5 (Dy), and -1.64 (Eu). These factors
were applied to all lines of the respective ions in the master line
list. Then this line list was processed by the line pre-selection
procedure in the LL MODELS code to
select only those lines that contribute non-negligibly to the total
line opacity coefficient for a given temperature-pressure distribution.
Applying this line scaling procedure allowed us to mimic the line
strengths correspond to the NLTE ionization equilibrium
abundance for each REE (see Shulyak
et al. 2009, for more details).
4 Results
4.1 Adopted atmospheric parameters
We started analysis with a homogeneous abundance model calculated with K,
,
and individual abundances taken from Cowley
et al. (2000). Once the stratification was
introduced in the model atmosphere, the fit to the hydrogen H
line
and the observed photometric parameters required decreasing
of
of the star down to approximately 6400 K. In total,
four iterations of the abundance analysis and the model
atmosphere calculation were performed to achieve a converged solution.
The abundances from the third iteration did not introduce noticeable
changes in the atmospheric structure.
The effective temperature obtained in our study is identical
to the
obtained by Kochukhov et al.
(2002), who fitted the H
and H
lines
using the same UVES spectra. However, Kochukhov
et al. relied on less advanced ATLAS9 model
atmospheres calculated using ODFs with artificially increased
Fe line absorption (Piskunov
& Kupka 2001) and also attempted to empirically
adjust the
structure
of their model.
Figure 1
illustrates the observed and predicted H line profiles
calculated with different atmospheric models. Unfortunately,
it is impossible to infer an accurate value of
from the H
profile
due to low temperature of the star and resulting poor sensitivity of
the hydrogen lines to the pressure stratification. For this
reason we kept this parameter fixed during the iterative procedure of
abundance analysis (however, see Discussion).
![]() |
Figure 1:
Comparison between the observed and predicted H |
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4.2 Abundance pattern
Table 4 (Online material) summarizes individual abundances of chemical elements in the atmosphere of PS derived with the final model atmosphere. Note the strong enrichment of REE elements compared to the solar composition. These elements dominate the spectrum of PS, playing a key role in the radiative energy balance (see below).
In Fig. 2 we illustrate abundance pattern of PS relative to the recent compilation of the solar abundances (Grevesse et al. 2007). The elements heavier than Ba exhibit an overabundance by 3 dex and larger. Several REEs show clear ionization anomaly related to stratification of these elements and departures from LTE.
Vertical distribution of Si, Fe, Ba, and Ca is presented in
Fig. 3.
This figure illustrates the difference between the initial and final
stratification as derived at the first and the last iterations
respectively. Ba seems to be the only element whose
stratification did not change appreciably during iterations. Only the
position of the abundance jump changed slightly.
Fe shows a change in the abundance in the upper atmosphere by
about 1 dex.
The position of the abundance jump for Si changed significantly.
In contrast, the Ca abundance in the upper
atmosphere changes dramatically. These results illustrate the
importance of taking into account a feedback of stratification on the
model structure.
The effect of introducing stratification on the fit to H line
appears to be not very strong. The maximum difference between the
stratified and non-stratified model predictions is at the level
of 1% as seen from Fig. 1. However,
the two profiles are still clearly distinguishable. Thus,
chemical stratification should be taken into account in accurate
modeling of the hydrogen line formation.
![]() |
Figure 2: Chemical composition of the atmosphere of HD 101065. Different symbols correspond to ions from I to III. Bars show abundance ranges for stratified elements. |
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The Fe and Ba abundance distributions agree with those derived previously by either trial-and-error method (Ryabchikova 2003) or based on the equivalent widths (Yushchenko et al. 2007). Figures 4-7 (Online material) illustrate the resulting fit to line profiles.
![]() |
Figure 3: Stratification of four elements in the atmosphere of HD 101065 derived at the first and last iteration of the abundance analysis. |
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4.3 Energy distribution and photometric colors
Unusual photometric parameters of PS clearly indicate that its energy
distribution should differ much from that of normal stars with
similar
.
Indeed, a dense forest of REE lines obtained from theoretical
computations should have a strong impact on the atmospheric energy
balance.
Figure 8
illustrates theoretical energy distributions of HD 101065 and
a normal F-type star HD 49933. Both stars have close effective
temperatures but very different atmospheric chemistry:
HD 49933 has a slight underabundance of iron-peak elements and
a nearly solar abundance of REEs as derived by Ryabchikova
et al. (2009). Model fluxes were computed with the
stratified distribution
of Si, Ca, Fe, and Ba as well as with the scaled
REE abundances as described above. In addition,
ATLAS9 ODF model from Piskunov
& Kupka (2001) calculated with K,
and
Fe opacity scaled to simulate an enhanced
REE absorption is shown for comparison purpose.
To keep the figure representative, here we show only models
with final
K,
and
different assumptions about REE opacity. Energy distributions
of models with
K
and
K
are displayed in Fig. 9
(Online Material).
Table 1: Observed and predicted colors of HD 101065.
![]() |
Figure 8: Observed and theoretical energy distribution of HD 101065 calculated using line lists from two datasets: VALD and ISAN. For comparison purpose the theoretical energy distribution of a normal F-type star HD 49933 is also shown. All theoretical calculations are smoothed with 20 Å Gaussian. |
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The observed energy distribution of PS is constructed combining the
large-aperture IUE observations extracted from the
INES database
and photometric data in the
(Wegner 1976), Geneva
,
(Catalano et al. 1998)
and 2MASS (Skrutskie et al. 2006)
systems converted to absolute fluxes. The model fluxes are scaled to
account for the distance to PS, which is 112
11 pc
according to the revised Hipparcos parallax of
0.87 mas
(van Leeuwen 2007).
Theoretical fluxes are further scaled to match the observed Paschen
continuum defined by the
and Geneva photometry. This scaling factor corresponds to the radius of
the star, for which we found R=1.98
.
The same scaling is applied to all theoretical fluxes presented in
Fig. 8.
For models with
K
and
K
we find
and
respectively (see Fig. 9, Online
Material).
It is evident that the enormous REE absorption leads to smoothing of the Balmer jump. This is clearly visible for both models computed with the new ISAN REE line list and with the original VALD REE line list. The only difference is that the former one predicts less flux redward of 4100 Å and more flux in the region 3000 Å and 3800 Å (note that the model computed with mean REE abundances should be used with big caution due to ignoring REE anomaly, and we present it in Fig. 8 for illustrative purpose only). Taking into account difficulties in predicting accurately the spectra of REEs, one can conclude that our models computed with the recent improvements of the line REE opacity provide a reasonably good fit to the observed fluxes. They are able to reproduce not only the shape of the observed energy distribution but also the amplitude of the Balmer jump reasonably well, much better than in the previous attempt by ODF-based modeling (Piskunov & Kupka 2001). However, there are still some discrepancies with observations, i.e. too high fluxes in the 2000-2500 Å region and around 4500 Å. The description of the energy distribution of PS can be probably improved in the future once more complete line lists of REEs will become available. A new well-calibrated observed spectrophotometry in the 3300-10 000 Å region would also be extremely useful for further modeling.
The impact of the peculiar energy distribution on the
photometric parameters of HD 101065 is illustrated in
Table 1.
Observations in the Strömgren system were taken from Hauck & Mermilliod (1998).
Predictions of Piskunov &
Kupka (2001) model calculated with the ODF line
opacity representation using abundances from Cowley
et al. (2000) are also given for comparison. We also
show theoretical photometric parameters for the models calculated with
the mean REE abundances corresponding to the first and second
ions, as well as for the models with the scaled
REE opacity, and for different
.
All models except the ODF-based one were computed with the chemical
stratification illustrated in Fig. 3.
There are few things to note from Table 1. First of all,
a strong impact of the REE opacity is clearly seen
for the photometric
index which reaches an even more negative value than the observed one
for the models with the ISAN data and the mean
REE abundances. Similarly, the recent version of VALD
already provides REE opacity that is enough to bring down
index
significantly compared to previous calculations and provides a result
very close
to the observed one. However, mean abundances can not be used for
accurate modeling due to significant REE ionization anomaly.
Accounting for this via the scaling of REE line opacity
decreases the flux redistribution effect and thus the impact on the
index.
The implementation of the new ISAN data gives a much better
fit to observations than using the data from VALD alone,
yet this fit is far from being fully satisfactory.
Table 2: Summary of REE lines in VALD and combined ISAN line lists before and after line preselection procedure.
Second, it remains difficult to obtain a good fit
simultaneously for all photometric indices. For instance, all
models fail to reproduce the U-B value,
while b-y is not
reproduced by models with the scaled REE opacity unless one
decreases
to
6200 K.
However, lowering
is not consistent with the observed H
line. Similar, worse
fit is obtained for models with
K,
which show reduced b-y
and increased
indicies.
Third, stratification of Fe, Si, Ca, and Ba considered in our
study does not play a significant role for the overall energy
redistribution and thus photometric colors as seen from the sixth raw
of Table 1.
Most of the photometric indicators are negligibly affected by the
presence of stratification. The index is again an
exception since it decreases by
0.03 mag if
stratification is introduced. However, this change is still fairly
small compared to typical precision of the observed
Strömgren photometry.
Finally, a high sensitivity of the photometric parameters to the new ISAN calculations indicates that the general disagreement between models and observations is the subject of future more precise and complete calculations of REE spectra. Using the data for more REE elements than those presented in this investigation can potentially improve the fit to the highly peculiar observed photometric properties of PS. Implementation of the ISAN line lists for only a few REE ions already allowed us to achieve significantly better results compared to previous modeling attempts and clearly showed that the ``enigmatic'' characteristics of PS likely result from the REE line opacity which was ignored in previous calculations. We can thus suggest that further step in the modeling of the observed properties of PS should be made in the direction of improved theoretical calculations of REE transitions.
The strong suppression of the Balmer jump and energy
redistribution to the infrared is a direct result of the unusually high
REE abundances in the stellar atmosphere. Table 2 summarizes
some statistics illustrating importance of the REE opacity in
the atmosphere of PS. It shows the number of lines of a given
ion before and after line preselection procedure
performed by the LL MODELS code.
This procedure selects from the master line list only those lines that
noticeably contribute to the line opacity for a given T-P structure.
Usually the preselection criterion is ,
where
and
are the line center and continuum opacity coefficients, respectively.
It is seen that, for instance, all lines
of Pr II and Nd II
present in the current version of VALD contribute to the opacity in the
atmosphere of PS. A larger number of lines is
selected from the more complete ISAN line lists.
As expected from the low magnitude of the index
and the energy distribution shown in Fig. 8, a great
majority of strong REE lines are concentrated around the
Balmer jump. Figure 10 illustrates
the line distribution for selected REE ions depending upon
their wavelength position and central intensity. We show the number of
lines per 50 Å wavelength bin for which the central
intensities are greater or equal to a given normalized intensity. The
latter ranges from 0 (fully saturated strong line)
to 1 (continuum level, weak line). One can seen that ions
like Ce II, Pr II,
Nd II/III, Sm II
have a large number of strong lines located exactly in the region of
Balmer jump and blueward, dramatically contributing to the
opacity and radiation field.
![]() |
Figure 10:
Distribution of lines for several REE ions depending on their position
and central depth in the ISAN line lists for the model
atmosphere with |
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5 Conclusions
HD 101065 is known to be one of the most peculiar stars ever found. Its unusual photometric colors and extremely rich spectra of REE elements represent a major challenge for modern stellar atmosphere theory. With the increasing number of the accurately measured and calculated REE transitions and the progress in the model atmosphere techniques it became possible to carry out this quantitatively new analysis of the spectrum of HD 101065 and to re-determine atmospheric parameters of this star.
In this study we investigated the impact of the new theoretically computed spectra of some REE elements on the atmospheric properties of HD 101065 and carried out a detailed spectroscopic analysis of the stratification of Si, Fe, Ba, and Ca in the stellar atmosphere. Theoretical computations of REE transitions have extended already existing line lists of REE lines by a factor of one thousand. Implementing an iterative procedure of the abundance analysis, we re-derived atmospheric parameters of the star in a self-consistent way, accounting for the ionization disequilibrium between the first and second ions of the REEs caused by significant NLTE effects.
The main results of our investigation can be summarized in the following:
- REEs appear to play a key role in the radiative energy balance in the atmosphere of HD 101065, leading to the strong suppression of the Balmer jump and energy redistribution from UV and visual to IR.
- The abnormal photometric color-indices of
HD 101065 arise due to a strong absorption in
REE lines. We showed that the introduction of the new
extensive line lists of REEs allow one to achieve a better agreement
between the unusually low magnitude of the Strömgren
index observed for this star and the model predictions. We also demonstrated, for the first time, a satisfactory agreement between the observed and computed spectral energy distribution of HD 101065.
- The remaining discrepancy between the observed and calculated photometric parameters indicates that some important opacity sources are still missing in the modeling process. For this reason the future extension of the REE line lists is needed for complete understanding of the observed properties of HD 101065.
- Using theoretical spectrum synthesis we derived
stratification profiles for Si, Ca, Fe, and Ba. We found a
strong depletion of all these elements in the upper atmosphere of
HD 101065. The effect of stratification on the H
line profile appears to be at the level of 1%, which is however important for accurate calculation of the hydrogen line profiles.
- There is not much influence of the stratification on the
considered photometric parameters of HD 101065,
except for the Strömregn
index, which is modified by stratification by 0.03 mag.
- Using combined photometric and spectroscopic analysis and
based on our novel iterative procedure of the abundance and
stratification analysis we find effective temperature of the
HD 101065 to be
K.

6 Discussion
We can compare the global parameters of HD 101065 derived in
the present study with the values obtained by an independent pulsation
modeling (Mkrtichian et al. 2008).
The best model explaining the observed frequency pattern
of HD 101065 is characterized by =
6622
100 K,
=
4.06
0.04,
R = 1.90
0.08
.
These parameters agree with ours within
.
However, to fit the observed frequencies in the framework of
current asterosismic models one needs dipolar magnetic field strength
= 8.7 kG,
which is incompatible with the observed field modulus of
= 2.3 kG.
One of the reasons for this significant discrepancy between the
observed and inferred magnetic field is the use of normal
relation
in the pulsation modeling. This relation differs significantly from a
more realistic one derived in the present paper, as illustrated in
Fig. 11
(Note that computing model with solar abundances we did not apply
scaling of the REE opacity due to the low abundance of
REE elements in solar atmosphere and thus its negligible
impact on model structure). As expected from the fit to H
line
and
photometric parameters presented above, the effect of
stratification on model T-P structure
is negligible and hardly distinguishable from the homogeneous abundance
model (dash-dotted line in Fig. 11). Note that it
is still considereably different from the solar abundance model, and is
in good agreement with the result of Monin
& LeBlanc (2007).
![]() |
Figure 11: Comparison in atmospheric temperature structure of theoretical models. Thick line - ODF model from Piskunov & Kupka (2001), dotted line - homogeneos solar abundances model, dashed line - stratified abundances final model adopted in the present study, dash-dotted line - same as before but assuming homogeneos abundances. |
Open with DEXTER |
In spite of our new results, illustrating an improved understanding of the physics of the atmosphere of HD 101065, it is still impossible to describe all observed properties of this star with the current state-of-the-art modeling. In this section we would like to discuss physical effects that we ignored in our analysis.
Taking into account a jungle of the REE lines seen in the spectrum of HD 101065 and their importance as an opacity source for the model atmosphere calculation it would be reasonable to search for the possible stratification of rare-earth elements. Considering the case of a hotter roAp star HD 24712, for which the accumulation of REEs in the upper atmosphere plays a critical role, producing a characteristic inverse temperature gradient (see Shulyak et al. 2009, for more details), one would expect the same mechanism to operate in the atmosphere of HD 101065 as well. However, the stratification analysis of REEs is highly complicated due to blending of REE lines with accurately known atomic parameters (that ideally should be used for such an analysis) by other REE lines for that no laboratory measurements exist and thus can not be included in the spectrum synthesis. At the same time, assuming that REEs are distributed similarly in both stars, we do not expect a strong impact of the REE stratification on the energy distribution of HD 101065 simply because it would affect only the optically thin atmospheric layers.
As mentioned above, due to the low
of PS, it is impossible to infer the
value from the
hydrogen lines. Furthermore, such
indicator as the
photometric
index is strongly affected by the REE opacity,
which is still far from being completely understood. The ISAN
calculations gave us information on several REE ions but the
role of other REE species is not known at this moment. On the
other hand, as seen from the high resolution spectra
of HD 101065, most of the line absorption is due to
such ions as Pr II/III, Nd II/III,
and Ce II/III that are represented
in our calculations by a large number of lines. This suggests that the
magnitude of the energy redistribution is well modeled. However,
extending our calculations using new line lists for other
REE ions will allow to improve the accuracy of theoretical
index
and constrain the
of the star. In principle, increasing
up to 4.5 and decreasing
down to 6200 K provides a better fit to the observed
photometric parameters as demonstrated in the last row of
Table 1.
Also the fit to the H
line
remains reasonable with these parameters. However,
the ionization equilibrium for different elements is not as
good as with the
K,
model.
This is why, and taking into account various uncertainties related to
the REE opacity, we did not attempt to derive surface
gravity in our study. This limitation does not affect any of the
results presented in this paper.
This work was supported by the following grants: FWF Lise Meitner grant No. M998-N16 and Deutsche Forschungsgemeinschaft (DFG) Research Grant RE1664/7-1 to DS, by RFBG grants (08-02-00469a, 09-02-00002a), Presidium RAS Programme ``Origin and evolution of stars and galaxies'' and Russian Federal Agency on Science and Innovation Programme (02.740.11.0247) to T.R. O.K. is a Royal Swedish Academy of Sciences Research Fellow supported by grants from the Knut and Alice Wallenberg Foundation and the Swedish Research Council. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. Based on INES data from the IUE satellite. We also acknowledge the use of electronic databases (VALD, SIMBAD, NIST, NASA's ADS).
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Online Material
![]() |
Figure 4: Comparison between the observed and theoretical profiles for the Ba lines. Observations are shown with symbols, thick full line corresponds to the synthetic spectrum calculated with the best-fitting stratified abundance distribution and dashed line shows prediction of the spectrum synthesis with a homogeneous abundance. |
Open with DEXTER |
![]() |
Figure 5: Same as in Fig. 4 but for Ca. |
Open with DEXTER |
![]() |
Figure 6: Same as in Fig. 4 but for Si. |
Open with DEXTER |
![]() |
Figure 7: Same as in Fig. 4 but for Fe. |
Open with DEXTER |
![]() |
Figure 9:
Observed and theoretical energy distribution of HD 101065
calculated with scaled REE opacity from ISAN line
list (see text). For comparison purpose the
theoretical energy distribution of a normal F-type star
HD 49933 is also shown. All theoretical calculations
are smoothed with 20 Å Gaussian. Theoretical fluxes
were scaled assuming radiuses: |
Open with DEXTER |
Table 3: List of Si, Ca, Fe, and Ba spectral lines used for reconstruction of chemical stratification.
Table 4: Abundances of individual elements.
Footnotes
- ... HD 101065
- Based on observations collected at the European Southern Observatory (Paranal, La Silla) and on data retrieved from the ESO Science Archive.
- ...
- Figures 4-7, 9 and Tables 3, 4 are only available in electronic form at http://www.aanda.org
- ... online
- http://www.astro.uu.se/ oleg/pf.html
- ... Kurucz
- http://kurucz.harvard.edu
- ... INES database
- http://sdc.laeff.inta.es/ines/index2.html
- ... Geneva
- http://obswww.unige.ch/gcpd/ph13.html
All Tables
Table 1: Observed and predicted colors of HD 101065.
Table 2: Summary of REE lines in VALD and combined ISAN line lists before and after line preselection procedure.
Table 3: List of Si, Ca, Fe, and Ba spectral lines used for reconstruction of chemical stratification.
Table 4: Abundances of individual elements.
All Figures
![]() |
Figure 1:
Comparison between the observed and predicted H |
Open with DEXTER | |
In the text |
![]() |
Figure 2: Chemical composition of the atmosphere of HD 101065. Different symbols correspond to ions from I to III. Bars show abundance ranges for stratified elements. |
Open with DEXTER | |
In the text |
![]() |
Figure 3: Stratification of four elements in the atmosphere of HD 101065 derived at the first and last iteration of the abundance analysis. |
Open with DEXTER | |
In the text |
![]() |
Figure 8: Observed and theoretical energy distribution of HD 101065 calculated using line lists from two datasets: VALD and ISAN. For comparison purpose the theoretical energy distribution of a normal F-type star HD 49933 is also shown. All theoretical calculations are smoothed with 20 Å Gaussian. |
Open with DEXTER | |
In the text |
![]() |
Figure 10:
Distribution of lines for several REE ions depending on their position
and central depth in the ISAN line lists for the model
atmosphere with |
Open with DEXTER | |
In the text |
![]() |
Figure 11: Comparison in atmospheric temperature structure of theoretical models. Thick line - ODF model from Piskunov & Kupka (2001), dotted line - homogeneos solar abundances model, dashed line - stratified abundances final model adopted in the present study, dash-dotted line - same as before but assuming homogeneos abundances. |
Open with DEXTER | |
In the text |
![]() |
Figure 4: Comparison between the observed and theoretical profiles for the Ba lines. Observations are shown with symbols, thick full line corresponds to the synthetic spectrum calculated with the best-fitting stratified abundance distribution and dashed line shows prediction of the spectrum synthesis with a homogeneous abundance. |
Open with DEXTER | |
In the text |
![]() |
Figure 5: Same as in Fig. 4 but for Ca. |
Open with DEXTER | |
In the text |
![]() |
Figure 6: Same as in Fig. 4 but for Si. |
Open with DEXTER | |
In the text |
![]() |
Figure 7: Same as in Fig. 4 but for Fe. |
Open with DEXTER | |
In the text |
![]() |
Figure 9:
Observed and theoretical energy distribution of HD 101065
calculated with scaled REE opacity from ISAN line
list (see text). For comparison purpose the
theoretical energy distribution of a normal F-type star
HD 49933 is also shown. All theoretical calculations
are smoothed with 20 Å Gaussian. Theoretical fluxes
were scaled assuming radiuses: |
Open with DEXTER | |
In the text |
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