Poster presented at the 198^th AAS meeting, Pasadena, CA, USA, june 2001

A spectroscopic study of the suspected chemically peculiar star HD207538.
FUSE and SARG-TNG combined spectra.

G. Catanzaro¹, M. Andre², F. Leone¹, P. Sonnentrucker²

1 - Catania Astrophysical Observatory, Via S. Sofia 78, 95125, Catania, Italy

2 - The Johns Hopkins University, 3400 N. Charles Street, 21218, Baltimore, MD, USA

Abstract

We present far UV and optical spectroscopic data of the suspected CP star HD207538.
FUSE observation (range 1000 - 1200 Å, R = 16000) and optical spectra obtained with the high resolution spectrograph (SARG) of the Italian "Telescopio Nazionale Galileo (TNG)" (range 4600 - 7000 Å, R = 144000) have been combined.
A spectroscopic analysis has been performed, with Kurucz codes, in order to better define the chemical peculiarity of this object with particular reference to abundance stratification.
As part of our fundamental analysis we also studied the gas-phase ISM content of that line of sight. From the analysis of these ISM lines we deduced a stellar + ISM model of the line of sight in the FUV.

Introduction

HD207538 is a B0 type star Teff=32190 K,  log g=4.32 and =10 km s-1 Daflon et al. (1999) classified as suspected chemically peculiar star in the "General Catalogue of Ap and Am stars" (Renson et al., 1991).
Chemically peculiar star are characterized by spectral, photometric and magnetic variations with a common period. In the oblique rotator model, proposed by Stibbs (1950), chemical elements are not homogeneously distributed on the stellar surface, and the observed variations are due to the stellar rotation. They also present anomalies in the intensity of spectral lines if compared with stars of the same temperature.
The anomalous abundances are caused by diffusion processes (Michaud, 1970).
Magnetic fields are suspected to influence the diffusion by suppressing mass-motions and changing the path of ionized species (Michaud et al. 1981), so that diffusion in CP stars results in a non-homogeneous distribution of elements over the stellar surface.
One of the most interesting results predicted by the theory of radiative diffusion process is the vertical stratification of chemical elements in the atmospheres of CP stars.
HD207538 has been observed with Far Ultraviolet Spectroscopic Explorer (FUSE), in the range spanning 1000 - 1200 Å and with the Telescopio Nazionale Galileo (hereafter TNG) equipped with the SARG spectrograph in the optical range.
Combining these two data sets we have the unique opportunity to look very deeply into the atmosphere of this star. Simply selecting a number of spectral lines whose region of formation span a wide range of depth in the atmosphere, we calculated chemical abundances related to different atmospheric layers.
For the sake of clarity, we show in Fig. 1 the contribution functions for two carbon lines, CII l4647.420 Å (optical) and CIII l1165.870 Å (FUV), calculated for an atmospheric model with Teff=32190 K and log g=4.32 .

Fig. 1: Contribution functions calculated for the central wavelength of two carbon lines

Tab. 1: Astrometric and photosperic properties of HD207538. Rotational velocity has been inferred using all the single lines observed in the optical region

Observations

HD207538 has been observed in time-tag mode on 8th December 1999 with FUSE.
This early-type (B0) star HD207538 at about 640 parsecs is shining through
a translucent cloud with a reddening E(B-V)=0.63. This target is one of the translucent clouds program (Snow 2000) bright enough to allow a good S/N and with sufficient foreground dust and gas to show large column densities of many atomic and molecular species. These FUSE spectra were extracted from 4 sub exposures totalizing 7736 seconds (3 orbits) in the large aperture with the version 1.8.4 of the pipeline.
The processing steps include data screening, thermal drift correction, geometric distortions correction, Doppler correction to heliocentric wavelength, dead time correction and wavelength calibration. At the time of the observation the focus was not done yet, making the astigmatism correction less critical and resulting in a 15000 resolution spectrum. The spectra were then coadded manually, using both exposure time and error weighting, to obtain a S/N of about 15 per resolution element. Strong interstellar extinction and misalignment of the SiC channels prevented us to use data shortward 980 Å.
The optical data were collected at TNG equipped with the SARG spectrograph operated on the Canarian islands (Spain). The echelle spectrum has been reduced using the IRAF packages. The calibration lamp lines show that the resolving power is always R > 120000 and the achieved S/N ratio is greater than 100. Our optical spectrum covers the region from 4600 Å to about 7000 Å. Because of the high effective temperature of this star there are a few lines strong enough to be useful for an abundance analysis.
For our goal we extracted a small region spanning from 4625 Å to 4680 Å containing a number of metallic lines, namely C, N, O and Si.

Analysis

For the spectrum synthesis analysis we adopted the above cited atmospheric parameters founded by Daflon et al. (1999). With these quantities we used ATLAS9 (Kurucz, 1993) to build the atmospheric model in the LTE approximation needed by SYNTHE (Kurucz & Avrett, 1981) in order to compute the synthetic spectrum.
To check the goodness of our model we compare the computed flux distribution with the observational data.
The observed flux (FUSE, IUE and optical) has been de-reddened
using the algorithm developed by Cardelli et al. (1989). In Fig. 2 we show such a comparison.

Fig. 2: Comparison between theoretical flux distribution (red line) and
observation. Sources of data are: FUSE (green dots), IUE (cyan dots: SWP0892 and LWR07751 data sets)
and optical photometry (blue dots). UBV photometry is taken from SIMBAD database.

Moreover, HD207538 belongs to the OB association Cep OB2 as stated by Grisby et al. (1999), Daflon et al. (1999) and De Zeeuw et al. (1999). In the last paper the authors found a new determination of the distance of this association d=615 ± 35 pc.
The portion of the optical spectra we reproduced theoretically spans the range from 4625 Å to 4680 Å. In this region C, N, O and Si lines have been identified and used to infer the abundance of these elements. The results of this analysis, reported in Tab. 2 and in Fig. 4, show that abundances are close to the solar value with a slight over-abundance.

Tab. 2: Chemical abundance of C, N, O and Si calculated and comparison with the values from Daflon et al.(1999)

Fig. 3: Match of the spectral region observed with the TNG. Solid line is the
observed spectrum, thin line is the spectrum calculated as described in the text. Labels on the top of the plot have the following meaning: last 3 digits of the line wavelegth expressed in nm, atomic number and ionization state, energy of the lower atomic state and residual intensity of the line.

The abundances inferred from the optical spectrum have been used also to calculate the synthetic spectrum in the FUSE range. It is worthy to note that CNO abundances that well reproduce the line strength in the TNG spectrum yield lines too strong in the FUSE region. In this region an under-abundance of about 0.3 dex with respect the values inferred by the optical lines is needed to take into account the strength of the FUV lines. This is not true for the silicon, the same abundance takes into account the strength of the lines in both the spectral region we examined. For the other elements identified in the FUSE range (i.e. iron and phosphorus) solar abundances have been used. The results are shown in Fig. 4.

Fig. 4: ISM (green) + star (red) model overplotted on the data

.
This fact could be a direct evidence for element stratification in the atmosphere, but because the low S/N and resolution of FUSE spectrum combined with the LTE approximation, this small discrepancy (0.3 dex) could be due to these various sources of error. Thus we need a more detailed analysis to ascertain if the differences are real or not.
For the ISM analysis, we used the profile fitting software called Owens and described in Hebrar (2001). We also used the curve-of-growth technique for FeII lines which have a wide range of f-values in FUSE. The f-values for FeII are revised f-values from Howk et al 2000, the others are from Morton 1991 and Welty 1999.
Many metallic lines in the FUSE range (essentially FeII, PII CI and NI) were fitted simultaneously in 20 windows between 1100 Å and 1200 Å. We assumed that the species were originating in one main component (b-value 6.23 kms, totally turbulent).
All the transitions are very strong except for a few of them. Although the S/N is low, we were able to get a good estimates of FeII thanks to many lines in the FUSE range. For NI and CI, we had to perform a more careful analysis and we ended up with larger error bars. As to PII, the column density is coming from a very weak transition (1124.94 Å) and we quote a 1 sigma upper limit.
It should be noted that the fits of these lines are not changing much with the column density (being on the flat part of the curve of growth). Hence, we have a good fit of the ISM lines that can easily be subtracted from the stellar spectrum. We detected also CI* and CI** along this line of sight but the transitions are blends of multiplet and without STIS data we are unable to distinguish the various excited states.

Fig. 5: Curve of growth for the FeII ion. The plotted c-o-g corresponds to a b-value of 6.2 km s-1 and a column density of 14.98 in log. Note that the only weakest transictions give the column density.

Tab. 3: ISM atomic column densities toward HD207538

Conclusions

We investigate the real nature of the suspected chemically peculiar star HD207538.
FUSE and SARG-TNG combined spectra have been used for this study.
One of the principal observational characteristic of chemically peculiar stars is the photometric variability. For this star, no evidence of light variations has been found using the Hipparcos data.
The chemical abundance of the elements identified are compatible with solar values with a slight over-abundance for oxygen and nitrogen.
Finally we did not found strong evidence about the chemically peculiar nature of this object.
Numerous ISM metallic lines are also seen along this line of sight that blend the stellar
features but which are of great interest for the study of translucent clouds (Sonnentrucker et al., 2001).

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