A SEARCH FOR DIFFUSE CIRCUMSTELLAR
BANDS IN THE CIRCUMSTELLAR ENVELOPES
OF POST-AGB STARS
R. Luna,1 M. A. Satorre,1 P. García Lario2
1 Laboratorio de Caracterizaciones de Interés Astrofísico. Escuela Politécnica Superior de Al-
coy, Spain
2 European Space Astronomy Centre. ISO Data Centre (ESA), Villafranca del Castillo, Spain
ralunam@fis.upv.es
Abstract
In this work we present a new procedure to identify Diffuse Bands (DBs hereafter) in the circumstellar envelopes of evolved stars. The method is based on the
systematic analysis of the strength of 9 of the strongest, best known DBs, usually found in the interstellar medium (hence the usual name of DIBs, or Diffuse
Interstellar Bands), in the high resolution optical spectra of a carefully selected
sample of candidate post-AGB stars.
Keywords:
Diffuse Interstellar Bands, post-AGB stars
1.
Introduction
DBs are bands of still unknown origin which appear overimposed on the
optical spectra of reddened stars. This is the reason why it has always been
thought that they must be formed in the interstellar medium. Although no
specific carrier for these spectroscopic features has unambiguously been identified yet, there are indications that they might be some kind of carbon-based
gas-phase molecules. If this is the case, it is expected that DBs could also be
formed and detected in the circumstellar environments of C-rich stars in their
late stages of evolution, while they are evolving from the AGB to the planetary
nebula stage, a possibility which has not been systematically explored so far.
In this work we present a new method to identify possible circumstellar DBs
(DCBs) from the analysis of their strength as a function of the extinction in a
sample of post-AGB stars.
2
Figure 1. The DBs selected to perform the analysis are nine of the strongest ones known, all
in the optical range: 5780, 5797, 5850, 6196, 6284, 6379, 6614, 6993 and 7224 Å. Note the
strong telluric contamination at 6284 Å.
2.
DBs selected
The DBs selected to perform the analysis are nine of the strongest ones
known in order to minimize the uncertainties in the detection and in the measurement of the EW (0.044 < EW/E(B − V ) < 1.1). All them are within the
optical range: 5780, 5797, 5850, 6196, 6284, 6379, 6614, 6993 and 7224 Å
(see Figure 1). Three of them are affected by telluric contamination: 6284,
6993 and 7224 Å (the two latter are showed after telluric removal in Figure 1).
Three other DBs have a substructure tentatively associated in the literature to
ro-vibrational bands: 5797, 6379 and 6614 Å.
3.
Selection of the sample of post-AGB stars
In order to select the sample of sources to be used in this analysis, we have
taken as starting point a catalogue of candidate post-AGB stars1 , identified as
such because of their characteristic infrared colours, spanning a wide range of
properties. These include:
Chemistry: carbon- and oxygen-rich stars have been chosen.
A search for Diffuse Circumstellar Bands in post-AGB stars
3
Figure 2. Example of two post-AGB stars with a very different strenght of the DB 6284 Å
feature before (left panel) and after (right panel) removing the telluric contribution.
Spectral types: covering from B-type to G-type stars.
Estimated contribution of the circumstellar dust to the overall extinction:
priority has been given to sources dominated by circumstellar extinction.
Velocities: we have selected high velocity stars (larger than 50 km s−1
in absolute value) whenever possible.
As a result of this selection we have obtained 33 post-AGB stars with a wide
variety of observational properties to start the search.
4.
Data Reduction
We have used high-resolution (R =λ/∆λ = 50000 – 100000 at 5500 Å) echelle
spectroscopy covering a wide wavelength range (4000 – 10000 Å). The IRAF
NOAO package was used to perform the reduction of the raw spectra. This
package allows to perform the usual tasks in echelle spectroscopy data reduction: bad pixel correction, bias correction, scattered light subtraction, flatfield correction, order extraction, wavelength calibration, telluric lines removal
(Figure 2), estimation of the equivalent width of individual features and velocity calculation.
4
Figure 3. Intensity (equivalent widths in Å) of the 9 DBs selected versus E(B-V) for the postAGB stars in the sample. Solid lines correspond to fits made using field stars taken from the
literature, and are representative of the behaviour of stars affected only by interstellar extinction.
The inverted triangles represent upper limits in case of no detections.
5.
Analysis of the sample: comparison with field stars
The DB intensities for all the stars in our sample were first compared with
the corresponding values derived for field reddened stars taken from the literature.
A search for Diffuse Circumstellar Bands in post-AGB stars
5
We have chosen two samples of reference, reddened O-B stars2,3 , hereafter Thorburn (Th) sample and Jenniskens (Je) sample, which seem to
be representative of the general behaviour of field stars affected only by
interstellar reddening. From the data in the literature and for each DB
under analysis we have derived the best linear fit which represents the
behaviour of the intensity of DBs vs. colour excess. The general trend
of the Th and Je samples are represented by solid lines in Figures 3 and
5.
We have determined the EW of each of the 9 DBs under study in every
post-AGB star in our sample and analysed the dependence of the measured equivalent widths with the observed colour excess E(B-V).
In most cases, the measured equivalent widths are well below the values
derived for field stars according to the extinction E(B-V) of the source. Only
a subset of sources seems to follow the same behaviour observed in field stars
(see Figure 3). We interpret this result as a consequence of the absence (or at
least the underabundance) of the DB carriers in the circumstellar envelopes of
some of these post-AGB stars.
6.
Analysis of the sources dominated by circumstellar
extinction
In order to determine whether our hypothesis is consistent with the observations we have represented together in Figure 4 the distribution of E(B-V)
versus Galactic Latitude for all stars in our sample and for a sample of reference stars taken from the catalog of Guarinos4 , (based on observations of hot
reddened stars used in the study of DB intensities). This way we have divided
our sample in two groups according to whether the overall extinction observed
in the post-AGB star in our sample is more likely to be dominated by the circumstellar contribution (≥ 50 %, DCS type stars) or just consistent (REST)
with the interstellar extinction expected according to the Galactic Latitude of
the post-AGB star considered.
A comparison of the results obtained in stars belonging to each of the two
groups considered above with those found in reference stars shows that there
is a general trend for the stars with a dominant circumstellar extinction contribution to show much weaker DB strengths compared to the rest of post-AGB
stars in the sample. In the most extreme cases there are stars in this group
with a large extinction in which surprisingly some DBs are completely absent.
In contrast, we find a significant number of sources belonging to the group
REST (which are supposed to be only slightly affected by circumstellar extinction) showing DB strengths fully in agreement with those observed in field
stars. The specific environmental conditions in the circumstellar envelopes of
6
Figure 4. E(B-V) distribution versus Galactic Latitude for the post-AGB stars in the sample
(circles) and for reference stars taken from Guarinos4 (squares). Post-AGB stars dominated by
circumstellar extinction (DCS) are indicated by filled circles and they are labelled with their
IRAS name.
the post-AGB stars under study could be the responsible for the differences
observed between individual stars within the DCS sample. Indeed, the results
obtained suggest that some of the DB carriers could be completely absent in
some of these envelopes while not in others.
Further analysis which take into account the different chemical properties
(C-rich vs. O-rich) of the circumstellar envelopes under study and the wide
variety of effective temperatures considered (early vs. intermediate and late
spectral type of the central stars) is on-going and will be presented elsewhere.
A search for Diffuse Circumstellar Bands in post-AGB stars
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Figure 5. Intensity (equivalent widths in A) of DBs versus E(B-V) for the post-AGB stars in
the sample. Filled circles represent post-AGB stars dominated by circumstellar extinction and
open triangles represent the rest of stars of the sample. The solid lines represent again the linear
fits made with field stars taken from the literature and represent the behaviour of stars affected
only by interstellar extinction. The arrows represent upper limits.
8
References
[1] Suárez, O.(2004). “Stellar evolution in the post-AGB stage”. PhD Thesis, U. Vigo.
[2] Thorburn J.A., Hobs L.M., McCall B. (2003). “Some diffuse interstellar bands related to
interstellar C2 Molecules”. The Astrophisycal Journal, 584: 339 − 356.
[3] Jenniskens P., Désert F. (1994). “A survey of Diffuse Interstellar Bands (3800 − 8680 Å)”.
Astronomy and Astrophysics, 106: 39 − 78.
[4] Guarinos J. (1988). “Catalogue of Diffuse Interstellar Band measurements of Snow, York
and Welty”. Bulletin d’information Centre Données Stellaires. 35, 161 − 164.