arXiv:astro-ph/0409665v1 28 Sep 2004
Astronomy & Astrophysics manuscript no. gsarkar
(DOI: will be inserted by hand later)
February 2, 2008
High resolution spectroscopy of the hot
post-AGB stars : IRAS13266-5551 (CPD-55 5588)
and IRAS17311-4924 (Hen3-1428)⋆
G. Sarkar1 ⋆⋆, M. Parthasarathy2, and Bacham E. Reddy2 ⋆⋆⋆
1Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind,
Pune - 411007, India
2Indian Institute of Astrophysics, Koramangala, Bangalore - 560034, India
Received / Accepted
Abstract. The high resolution spectra covering the wavelength range 4900 Å to 8250 Å of
the hot post-AGB stars IRAS13266-5551 (CPD-55 5588) and IRAS17311-4924 (Hen3-
1428) reveal absorption lines of C II, N II, O II, Al III, Si III and Fe III and a rich emission
line spectrum consisting of H I, He I, C II, N I, O I, Mg II, Al II, Si II, V I, Mn I, Fe III,
[Fe II] and [Cr II]. The presence of [N II] and [O I] lines and absence of [O III] indicate low
excitation nebulae around these stars. The components of Na I absorption lines indicate the
presence of neutral circumstellar envelopes in addition to the low excitation nebulae around
these two hot post-AGB stars. The Hαlines show P-Cygni profiles indicating ongoing post-
AGB mass loss. Fromthe absorption lines, wederived heliocentric radial velocities of 65.31
± 0.34 km s−1and 27.55 ± 0.74 km s−1for IRAS13266-5551 and IRAS17311-4924 respec-
tively. High galactic latitude and large radial velocity of IRAS13266-5551 indicate that
it belongs to the old disk population. Preliminary estimates for the CNO abundances in
IRAS13266-5551 are obtained.
Key words. Stars: AGB and post-AGB — Stars: early-type — Stars: abundances — Stars:
2 G. Sarkar et al.: Hot post-AGB stars : IRAS13266-5551 and IRAS17311-4924
From the study of IRAS sources with far-IR colours similar to that of planetary nebulae (PNe)
several cool and hot post-AGB stars have been discovered (Parthasarathy and Pottasch, 1986;
Parthasarathy et al., 2000a, Parthasarathy et al., 2001) forming an evolutionary sequence in the
transition region from the tip of the AGB to the early stages of PNe (Parthasarathy, 1993a,
b). IRAS13266-5551 (CPD-55 5588) and IRAS17311-4924 (Hen3-1428) were identified as
hot post-AGB stars (Table 1) based on their far-IR flux distribution, high galactic latitudes
and B-supergiant spectra in the optical (Parthasarathy & Pottasch, 1989; Parthasarathy, 1993a;
Parthasarathyet al.,2000a).TheUV (IUE)spectraofthese stars showC II(1335Å), Si IV(1394
Å , 1403 Å ), C IV (1550 Å ) and N IV (1718 Å ) lines typical of the central stars of PNe. The
C IV (1550Å ) resonancelines areblueshiftedindicatingstellar windvelocitiesof−1821kms−1
(CPD-55 5588) and −1066 km s−1(Hen3-1428) respectively (Gauba and Parthasarathy, 2003).
The “30 µ feature”, SiC emission at 11.5 µ, and UIR band at 7.7 µ were detected in the ISO spec-
trum of IRAS17311-4924(Gaubaand Parthasarathy,2004).These features have been detectedin
the circumstellar dust shells of carbon rich AGB stars (C-stars), post-AGB stars, proto-planetary
nebulae (PPNe) and planetary nebulae (PNe) (see e.g. Hony et al., 2002; Hrivnak et al., 2000,
Volk et al., 2000, 2002). Loup et al. (1990) detected CO emission in IRAS17311-4924 typical
for circumstellar shells around evolved objects.
High resolution optical spectra of only a few hot post-AGB stars have been analysed.
These include IRAS01005+7910 (Klochkova et al., 2002), IRAS18062+2410 (SAO85766,
Parthasarathy et al., 2000b; Arkhipova et al., 2001a; Mooney et al., 2002, Ryans et al., 2003),
IRAS19590-1249 (LSIV−12◦111, McCausland et al., 1992; Conlon et al., 1993a; Ryans et al.,
2003) and IRAS20462+3416 (LSII+34◦26, Parthasarathy, 1993b; Garc´ia-Lario et al., 1997;
Arkhipova et al., 2001b). The optical spectra of these stars show absorption lines due to C II,
N II, O II, Si II, Si III, Fe III etc. Emission lines of He I, Fe I, II and III, N I, Ni I, O I have also
been detected. Nebular emission lines of [O II], [N II], [S II] etc., detached cold circumstellar
dust shells, OB-supergiant spectral types, high galactic latitudes and chemical composition in-
dicate that these are PPNe (Parthasarathy et al., 1993c, 1995 and 2000b). IRAS13266-5551 and
IRAS17311-4924are found to be similar to the objects mentioned above. In this paper we report
an analysis of their high resolution spectra.
Send offprint requests to: G. Sarkar e-mail: email@example.com
⋆Based on observations made with the Victor M. Blanco 4m telescope of the Cerro Tololo Inter-
American Observatory, Chile.
⋆⋆Present address : Indian Institute of Technology, Kanpur - 208016, India
⋆⋆⋆Visiting Observer, Cerro Tololo Inter-American Observatory, which is operated by the Association of
Universities for Research in Astronomy Inc., under contract with the U.S. National Science Foundation.
G. Sarkar et al.: Hot post-AGB stars : IRAS13266-5551 and IRAS17311-49243
Table 1. Details of the stars
IRAS NameRA DEClb Sp.TypeV B-V IRAS Fluxes (Jy.)
(2000) (2000) Optical12 µ
13266-5551CPD-55 558813:29:50.8 -56:06:53308.30
0.76 35.90 35.43 11.66
Photometry is from :aReed (1998) andbKozok (1985).
Hen3-1428 17:35:02.49-49:26:26.4 341.41-9.04 B1IIe10.68b
18.34 150.7058.74 17.78
Spectral types are from Parthasarathy et al. (2000a).
Highresolution(R ∼ 30,000)spectra ofIRAS13266-5551andIRAS17311-4924from4900Å to
8250Å wereobtainedon 22ndJune,2002.Eachobjectwas observedtwice duringthe night.The
echelle spectrograph at the f/7.8 Ritchey-Chretien focus of the Victor M. Blanco 4m. telescope
of the Cerro Tololo Inter-American Observatory (CTIO), Chile was used for the purpose. The
spectra were recorded using a Tektronix 2048X2048CCD. The slit width was 150 µ correspond-
ing to 1′′on the sky. Appropriate number of bias frames and flat fields were observed. A Th-Ar
comparison lamp was used for wavelength calibration.
As these are hot stars, spectra in the blue would contain more number of absorption lines.
However, our observing program and the spectrograph setup did not allow us to go shortward
of 4900 Å . Therefore, the analysis reported in this paper is based on the spectra covering the
wavelength range 4900 Å to 8250 Å .
The spectra were processed using standard IRAF routines. They were corrected using data in the
overscan region of the CCD chip. The other reduction steps included trimming, bias subtraction,
flat field correction, correction for scattered light and wavelength calibration. The two sets of
reduced spectra for each object were then combined to increase the signal-to-noise (S/N) ratio.
The final S/N ratios for IRAS13266-5551and IRAS17311-4924were estimated to be ∼ 120.The
emissionlines weremeasured.Wheneverrequired,deblendingwas donetoobtaingaussianfits to
the blended line profiles. The continuum normalised spectra are presented in Appendices A and
B (Figs. A and B). The line identifications (Tables 2a, 2b, 2c, 2d, 3a, 3b, 3c and 3d) are based on
the Moore multiplet table (1945) and the linelists of Parthasarathy et al. (2000b) and Klochkova
et al. (2002). Unidentified lines are denoted by “UN”. Night sky emission lines (atmospheric
emission) were identified from Osterbrock and Martel (1992) and Osterbrock et al. (1996) and
are listed as “atmos.” in the tables. The laboratory wavelengths, log (gf) values and excitation
potentials (χ) are from the linelist compiled by Ivan Hubeny and retrieved from the directory
/pub/hubeny/synplotby anonymous ftp from tlusty.gsfc.nasa.gov.
4G. Sarkar et al.: Hot post-AGB stars : IRAS13266-5551 and IRAS17311-4924
3.1. Description of the spectra
The high resolution optical spectra of IRAS13266-5551 and IRAS17311-4924 show absorption
lines due to C II, N II, O II, Ne I, Al III and Si III. The O I triplet at ∼ 7773 Å was detected
in both stars. Both stars show a rich emission line spectrum with lines of C II, Mg II, Al II,
Si II, Fe III and [Cr II] in emission. Emission lines of N I, O I, [O I], V I, Mn I and [Fe II] in
IRAS17311-4924 were also detected. The presence of low excitation nebular lines of [N II] in
the spectra of both stars and the absence of [O III] 5007 Å indicate that photoionisation has just
The He I lines in the two stars show a variety of profiles. They appear in absorption, in
emission and also show P-Cygni profiles indicating post-AGB mass-loss. The He I(4) 5015.678
Å and He I(45) 7281.349 Å emission lines in IRAS13266-5551 are superposed on the corre-
sponding absorption components. The asymmetric nature of these emission lines suggests that
they may have P-Cygni profiles. The presence of high excitation lines of He I and low excitation
emission lines of Na I (see Sec. 3.4) and V I indicate a range of temperatures for the circumstel-
lar material around these stars. The circumstellar envelope around these stars may be extended
and the outermost regions may be cooler. The Hαlines in both stars show P-Cygni profiles. The
emission peak of the line in IRAS17311-4924is asymmetric.
3.2. Radial velocities
Heliocentric radial velocities (Vr) for the well defined absorption and emission lines are pre-
sented in Tables 2a, 2b, 2d, 3a, 3b and 3d. The radial velocities of the Fe III (5) absorption lines
in IRAS13266-5551 are relatively larger than the rest and those of the Ne I absorption lines in
IRAS17311-4924 are relatively smaller suggesting that these lines may be formed in different
regions in the atmospheres of these stars. Therefore, in estimating the mean heliocentric radial
velocities, we have excluded the above lines. The radial velocity of C II (2) 6578.052Å absorp-
tion line has also beenneglected(see footnoteto Table 2a). We obtainedmeanradial velocities of
65.31 ± 0.34 km s−1and 27.55 ± 0.74 km s−1from the absorption lines in IRAS13266-5551and
IRAS17311-4924 respectively. The mean heliocentric radial velocities of the emission lines are
58.32 ± 0.65 km s−1and 32.74 ± 0.43 for IRAS13266-5551 and IRAS17311-4924 respectively.
In estimating the mean radial velocity of the emission lines we have excluded the radial velocity
measurementsof the forbiddenlines. The errors given here refer to the probableerrors of estima-
tion. Figs. 1 a and b show the overall radial velocity trend for the absorption and emission lines
with respect to the equivalent widths (Wλ) and lower excitation potentials of these lines.
The mean heliocentric radial velocity from absorption lines in the case of IRAS17311-4924
corresponds to VLSR= 31.13 km s−1. This value may be compared with the velocity (VLSR=) of
36 km s−1derived from CO observations of the star by Loup et al. (1990).
G. Sarkar et al.: Hot post-AGB stars : IRAS13266-5551 and IRAS17311-49245
Fig.1a.. Radial velocity trends of the absorption and emission lines in IRAS13266-5551.Radial
velocity measurements of the forbidden lines have not been plotted.
3.3. Wind velocities from the P-Cygni profiles
We estimated wind velocities from the well defined and unblended blue absorption edges of the
P-Cygni profiles of He I, C II and Fe III (Tables 2c and 3c). The absorption components of the
HαP-Cygni profiles are affected by the broad wings of the Hαemission components (Fig. 3) and
hence could not be used to estimate the wind velocities in these stars. The absorption component
of He I(45) 7281.349 Å P-Cygni profile in IRAS17311-4924 may be affected by atmospheric
absorption lines in this region. The wind velocities in IRAS17311-4924 increase with the LEP
of the species involved.
3.4. Diffuse interstellar bands (DIBs)
DIBs are absorptionfeatures in the spectra of reddenedstars and havetheir originin the interstel-
lar and circumstellar medium. They are typically broader than expected from the Doppler broad-
ening of turbulent gas motions in the interstellar and circumstellar medium. DIB at 5780.410
Å was identified in the spectra of IRAS13266-5551 and IRAS17311-4924. IRAS13266-5551