Infrared Properties of Barium Stars?

A&A 372, 245–248 (2001) Astronomy DOI: 10.1051/0004-6361:20010508 & c ESO 2001 Astrophysics

Infrared properties of barium stars?

P. S. Chen??

Yunnan Observatory & United Laboratory of Optical Astronomy, CAS, Kunming 650011, PR China

Received 25 September 2000 / Accepted 29 November 2000

Abstract. We present the results of a systematic survey for IRAS associations of barium stars. A total of 155 associations were detected, and IRAS low-resolution spectra exist for 50 barium stars. We use different color- color diagrams from the visual band to 60 µm, relations between these colors and the spectral type, the barium intensity, and the IRAS low-resolution spectra to discuss physical properties of barium stars in the infrared. It is confirmed that most barium stars have infrared excesses in the near infrared. However, a new result of this work is that most barium stars have no excesses in the far infrared. This fact may imply that infrared excesses of barium stars are mainly due to the re-emission of energy lost from the Bond-Neff depression. It is also shown that the spectral type and the barium intensity of barium stars are not correlated with infrared colors, but may be correlated with V − K color.

Key words. stars: late-type – infrared: stars – stars: chemically peculiar

1. Introduction The most comprehensive list of barium stars in the lit- erature is that published by Lü et al. (1983) and Lü (1991). Barium stars (also known as Ba ii stars) were first iden- In the latter paper, 389 barium stars were listed with bar- tified as a class of peculiar red giants by Bidelman and ium intensity from 0.1 to 5 (Warner 1965; Keenan & Pitts Keenan (1951). Barium stars are thought to be evolved G, 1980). In the earlier years, Neugebauer & Leighton (1969, K and M stars with luminosity classes I to IV, and they hereafter IRC) observed some bright barium stars in the exhibit strong spectral lines of s-process elements, partic- I and K bands. The first systematic photometry in the ularly Ba ii at 4554 A˚ and Sr ii at 4216 A,˚ and carbon-rich JHK bands was given by Feast & Catchpole (1977), who molecules such as CH, CN and C2 (Bidelman & Keenan showed that barium stars have infrared excesses in the 1951; McClure 1984). Recently, it was confirmed that bar- near infrared, compared to normal giants. After about a ium stars are hotter analogues and progenitors of Tc-poor decade, Hakkila & McNamara (1987) and Hakkila (1989) (also called extrinsic) S stars in binary systems (McClure made more intensive studies of barium stars in the near 1984; Han et al. 1995; Jorissen et al. 1998). With the dis- and middle infrared. From these studies, it was shown that covery of the binary nature of barium stars, their chem- most barium stars exhibit weak (<0m. 1) infrared excesses ical peculiarities have been attributed to mass transfer in the H and M bands, and some barium stars show rather across the binary system. When the current white dwarf large (>0m. 2) infrared excess in the N band. These au- (WD) companion of a barium star was a thermal pulsing thors suggested that infrared excesses in such bands may AGB star, it transferred s-process and carbon-rich mate- be caused by: (1) redistribution of energy into the infrared rial to its companion, which now appears as a barium star from the so called Bond-Neff depression, which is a broad (Bergeat & Knapik 1997; Jorissen et al. 1998). The sce- drop in the continuum between 3500 A˚ and 4500 A(Bond˚ nario has recently been confirmed by Bohm-Vitense et al. & Neff 1969) or (2) thermal emission in the circumstellar (2000). Most barium stars they studied appear to have material caused by mass transfer in the binary system. excess flux in the UV, which can be attributed to their Hakkila & McNamara (1987) pointed out that it is nec- WD companions. Therefore barium stars are very impor- essary to study the physical properties of barium stars in tant to our understanding of nucleosynthesis and stellar the far infrared region in order to test these mechanisms evolution in the post-main sequence stage. of production of such infrared excesses.

? Table 1 is only available in electronic form at the CDS In this paper, a survey of IRAS associations of barium via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) stars is presented. Diﬀerent color-color diagrams, relations or via between these colors and the spectral type, and the bar- http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/372/245 ium intensity are used to outline the physical properties ?? e-mail: [email protected] of barium stars in the infrared.

Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20010508 246 P. S. Chen: Infrared properties of barium stars

2. Samples and data processing

As mentioned above, Lü (1991) presented a comprehensive catalog of barium stars, in which a total of 389 barium stars are listed with the HD number and the position from the HD catalog in the epoch of 1900. This is our working sample. Based on the positions and magnitudes in V from Lü (1991), all barium stars were checked and identified with the HST Guide Star Catalog (1992, hereafter GSC). Because all barium stars listed are fairly bright (magnitude in V brighter than 10m. 0), this identification is easily made. Then, using the method for the IRAS identification of stars from Chen (1996), IRAS associations of these Fig. 1. K − [12] versus V − K diagram for barium stars. barium stars were obtained. This method is as follows: (1) there is a positional error ellipse for each IRAS source 3. Discussion in the IRAS Point Source Catalog (1988, hereafter PSC), and this error ellipse has a reliability of over 95% (IRAS 3.1. Color-color diagrams Catalogs and Atlas: Explanatory Supplement 1988, hereafter IRAS ES); (2) On average, the positional accuracy From Table 1, the flux densities at 12, 25 and 60 µmcan in the GSC is better than 000. 3 (GSC 1992) so that if the be transferred into magnitudes according to the following position of a barium star identified in the GSC is located expressions without color correction (IRAS ES): in a certain error ellipse of an IRAS source, the associa- [12] = 3.63 − 2.5logF12 (1) tion is confirmed, otherwise there is no such association between the barium star and the IRAS source. [25] = 2.07 − 2.5logF25 (2)

155 IRAS associations of barium stars were found and [60] = 0.19 − 2.5logF60. (3) are listed in Table 1. In addition, IRAS carried on board a slitless spectrometer which recorded the low-resolution If V magnitudes from the GSC observations and K mag- spectra of many sources in the 8–23 µm region with the nitudes from the IRC or others in Table 1 are taken into spectral resolution of about 20–60 (IRAS ES). The IRAS account, the K − [12] versus V − K color-color diagram Atlas of low-resolution spectra (LRS) (IRAS Science Team can be plotted in Fig. 1. The blackbody line is also shown 1986, hereafter LRS Atlas) contained spectra of 5425 in Fig. 1 with some temperature indications. It can be sources. This database was extended by Kwok et al. (1997) seen from Fig. 1 that the objects of our sample cover a to a total of 11 224 sources. Therefore LRS spectra of these large interval in V − K but only a small K − [12] interval 155 barium stars were checked and extracted from Kwok with about 2 magnitude 5000 K. If the “standard” IRAS two-color diagram from origination, except for those from IRC. van der Veen & Habing (1988, Fig. 5b, note that the color P. S. Chen: Infrared properties of barium stars 247

Fig. 2. [12] − [25] versus K − [12] diagram for barium stars. Fig. 4. V − K versus spectral type for barium stars.

Fig. 3. [25] − [60] versus [12] − [25] diagram for barium stars. Fig. 5. K − [12] versus spectral type for barium stars. definition in this figure is slightly different from Fig. 3 in this paper) is taken into account to compare with Fig. 3, it is clear that all barium stars here analyzed are located 3.3. Barium intensity and colors in the region of I in the “standard” IRAS two-color diagram, which is the region of “non-variable stars without Taking data from Table 1, the V − K color versus bar- circumstellar shells”. From the distributions above, it may ium intensity diagram is plotted in Fig. 6. For barium be concluded that most barium stars have some infrared intensities less than one, i.e. for so called “weak barium excesses in the near infrared, but no infrared excesses in stars” (Lü 1991), there is a wide distribution of V − K the IRAS region can be found. color from about 2 to almost 6, but no correlation between the barium intensity and the V −K color. However, 3.2. Spectral type and colors as the barium intensity increases from 2 to 5, on average, the V − K color seems to gradually increase, but the sta- Taking the spectral type from Lü (1991), the V − K ver- tistical significance is small because of the small available sus Spectral type (Sp.) diagram for barium stars can be sample size. The definition of barium intensity is based on plotted (Fig. 4). Here we indicate the intrinsic V − K the Ba ii line strength at 4554 A˚ (McClure 1984; Lü 1991), colors for normal giant stars from Bessell & Brett (1988) however, this line is out of the range of the pass-band in with open squares. Compared with the normal giant stars, V for the standard system (Bessell 1990). Therefore, the most barium stars with different spectral types show in- possible weak correlation between V −K color and barium frared excesses in the K band. In addition, V − K colors intensity for barium stars with a barium intensity from 2 for either barium stars or normal giant stars are all in- to 5 is not easy to explain. The barium line strength is creased as spectral types become later. This mainly may (besides barium abundance) a function of effective tem- be due to the decrease in the photospheric temperature perature and gravity of the star, and the V − K color of the stars. The K − [12] versus spectral type diagram can be considered as a temperature indicator. However, for barium stars can be plotted in Fig. 5. It can be seen the situation is very complicated because the cooler bar- from Fig. 5 that there is no correlation between K − [12] ium stars have higher luminosities than the hotter barium color and spectral type for barium stars. The [12] − [25] stars (Bergeat & Knapik 1997). Thus, it is possible that and [25] − [60] versus spectral type diagrams are similar a very complicated combination of temperature and lu- to Fig. 5. minosity/gravity effects taken place. Detailed models are 248 P. S. Chen: Infrared properties of barium stars

Fig. 6. V − K versus barium intensity for barium stars. Fig. 7. K − [12] versus barium intensity for barium stars. required to check this problem. Thus, the weak correlation Acknowledgements. We thank the referee for his/her sugges- of barium intensity with V −K color may not reflect a real tions. This work is supported by the National Natural Science correlation between barium abundance and V − K color. Foundation of China and the Chinese Academy of Sciences. This work has made use of NASA’s ADS database. In addition, the K − [12] color versus barium intensity diagram, shown in Fig. 7, shows no such correlation. The References [12]−[25], and [25]−[60] versus barium intensity diagrams are similar to that of Fig. 7. Bessell, M. S. 1990, PASP, 102, 1181 Bessell, M. S., & Brett, J. M. 1988, PASP, 100, 1134 Bidelman, W. P., & Keenan, P. C. 1951, ApJ, 114, 473 3.4. LRS spectra Bond, H. E., & Neff, J. S. 1969, ApJ, 158, 1235 Bergeat, J., & Knapik, A. 1997, A&A, 321, L9 From Table 1 it can be seen that there are 50 LRS spectra Bohm-Vitense, E., Carpenter, K., Robinson, R., Ake, T., & for barium stars, among which 45 are classified as S, indi- Brown, J. 2000, ApJ, 533, 969 cating the Rayleigh-Jeans tails of the stellar photospheric Chen, P. S. 1996, ChA&Ap, 20, 169 continuum; 4 are in the group F, indicating a featureless Elias, J. H. 1978, AJ, 83, 791 continuum with small amounts of circumstellar dust and Engels, D., et al. 1981, A&AS, 45, 5 one belongs to the group I, indicating a noisy or incom- Feast, M. W., & Catchpole, R. M. 1977, MNRAS, 180, 61 plete spectrum (Kwok et al. 1997). Note that V − K and Hakkila, J., & McNamara, B. J. 1987, A&A, 186, 255 K −[12] colors for F sources and S sources are not very dif- Hakkila, J. 1989, A&A, 213, 204 Han, Z., Eggleton, P. P., Podsiadlowski, P., & Tout, C. A. 1995, ferent. From these results it is again confirmed that most MNRAS, 277, 1443 barium stars have no infrared excesses beyond the near Hartoog, M. R., et al. 1977, PASP, 89, 660 infrared region (at least in the 8–23 µm range). STScI 1992, HST Guide Star Catalog (CD-ROM), Version 1.1 (GSC) Joint IRAS Science Working Group 1988, IRAS Point Source 4. Conclusions Catalog, Version 2, (GPO, Washington DC) (PSC) From the discussion in Sect. 3 we conclude: Joint IRAS Science Working Group 1988, IRAS Catalog and Atlas Explanatory Supplement, (GPO, Washington DC) (1) Most barium stars have infrared excesses in the near (IRAS ES) infrared, but not in the IRAS region. This implies that IRAS Science Team 1986, A&AS, 65, 607 (LRS Atlas) the excess infrared flux is mainly due to the re-emission Jorissen, A., van Eck, S., Mayor, M., & Udry, S. 1998, A&A, 332, 877 of energy lost from the Bond-Neff depression. The ther- Kwok, S., Volk, K., & Bidelman, W. P. 1997, ApJS, 112, 557 mal emission caused by mass transfer in the binary Keenan, P. C., & Pitts, R. E. 1980, ApJS, 51, 489 system is not the main contributor; Leggett, S. K., & Hawkins, M. R. S. 1988, MNRAS, 234, 1065 (2) Spectral types of barium stars are correlated with the Lü, P. K., Dawson, D. W., Upgren, A. R., & Weis, E. W. 1983, color V − K, indicating the photospheric temperature ApJS, 52, 169 difference. However, spectral types of barium stars are Lü, P. K. 1991, AJ, 101, 2229 not correlated with K − [12], [12] − [25] and [25] − [60] McClure, R. D. 1984, PASP, 96, 117 colors; McGregor, P. J., & Hyland, A. R. 1984, ApJ, 277, 149 (3) barium intensities of stars with barium intensity Neugebauer, G., & Leighton, R. B. 1969, NASA SP-3047 (IRC) Price, S. D. 1968, AJ, 73, 431 from 2 to 5 may be weakly correlated with V − K − − van der Veen, W. E. C. J., & Habing, H. J. 1988, A&A, 194, color, but not correlated with K [12], [12] [25] and 125 − [25] [60] colors. Warner, B. 1965, MNRAS, 129, 263