Observational survey of the puzzling star HD 179821: Photometric variations and period analysis H. Le Coroller, A. Lèbre, D. Gillet, E. Chapellier To cite this version: H. Le Coroller, A. Lèbre, D. Gillet, E. Chapellier. Observational survey of the puzzling star HD 179821: Photometric variations and period analysis. Astronomy and Astrophysics - A&A, EDP Sciences, 2003, 400, pp.613-621. 10.1051/0004-6361:20030076. hal-00418974 HAL Id: hal-00418974 https://hal.archives-ouvertes.fr/hal-00418974 Submitted on 27 Jan 2021 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. A&A 400, 613–621 (2003) Astronomy DOI: 10.1051/0004-6361:20030076 & c ESO 2003 Astrophysics Observational survey of the puzzling star HD 179821: Photometric variations and period analysis? H. Le Coroller1,A.L`ebre2, D. Gillet1, and E. Chapellier3 1 Observatoire de Haute-Provence (CNRS), 04870 Saint-Michel-l’Observatoire, France 2 Groupe de Recherche en Astronomie et Astrophysique du Languedoc–UMR 5024/ISTEEM (CNRS), Universit´e Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex 05, France 3 Observatoire de la Cˆote d’Azur, D´epartement Fresnel, UMR 6528, BP 4229, 06304 Nice Cedex 4, France Received 8 August 2002 / Accepted 19 December 2002 Abstract. From new photometric observations (UBVRI), we present the characteristic features of the light variations of the evolved star HD 179821 (= SAO 124414 = IRAS 19114+0002). Our data, collected through 1999 and 2000, have been com- bined together with previous photometric measurements available in the literature. Thus, a long term V-light curve (gathering more than 10 years of observations for HD 179821) has been composed. We have analysed it with the Fourier transform method. Two main frequencies are present in the resulting power spectrum, reflecting a dominant bimodal pulsator behavior. A long term phenomenon is also found, but it is not possible to decide whether it is periodic. The Fourier analysis has also been applied on two other filters (U and B) and confirms the detected frequencies. On the basis of our period analysis, we discuss the nature of HD 179821: low-mass post-AGB star or high-mass star. Key words. stars: atmospheres – stars: AGB and post-AGB – shock waves – stars: oscillations – stars: fundamental parameters – stars: individual: HD 179821 1 1. Introduction the large outflow velocity of HD 179821 (34 2kms− , Zuckerman & Dyck 1986) would be rather in favor± of the mas- HD 179821 (= SAO 124414 = IRAS 19114+0002 = AFGL sive hypothesis. However Josselin & L`ebre (2001) have given 2343 = V1427 Aql = HIP 94496) is an oxygen-rich evolved an example of another post-AGB object (OH 231.8+4.2) with star surrounded by an extended envelope of gas and dust. This such a large outflow velocity value (30 km s 1). From radio ob- object is still the subject of debate to know whether it is an − servations, these authors have also derived a very low value for intermediate-mass post-AGB star or a massive supergiant. the 12C / 13C ratio which is not common for massive stars. One way to classify this kind of object is to study its chem- ical composition and/or the characteristic features of the shell The distance and consequently the luminosity of the star is surrounding the star. Indeed, the abundances of s-elements another argument to help in its classification. If HD 179821 lies synthetized during the third dredge-up of the AGB life are at 1 kpc, it is clearly a post-AGB star while if it is located at good indicators to decide between a post-AGB or a hyper- 6 kpc, its luminosity thus being larger, its initial mass is conse- giant status (Busso et al. 1995; Van Winckel et al. 1996). quently over 20 M . Nevertheless, the distance of HD 179821 In HD 179821 however, the s-element abundances are not is not well determined by the trigonometric parallax given by known with very good accuracy. For instance and depending Hipparcos. Consequently, an indirect way must be used to es- on the adopted effective temperature for HD 179821, Zacs et al. timate this distance. Zuckerman & Dyck (1986) have noted 1 (1996) and Reddy & Hrivnak (1999) have found s-elements in that the high LSR velocity of HD 179821 (105 1kms− ) over-abundance, while Th`evenin et al. (2000) have obtained a would place this star at 6 kpc. Jura & Werner (1999)± have slight deficiency. also claimed that the OH-maser ring is best understood if HD On another hand, the expansion velocity of the shell can 179821 lies at 6 kpc. Indeed, at 1 kpc the ultraviolet dust opac- also help to classify the star. The typical expansion velocity ity deduced from the OH-radius is too large. Moreover, using 1 of the shell of post-AGB stars is about 10–20 km s− while a radiative transfer model of the nebula, Hawkins et al. (1995) Send offprint requests to: H. Le Coroller, have obtained a good fit to the observed inner and outer ra- e-mail: [email protected] dius of the shell and to its spectral energy distribution, if HD ? based on observations carried out at the Observatoire de Haute 179821 lies at a distance of about 6 kpc. But for this distance, Provence, France, operated by the Centre National de la Recherche the height of HD 179821 above the galactic plane would be Scientifique (CNRS). around 524 pc (Josselin & L`ebre 2001) which is quite above Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20030076 614 H. Le Coroller et al.: Observational survey of the puzzling star HD 179821 the scale height for 20 M stars (Miller & Scalo 1979). Thus, For all these photometric observations, we used the 80 cm arguments pointing≥ to a large distance and a high mass are still and the 1.20 m telescopes at the Observatoire de Haute- open to criticism (Josselin & L`ebre 2001). Notably, the distance Provence (OHP, France), in 1999 and in 2000 respectively. 1 of 6 kpc, deduced from the Vlsr = 105 km s− (Zuckerman & In 1999, we observed HD 179821 very regularly (every clear Dyck 1986) is not obvious because the star would then be out- night), while in 2000, we observed the star once a week only side the galactic disk. but over four months. In both cases we performed differential Very recently Gledhill (Gledhill et al. 2001; Gledhill & photometry with Cousin filters Bc, Vc, Rc, Ic and in U 0 .The Takami 2001) found some new arguments in favor of the mas- transmission curves of these filters and the CCD characteristics sive status of HD 179821. Notably, these authors show that are given on the OHP WEB server (http://www.obs-hp.fr). the OH-shell feature is consistent with a distance estimate to Note that we did not work in a standard photometric system. 6 kpc. Moreover, they find that the SiO emission (as the OH- Moreover, in order to spread the flux on the CCD, the im- maser emission) arises in a large-scale detached shell, rather ages were acquired out of focus. Thus, we increased the ex- than from a location close to the star. This effect is also ob- posure time to obtain a good signal-to-noise ratio. The ta- served in a yellow hyper-giant star, IRC +10420. The connec- bles with all our measurements (data collected in 1999 and tion between these two stars is a strong argument in favor of the in 2000) are available upon request from H. Le Coroller hyper-giant status for HD 179821. In the same way, Molster ([email protected]). et al. (2002) have concluded that the circumstellar dust shell of HD 179821 is comparable to the ones of two other massive stars: AFGL 4106 and IRC +10420. Notably, for HD 179821, 2.1. Photometric observations in 1999 the detected 10 µm complex is dominated by the red-shifted amorphous silicate feature and its shape and position strongly In 1999, we performed on HD 179821 one or two exposures resemble to the feature detected in AFGL 4106. Also, the per night in each filter (UBVRI). With the 80 cm telescope at strength of the ratio of the 32.8 µm to 33.6 µm features suggests OHP, the CCD field is 3:6 3:6 .Insuchareducedfield, that HD 179821 is massive and likely a post-red supergiant. 0 0 there was only one reference× star (HD 179796) and we could HD 179821 is one of these puzzling objects where the dis- not be sure about its stability. Fortunately, for the observations tinction between low mass post-AGB and massive stars is not collected in 2000, the CCD field being larger, it was therefore easy. Furthermore, both post-AGB and massive evolved stars possible to check the stability of this reference star (see next are statistically rare and their pulsation characteristics are still section). Hence, for the 1999 observations, in each filter, we poorly investigated and not completely understood. To better found the difference in magnitude between HD 179821 and the understand this kind of object, we have undertaken long term reference star (HD 179796).