Ecology of Blue Straggler Stars by Henri M. J. Boffin Giovanni Carraro & Giacomo Beccari

Author:Henri M. J. Boffin, Giovanni Carraro & Giacomo Beccari
Language: eng
Format: epub
Publisher: Springer Berlin Heidelberg, Berlin, Heidelberg

7.4.3 The Fellowship of the Ring

A small group of planetary nebulae (PNe) are known to harbour binary central stars where a subgiant or giant companion is enriched in carbon and s-process elements (see Boffin and Miszalski 2011; Miszalski et al. 2012 and ref. therein). These are the progenitors of the barium stars, caught during the short-lived phase ( ∼ 104 years) when the PN ejected by the white dwarf is still visible. These cool central stars are those of the (unfortunately) denominated A 35-type (Bond et al. 1993). The initial list included A 35 , LoTr 1 and LoTr 5 which have rapidly rotating sub-giants or giants accompanied by very hot white dwarfs peaking at UV wavelengths (T eff ≥ 100 kK). Since these initial discoveries, Bond et al. (2003) added WeBo 1 to the list while Frew (2008) raised suspicions that the nebula of A 35 may not be anymore a bona-fide PN, even if it is likely to have passed through a PN phase at some stage to create the stellar abundances currently observed (hence, the group should be named differently!). Further additions of HD 330036 and AS 201 may be drawn from barium enhanced yellow or D’-type symbiotic stars which show extended nebulae, but the classification of such objects as PNe is controversial (Corradi 2003; Jorissen et al. 2005). On the other hand, Tyndall et al. (2013) showed that the cool central star of LoTr 1 is not enhanced in barium and is therefore not a real member of the group. It is also interesting to note that van Winckel et al. (2014) reported the first detection of orbital motion in LoTr5 . Despite a 1807-day coverage, a full period was not yet covered, indicating that the orbital period may be as much as 10 years. This makes for a direct link with barium stars.

Miszalski et al. (2012) present UV and optical observations of the diamond ring PN A 70 (PN G038.1−25.4 ) that prove the existence of a barium star binary CSPN. The apparent morphology of A 70 is that of a ring nebula, although on closer inspection the [O III] image shows a ridged appearance similar to Sp 1 which is a bipolar torus viewed close to pole-on.

More recently, Miszalski et al. (2013a) also showed that the cool central star of the planetary nebula Hen 2-39 (Fig. 7.10) is carbon and s-process enriched. Weak Ca II K emission is detected, indicating some chromospheric activity may be present. Time-resolved photometry revealed a short-term periodicity, indicating that the nucleus of Hen 2-39 is a fast rotating giant (with a period of a few days), consistent with the periods found for similar stars. These include 5.9 days for LoTr 5 and 4.7 days for WeBo 1. A rotation period of 6.4 days was also determined for LoTr 1 (Tyndall et al. 2013). Given our discussion in Sect. 7.3.3, one could conjecture that these stars were spun-up during a wind accretion phase.

Fig. 7.10VLT FORS2 colour-composite image of Hen 2-39 made from Hα+[N II] (red), [O III] (green) and [O II] (blue).

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.