Astron. Astrophys. 322, 511-522 (1997)

2. The IN Comae system

IN Comae ( mag, G5III-IV) has been supposed to be the primary component of a binary system with one of the hottest known stars as its secondary ( mag, sdO,  K; Feibelman & Kaler 1983). This O subdwarf itself being the central star of the planetary nebula LoTr-5. As such, the evolutionary state of the G5 component (which we will call IN Comae hereafter ¹) is possibly unusual but, as we will show, it exhibits all the commonly known activity phenomena of the RS CVn or FK Comae type stars. It is, though, still not fully clear whether IN Comae is indeed physically related to the hot subdwarf.

2.1. History of orbital solutions

Originally, Acker et al. (1985) detected radial velocity variation with an amplitude of up to 100 km s^-1 and a probable period of 0.35 days. Jasniewicz et al. (1987) obtained a series of CORAVEL radial velocities and concluded that the IN Comae system consists of an inner 1.99-day orbital-period binary with two, approximately equal, components in the optical spectrum and a third component - the hot subdwarf - in an outer, most likely 540-day period orbit. Malasan et al. (1989) also concluded that IN Comae is a triple system, but found just one component in the optical spectrum - the G5 star - and it was not clear to them whether the hot subdwarf belongs to the inner or the outer system. Also, they obtained rather different orbital periods for both the inner and the outer system (1.75 days and 2000 days, respectively) than Jasniewicz et al. (1987). Later, Jasniewicz et al. (1994) presented further velocities and concluded that the inner system is a single-lined spectroscopic binary but with no well established period, that the absorption line profiles just mimic a double-lined spectrum and that in reality the profiles from the G5 component were distorted due to starspots. Most recently, Jasniewicz et al. (1996) suggested that the rotational and orbital planes are not coplanar.

2.2. The quest for the true photometric period

The history of the period determinations of IN Comae is long and checkered. Originally, its photometric variability was discovered by Schnell & Purgathofer (1983) at Vienna Observatory who also found four periods from their Fourier analysis (1.2001, 0.8567, 0.5453, and 0.3528 days) but most likely all of them were aliases of the true 5.9-day period. This 5.9-day period was first detected by Noskova (1989) and Bond & Livio (1990) and confirmed by Kuczawska & Mikolajewski (1993) although Jasniewicz et al. (1994) claimed that the 1.2-day period (frequency d^-1) is the correct one and 5.9 days the alias. Their conclusion was based on 25 photometric measurements during a two-month interval in 1989, and on H -line monitoring during three intervals from May 1990 through May 1991 at high spectral resolution. On the other hand, their new CORAVEL radial velocities did not show any convincing period at that time and the 1.2-day period seems to have appeared only in a limited interval of time. In their most recent paper, however, Jasniewicz et al. (1996) concluded that the 5.9-day period is more realistic but still use the 1.2-day period to phase their data.

Kuczawska & Mikolajewski (1993) obtained UBV photometry on seven consecutive nights with high time resolution and demonstrated the existence of yet another period in the data with 0.2504 days and also confirm the 5.9-day period. If we assume the 0.25-day period to be real and due to a reflection effect caused by the hot subdwarf onto its, so far unknown, companion star then this period would be the orbital period of the "inner" system and the G5 star would be the third star in the system with a rotation period of 5.9 days; a configuration very similar to HR 6469 (see Van Hamme et al. 1994), a RS CVn binary with an active G5 giant as the outer component in a triple system. However, this 0.25-day period of IN Comae was neither searched for nor noticed in recent photometry by Jasniewicz et al. (1996) and thus remains unconfirmed.

2.3. The planetary nebula LoTr-5

Longmore & Tritton (1980) discovered LoTr-5 as the planetary nebula with the highest galactic latitude of all known planetary nebulae ( = ) and measured its apparant size to 560" 490". To obtain its distance, Longmore & Tritton simply adopted = 0.7 for a G5III star and an observed apparent brightness of IN Comae to obtain 400 200 pc as the distance to LoTr-5. Kaler (1983) used a statistical relation for the total H flux of a planetary nebula to obtain its distance of 620 200 pc, while Malasan et al. (1989) applied a statistical relation between the apparent size and the distance and obtained 100-170 pc. The catalog of absolute fluxes and distances of planetary nebulae of Cahn et al. (1992) lists LoTr-5 with a distance of 6297 pc (also adopted in the Strasbourg-ESO catalogue by Acker et al. 1993), and is also based on a relation between total H flux and distance. However, this value is likely to be wrong because the catalog erroneously lists an apparent radius for LoTr-5 of 5.3" instead of the measured 260". Also, a typical linear diameter for a large planetary nebula is around 0.5 pc, while a 6-kpc distance would imply a linear diameter of around 20 pc (Longmore & Tritton 1980).

The central system was already discovered as an X-ray source by EXOSAT and Einstein (see Apparao et al. 1992). But the ROSAT all-sky survey resolved both X-ray emission components (Kreysing et al. 1992), i.e. the emission from the nebula itself caused by the interacting stellar wind from the central star, and the emission that originates directly from the central star (the subdwarf plus IN Comae).

2.4. The active-chromosphere character of IN Comae

Feibelman & Kaler (1983) discovered the composite nature of the ultraviolet spectrum of IN Comae, the hot subdwarf clearly dominating the spectrum below 2000Å . But the flux increase at longer wavelengths and the strong Mg II h&k emission lines indicate a very active chromosphere for the G5 component (the brightness of the subdwarf is 14.7 mag, thus no contribution at optical wavelengths is expected). The active-chromosphere character has been confirmed by the existence of strong Ca II H&K (Jasniewicz et al. 1987, Strassmeier 1994a) and H emission lines (Jasniewicz et al. 1994).

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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