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PV Cep: rapid changes of the on-source polarization

ATEL # 1256; V.M. Larionov, E.N. Kopatskaya, L.V.Larionova, T.S.Konstantinova (Sobolev Astronomical Institute, St.Petersburg, Russia), D.Lorenzetti (Osservatorio Astronomico di Roma, Monte Porzio, Italy)
on 2 Nov 2007; 6:35 UT
Password Certification: Dario Lorenzetti (dloren@mporzio.astro.it)

Subjects: Optical, Variables, Stars

From the Crimean telescope AZT-8 + ST-7XME CCD we have observed the variable PMS star PV Cep (Cohen, Kuhi & Harlan 1977 ApJ 215, L127; Gyulbudaghian et al. 1977 SvAL 3 84), obtaining a series of imaging (BVRI bands) and polarimetric (I band) data starting from 2007 June. This source belongs to a sample of young objects (EXor variables) that we are monitoring in IR since a couple of years (Lorenzetti et al. 2006, 2007). Our data evidence two important facts: (i) the well known variability of the associated biconical nebula (Gledhill, Warren-Smith & Scarrott 1987, MNRAS 229, 643) occurs even on less then month time-scale. The nebula, quite bright until mid-June, almost totally disappeared up to the end of July and slowly restored starting from mid-September, these variations beeing in phase with brightness variations (1 mag amplitude in R and I bands) of PV Cep itself; (ii) when the nebula faded we found that the polarization of the star itself spread into the range 18 - 22 % and PA of 64 68 deg. Previous determinations of the on-source visual polarization are 10% , PA = 71 deg; and 16%, PA = 90 deg; in 1984 June and 1989 July, respectively (Scarrott, Rolph & Tadhunter 1991, MNRAS 249, 131). Our measurements provide the largest value never recorded. While a variation of the degree of polarization occurring in a relatively short time-scale (years) can be still reconciled with effects due to both extinction or phenomena intrinsic to the source, the significant change in position angle (in a similar time-scale) confirms the interpretation given by (Scarrott et al. 1991) in favour of the existence of, at least, two sets of polarizing grains with different alignments that alternately dominate the polarization.


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