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New observations of the PZ Tel system, its substellar companion and debris disc

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  • ★Based on observations obtained at Paranal Observatory in European Southern Observatory (ESO) programmes 079.C-0908(A), 083.C-0150(B), 085.C-0012(A), 086.C-0600(A), 086.C-0600(B) and 087.C-0111(A).

E-mail: markus@astro.uni-jena.de

ABSTRACT

We present follow-up high-contrast imaging data of PZ Tel B, the substellar companion of a solar analogue pre-main-sequence star and member of the approximately 12-Myr-old β Pic moving group. Between 2010 October and 2011 June, we observed PZ Tel in three observing epochs with NACO/ESO-VLT, which proves the companionship of PZ Tel B on a very high significance level and shows that the photometry of the PZ Tel system is stable on the 10 per cent level. The orbital motion of PZ Tel B relative to its primary is clearly detected between all observing epochs, and we even find evidence for deceleration of its orbital motion, as expected for an object on a Keplerian orbit moving towards its apastron.

We also present new photometric measurements of PZ Tel just recently obtained by WISE, which clearly confirm that there is no excess emission in the spectral energy distribution (SED) of the star up to 24  μ m, while a prominent excess is detected at 70 μm with MIPS/Spitzer.

To quantify the orbit elements of the PZ Tel system, we applied a least-squares Monte Carlo (LSMC) fit on all astrometric measures taken with NACO. The results of the performed LSMC fit are that one cannot yet determine a single ‘best'-fitting orbital solution, due to the poor astrometric coverage of the orbit. However, the obtained good LSMC solutions (inline image) give an overview of the distributions and correlations of all orbit elements. According to our LSMC fit, PZ Tel is an eccentric binary (e ≳ 0.6), and the orbit elements degenerate with growing eccentricity.

For e < 0.95, the PZ Tel system is seen nearly edge-on (92° ≲ i ≲ 110°), has a longitude of the ascending node of 50° ≲ Ω ≲ 65°, an argument of periastron 170° ≲ ω ≲ 290° and exhibits a semimajor axis a larger than about 20 au, with the highest peak in the a-histogram located at about 25 au (or about 110 yr of orbital period), up to a few tens of au, as orbital solutions with a significantly larger semimajor axis would not be able to harbour a circumbinary disc which excites the observed excess in the SED of PZ Tel A.

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