Talk:BP Psc project

VsopMember Use this page for writing the DDT proposal. I start with a skeleton of the DDT here, please fill it and add at least you related publications. Once, we agree on the final version, we can just cut and paste into the proposal template. 14:50, 7 Nov. 2007 (CLST)

Title
BP Psc: an infant star or an evolved binary?

ScientificRationale
BP Psc is classified as a T-Tauri star, showing strong H-alpha emission, and excess IR flux due to the presence of dust. According to Torres et al (2006), it is a single-lined spectroscopic binary (SB1), based on RV measurements of the absorption line spectrum of the G2-type main companion. The orbital period is very uncertain, although likely in the range 1-3 days. Recently, Zuckerman et al (2007, priv.comm.; in prep. for ApJ) has discovered optical jets emanating from the object. They also performed AO-assissted IR imaging, which showed no companion, but revealed a near edge-on disk.

During 2006 BP Psc was observed by us as part of VSOP (Variable Star One-shot Project; Dall et al. 2007) using HARPS. As already noted by Torres et al, the spectum exhibits many forbidden emission lines typical of planetary nebulae (PNe), among them [O I] 6300, [SII] 6731. We have confimed that these lines do not change RV, but maintain a velocity which is blue-shifted with repect to the star. The CCF-derived RV from the stellar absorption spectrum varies with an amplitude of about 1km/s However, the big mover is the H-alpha line, which in the high-resolution HARPS spectra reveal a near-perfect accretion profile. The amplitude of the H-alpha RV is at least 20km/s peak-to-peak. This is very surprising, because it points to the accretion taking place not on the G2 star, but onto the unseen companion, and a very small companion too. Such a small accreting companion points to a white dwarf star, which would explain the presence of PN lines in the spectrum.

The final argument for an accreting companion is the evolution of the spectral lines, best visible in the strong Na D doublet. On 2006-07-14, the lines are heavily broadened, corresponding to very high rotational velocity, approaching 100km/s (Fig.xxx). On 2007-11-06, the lines have changed dramatically (Fig.xxx) leaving the appearence of the star spinning down at an impossible rate, now at 42km/s! The only likely explanation for this, is that the stellar disk has been partly eclipsed, which is consistent with the H-alpha RV being close to the CCF RV on that date. Thus, in all likelyhood, the G2 star is an evolved star, orbited by a low-mas compact object which is accreting material, and which recently shed part of its material into an expanding shell. Thus, rather than a star in its infancy, we are witnessing an evolved binary system in a very rare evolutionary stage.

In order to determine the nature of the system, RV monitoring and high resolution spectroscopy is vitally importent, and we propose to pursue this unique opportunity for characterizing a rare state of binary evolution using the unrivaled precision of HARPS.

Other ponits not used in above text:
 * Strongly reddned spectra indicates dust.
 * Ca II H+K emission cores following the H-alpha, although at lower RV amplitudes. Are not following the stellar RV. Indicates activity is not associated with the G2 star.
 * Moderately strong lithium line. Could be from youth or more generally from recent accretion.
 * Na D emission at the velocity of the star (need to check!) and narrow self-absorption from cool material at velocities around this... from the shell?

So, it may in fact be an evolved binary system...

ImmediateObjective
We want to:
 * determine whether the object is a binary or not, thereby...
 * determine whether it is a YSO or if it's a post-MS star with a compact companion, captured in a very brief evolutionary stage.

We are asking for DDT since this is a highly competitive area and the object in question is one of the most interesting of its kind. A paper will soon appear in ApJ describing a multi-wavelength study and the discovery of the optical jets (I. Seong, B. Zuckerman, priv. comm.). Furthermore, Chandra observations have been granted for a study dedicated to this object only (PI: Kastner, Chandra proposal 09200153).

TelescopeJustification
HARPS is the only facility which provides both the necessary spectral resolution to resolve all fine structured lines, as well as the necessary radial velocity stability to restrict the orbital parameters.

Strategy
Data will be reduced automatically by the HARPS pipeline and then transferred into the VSOP machine, hosted at ESO-Santiago. We will observe the object in OBJ-SKY mode, i.e. with a sky spectrum recorded alongside the object spectrum in order to correct for the changing strength of the sky emission lines.

We will analyse the RV data and determine whether the object is a binary or not. If binary, we will extract the orbital period based of the movement of the photospheric (stellar) lines and the (accretion disk) emission lines. The nebular/shell lines will be used as independent standard RV stability checks.

WhyNights
Based on our VSOP spectrum, we can expect S/N=100 in one 1800 sec exposure, which is required to obtain the necessary RV accuracy. Since the orbit must be significantly longer than 3 hours, we request that only one spectrum be taken per service night until the star disappears in the evening twilight around mid-January. Given the 3.6m schedule, we thus ask for observations on N nights, or a total of M hours.

Publications

 * Dall, T. H. et al (incl. Foellmi, Depagne, Lo Curto, Saviane, Schmidtobreick, Monaco, Schuetz) "VSOP: the variable star one-shot project. I. Project presentation and first data release", 2007A&A...470.1201