On the Distance to the 2M1207 System

Eric Mamajek (CfA, now at UR)

Last updated: 8 November 2007

Introduction

I have been emailed by a few people asking what the current status is of our knowledge of the distance to the 2M1207 system. There has been a recent flurry of trigonometric parallax distance estimates to this system, and they appear to have converged on 53 (+-1) parsecs, which is remarkably (and thankfully!) consistent with my original indirect distance estimate via the moving cluster method (53 +- 6 pc; Mamajek 2005), but which was admittedly 2.5 sigma lower than my most recent moving cluster method estimate (66 +- 5 pc; Mamajek & Meyer 2007). The new published trigonometric parallax estimates are self-consistent, and clearly now provide the preferred distance to this system.

Background

Chauvin et al. 2004 discovered a "planetary mass object" orbiting the young brown dwarf 2M1207. Although often called a "planet", the status of this object as such is in doubt (although it depends on your definition). The non-controversial facts that the majority of astronomers appear to agree on is the following:

(1) that the 2M1207 system is a member of the TW Hya association, which is probably ~8 Myr (with perhaps ~50% uncertainty in the age from the evolutionary tracks),

(2) 2M1207A is a ~25 Jupiter-mass brown dwarf with an accretion disk (with perhaps ~25% uncertainty in the mass from the evolutionary tracks), and

(3) The faint companion 2M1207B is clearly co-moving with 2M1207A, and they clearly constitute a physical binary system,

(4) that 2M1207B probably has a mass below the deuterium burning limit (i.e. it is not a "fusor").

As mass is usually not a directly determinable quantity for celestial objects, an object's luminosity and effective temperature is usually critical to estimating its mass (via comparison with theoretical evolutionary tracks). To constrain an object's luminosity, one needs an accurate distance -- hence the interest in deriving an accurate distance to this astrophysically interesting system.

For recent studies of the 2M1207 system and the faint companion, I refer the reader to Chauvin et al. 2004, Chauvin et al. 2005, Mamajek 2005, Song et al. 2006, Mohanty et al. 2007, Mamajek & Meyer 2007. 2M1207A was discovered by Gizis 2002, and for studies on the primary and its accretion disk & activity, I refer the reader to selected publications by Mohanty et al. 2003, Sterzik et al. 2004, Whelen et al. 2007, Stelzer et al. 2007.

Summary of Published Distances

A table summarizing the distance estimates to 2M1207 through the end of 2007 are listed in the table below from Ducourant et al., A&A, in press.

Table from Ducourant et al., in press

The initial distance (70 +- 20 pc) to the system by Chauvin et al. 2004 was estimated photometrically by assuming that the primary should lie on an empirical ~10 Myr isochrone. Mamajek 2005 used the moving cluster method to estimate individual distances to all of the TW Hya association members, and derived 53 +- 6 pc. The moving cluster distance was dependent on a proper motion for 2M1207 based on positions on photographic plates and the 2MASS survey, as well as an estimated mean space motion vector for the TW Hya association which was dependent on the Hipparcos parallaxes for a small number of bright group members. Song et al. 2006 calculated an improved proper motion for the 2M1207 system, and estimated a distance by taking the Hipparcos distance to the association member HR 4796 (TWA 11) and scaling its distance by the ratio of its proper motion and that of 2M1207b (this is not explicitly a moving cluster parallax, but should give a reasonable estimate to first order). Mamajek & Meyer 2007 used the same technique as Mamajek 2005, and included the improved Song et al. proper motion and a revised estimate of the TW Hya association group velocity vector (which dumped TWA 9, which may have a poor Hipparcos parallax due to binarity). This lead to an estimated distance of 66 +- 5 pc. The reason for the 2sigma increase in distance from Mamajek 2005 was a ~+1 sigma bump from the revised (smaller) proper motion, and a ~+1 sigma bump from the revised space motion.

Fortunately, we now have direct distance estimates to the 2M1207 system. In 2007, three groups independently published trigonometric parallax distances to 2M1207, all of which provide direct distance estimates independent of any velocity vector or evolutionary track models (except for the small conversion between relative and absolute parallaxes, but this should be well characterized). The three parallaxes from Gizis et al. 2007, Biller & Close 2007, and Ducourant et al, in press are listed in the table, and agree with one another within the errors, and agree well with the original moving cluster distance estimate from Mamajek 2005. As the three direct distance estimates from Gizis et al. 2007, Biller & Close 2007, and Ducourant et al, in press are independently derived and self-consistent, one should be able to take the weighted mean as robust estimate. The variance-weighted mean (i.e. weighted as 1/sigma^2) of these three parallaxes is:

52.75 (+1.04, -1.00) pc (1.9% error)

where the Ducourant et al parallax provides ~85% of the weight. Remarkably, this faint object now has the best determined distance in the TW Hya association, or of any known <10-Myr-old star within 100 pc (to my knowledge).

As this agrees well with previous indirect distance estimates from 2005 and 2006, the implications on the luminosity and mass of the system have changed little over the past two years. There luminosity and temperature estimates for the companion are still greatly at odds with the evolutionary tracks for substellar objects (either a factor of ~8 too underluminous or ~500K too hot?). The source of this discrepancy will have interesting astrophysical implications, no matter how it is resolved. For further discussion on this topic, I refer the readers to discussions in the following papers: Mamajek 2005, Song et al. 2006, Mohanty et al. 2007, Mamajek & Meyer 2007, Gizis et al. 2007, Biller & Close 2007, and Ducourant et al, in press.