Protostars and Planets V 2005 8522.pdf

THE DISTANCE TO THE YOUNG EXOPLANET 2M1207 B IN THE TW HYA ASSOCIATION. E. E. Mamajek, Harvard- Smithsonian Center for Astrophysics, 60 Garden St., MS-42, Cambridge MA 02138, USA, ([email protected]).

Introduction: Results: Recently, a faint companion to the young brown dwarf The proper motion and radial velocity of 2M1207 are statisti- 2MASSW J1207334-393254 (2M1207) was imaged [1], and cally consistent with TWAmembership (quantitatively strength- found to have common proper motion with its primary [2]. ening claims by [1,4]). The brown dwarf and companion are purported to be members The moving cluster method predicts a distance of 53 6 pc of the 10 Myr-old TW Hya Association (TWA) [3,4]. As- to the 2M1207 system. suming a distance of 70 20 pc and age of 10 Myr, the brown The improved distance roughly halves the previously cal- dwarf and companion are consistent with masses of 25 MJup culated luminosities for 2M1207 A and B, and reduces their and 5 MJup [1]. There is currently little constraint on the inferred masses to 21 MJup and 3 MJup using modern distance to this astrophysically interesting system (with the evolutionary tracks [9]. The current projected separation be- secondary possibly being the first imaged extrasolar planet). tween A and B is 41 5 AU. Although a trigonometric parallax is not yet available, it is pos- Objects in the literature with the “TWA” acronym seem to sible to use the proper motion and putative cluster membership be segregated by distance into at least two groups, with TWA of the 2M1207 system to estimate the distance using the mov- 12, 17, 18, 19, 24 appearing to be more distant (d ' 100- ing cluster method. This technique was historically valuable 150 pc) members of the Lower Cen-Cru OB association. The for establishing the bottom rungs of the cosmic distance ladder remaining TWA objects have moving cluster distances of d = in the pre-Hipparcos era, through geometrically determining 49 12 pc (1). the distances to members of nearby clusters (e.g. Hyades). The proper motion data for TWA are consistent with an +0.3 1 In this poster contribution, I (1) improve the kinematic internal 1D velocity dispersion of 0.80.2 km s . model and membership for the TWA,(2) calculate an improved There is some evidence for expansion in the TWA. This proper motion for the 2M1207 system, (3) test 2M1207’s mem- analysis places a strong lower limit on the linear expansion bership to the TWA, (4) use the moving cluster method to age of the TWA(>10.4 Myr; 95% confidence). Higher qual- estimate the distance to the 2M1207 system and other TWA ity radial velocities and trigonometric parallaxes are needed to members, (5) investigate the relation between the TWA and further constrain if, and how fast, TWA is expanding. the Lower Centaurus-Crux OB association, and (6) estimate A previously published expansion age estimate for TWA by the expansion age of the TWA. The results from this poster [10] (8.3 Myr), can be statistically ruled out. will appear in upcoming ApJ paper [5]. A kinematic analysis of B- and A-type stars in the Hippar- cos catalog within a 15 radius of the TWA center identified Analysis only one new TWA candidate: HIP 54477 (A1V). Further Members of a moving group, whose motions reflect either observations (radial velocity measurements, search for Li-rich parallel motion, expansion, or contraction, will have proper companions) should be undertaken to confirm or reject its TWA motions which appear to converge to a point on the sky. The membership. Other than the famous debris disk star HR 4796 positions and proper motions of candidate members of a mov- (TWA 11; A0V), and possibly HIP 54477, there is no other ing group can be used, in conjunction with some combination evidence for other high mass B- or A-type members of TWA. of available parallax and/or radial velocity data, to determine the space velocity of the group, as well as test for expan- Acknowledgments: EM is supported by a Clay Postdoctoral sion/contraction. Using the most recent TWA membership Fellowship from the Smithsonian Astrophysical Observatory list [6], along with astrometric data from the literature, and (SAO). new proper motions calculated by the author, I estimated the TWA convergent point using a 2 minimization grid technique References: (similar to that of [7,8]). With a convergent point solution, the [1] Chauvin, G., et al. 2004, A&A, 425, L29; [2] Chau- stellar proper motions can be used to estimate individual dis- vin, G., et al. 2005, A&A, 438, L25; [3] Webb, R. A., et al. tances to each TWA member. As a check, the moving cluster 1999, ApJ, 512, L63; [4] Gizis, J. E. 2002, ApJ, 575, 484; [5] distances can be compared to trigonometric parallax distances Mamajek, E. E. 2005, ApJ, in press (astro-ph/0507416); [6] (when available), and the predicted moving cluster radial ve- Zuckerman, B., & Song, I. 2004, ARA&A, 42, 685; [7] Jones, locities can be compared to measured values. D. H. P. 1971, MNRAS, 152, 231; [8] de Bruijne, J. H. J. 1999, MNRAS, 306, 381; [9] Baraffe, I., et al. 2003, A&A 402, 701; [10] Makarov, V. V., & Fabricius, C. 2001, A&A, 368, 866;