A Moving Cluster Distance to the Exoplanet 2M1207b in the TW Hydrae Association

“Using the constraints on luminosity and age (Chauvin et al. 2005), I interpolate masses from the non-gray evolutionary tracks of Burrows et al. (1997), the DUSTY tracks of Chabrier et al. (2000), and the COND tracks of Baraffe et al. (2003).”

“For all three models, the inferred upper mass limit of 2M1207 B (∼5-7MJup) is less than half of the deuterium-burning mass limit (∼13MJup; Burrows et al. 1997), and less than half of the maximum mass of Doppler velocity planets (∼15MJup; Marcy et al. 2005).”

“Using the constraints on luminosity and age (Chauvin et al. 2005), I interpolate masses from the non-gray evolutionary tracks of Burrows et al. (1997), the DUSTY tracks of Chabrier et al. (2000), and the COND tracks of Baraffe et al. (2003).”

“With the previous distance estimates (∼70 pc), Chauvin et al. (2004) estimates mass of 25MJup and 5MJup for A and B, respectively.”

“None have been directly imaged, however two have had their light detected through transit studies with the Spitzer Space Telescope (HD 209458b and TRes-1b; Charbonneau et al. 2005; Deming et al. 2005).”

“None have been directly imaged, however two have had their light detected through transit studies with the Spitzer Space Telescope (HD 209458b and TRes-1b; Charbonneau et al. 2005; Deming et al. 2005).”

“I add to TWA numbers 1 through 25 (Zuckerman & Song 2004) the three, new, low-mass candidate members 2M1207 and 2MASSW J1139511-315921 from Gizis (2002), and SSSPM J1102-3431 from Scholz et al. (2005).”

“I calculate luminosities using these absolute magnitudes, and the bolometric correction estimates of Golimowski et al. (2004).”

“Gizis (2002) discovered 2M1207 in a spectroscopic survey of red 2MASS sources, and claimed that the object was a ∼10 Myr-old, ∼25MJup member of the nearby (∼55 pc) TW Hya association (TWA; Webb et al. 1999b).”

“While astrophysically interesting in its own right as a representative of the new class of young, accreting brown dwarfs (e.g. Muzerolle et al. 2005; Mohanty et al. 2005), it appears that 2M1207 may become most famous for being the host “Sun” for the first imaged extrasolar planet – if indeed…”

“Considering the variations between published evolutionary tracks, especially for stars which are young and low-mass (Hillenbrand & White 2004), the cluster parallax distance should be considered an improved estimate.”

“There have been suggestions that some TWA stars may actually be background members of the ∼16-Myr-old Lower Centaurus-Crux (LCC) OB subgroup at d ≃ 110 pc (e.g. Mamajek & Feigelson 2001; Mamajek et al. 2002).”

“Such cloudlets could have been accelerated away from the LCC O-star population through the rocket effect (Oort & Spitzer 1955), and compressed to form stars due to radiation-driven implosion (Bertoldi & McKee 1990).”

“I add to TWA numbers 1 through 25 (Zuckerman & Song 2004) the three, new, low-mass candidate members 2M1207 and 2MASSW J1139511-315921 from Gizis (2002), and SSSPM J1102-3431 from Scholz et al. (2005).”

“The only published distance estimates to 2M1207 are ∼70 pc (Chauvin et al. 2004) and 70± 20 pc (Chauvin et al. 2005).”

“While astrophysically interesting in its own right as a representative of the new class of young, accreting brown dwarfs (e.g. Muzerolle et al. 2005; Mohanty et al. 2005), it appears that 2M1207 may become most famous for being the host “Sun” for the first imaged extrasolar planet – if indeed…”

“Such cloudlets could have been accelerated away from the LCC O-star population through the rocket effect (Oort & Spitzer 1955), and compressed to form stars due to radiation-driven implosion (Bertoldi & McKee 1990).”

“The radial velocity of HIP 54477 is not well constrained (vrad=+16.2± 10 km s −1; Barbier-Brossat & Figon 2000), but consistent with that for a TWA member at its position (+12.2± 1.6 km s−1).”

“These errors are very consistent with the SuperCOSMOS positional errors quoted by Hambly et al. (2001).”

“The small, young stellar groups associated with η Cha, ǫ Cha, and β Pic show many of these same symptoms (Mamajek et al. 1999, 2000; Ortega et al. 2002; Jilinski et al. 2005), although the η and ǫ Cha clusters appear to be more strongly bound than the TWA and β Pic groups.”

“Further observations of its radial velocity (Mohanty et al. 2003) and proper motion (Scholz et al. 2005) are roughly consistent with TWA membership.”

“If one calculates cluster parallax distances to “TWA” objects using the space motion vector of LCC (Madsen et al. 2002), the mean distances in column 7 of Table 1 change by less than ±5% (rms).”

“2 catalog (Urban et al. 1998) at http://ad.usno.navy.mil/ac/ squares fit to the 4 SuperCOSMOS points, and found their scatter consistent with positional errors of σα∗ = 143 mas and σδ = 196 mas.”

“The small, young stellar groups associated with η Cha, ǫ Cha, and β Pic show many of these same symptoms (Mamajek et al. 1999, 2000; Ortega et al. 2002; Jilinski et al. 2005), although the η and ǫ Cha clusters appear to be more strongly bound than the TWA and β Pic groups.”

“…globule formation scenario for TWA might explain a few observational quirks of the group, namely its location (∼70 pc away from the nearby LCC OB subgroup), age (∼7 Myr younger than LCC), space motion vector (directed ∼5 km s−1 away from the LCC; Mamajek et al. 2000), and low stellar density.”

“Part of the offset may be due to gravitational redshift, which for the typical ∼10Myr-old TWA member with mass ∼0.5M⊙ should be of order ∼0.4 km s−1(Greenstein & Trimble 1967, using radii from the D’Antona & Mazzitelli 1997 tracks), compared to ∼0.6 km s−1 for the Sun.”

“The analysis of MF01 included tens of X-ray-selected stars in their analysis which have been since shown to not be pre-MS stars (SZB03, Torres et al. 2003).”

“ANALYSIS Although a trigonometric parallax is not yet available for 2M1207, one can exploit the star’s putative membership in the TW Hya association to derive a distance using the cluster parallax (or “moving-cluster”) method (e.g. Atanasijevic 1971; de Bruijne 1999a).”

“If one calculates cluster parallax distances to “TWA” objects using the space motion vector of LCC (Madsen et al. 2002), the mean distances in column 7 of Table 1 change by less than ±5% (rms).”

“Cloudlets analogous to those on the periphery of Vel OB2 (Kim et al. 2005) and Ori OB1 (Lee et al. 2005) may be the evolutionary predecessors of small, unbound, ∼10 Myr-old associations like TWA.”

“Further observations of its radial velocity (Mohanty et al. 2003) and proper motion (Scholz et al. 2005) are roughly consistent with TWA membership.”

“Blaauw (1956, 1964) suggested that linear expansion or contraction may be detectable if deviations are present between the observed spectroscopic radial velocities, and those predicted from the moving group method for parallel motion.”

“For discussions on trying to detect the linear expansions of unbound associations, see Blaauw (1956, 1964), Bertiau (1958), Jones (1971), Brown et al. (1997), Dravins et al. (1999), and Madsen et al. (2002).”

“For discussions on trying to detect the linear expansions of unbound associations, see Blaauw (1956, 1964), Bertiau (1958), Jones (1971), Brown et al. (1997), Dravins et al. (1999), and Madsen et al. (2002).”

“An unexplained radial velocity offset of 0.4 km s−1 appears to be present among low-mass Hyades members (Gunn et al. 1988), even after accounting for gravitational redshift.”

“I approximately follow the convergent point grid technique of Jones (1971).”

“Mamajek & Feigelson (2001) and Lawson & Crause (2005) have suggested that TWA 14-19 are probably members of the more distant (∼120 pc; de Zeeuw et al. 1999) and older (∼16 Myr; Mamajek et al. 2002) Lower Centaurus Crux OB association (LCC).”

“This fact, combined with the finding that most of the stars in the MF01 convergent point analysis are not pre-MS (Song et al. 2002), suggests that their convergent point and dynamical age for the TWA (8.3 Myr) are not valid.”

“Small-scale star-formation in cometary globules on the edge of OB associations has strong observational support (Reipurth 1983; Ogura & Sugitani 1998), and there is strong evidence for triggering by the massive stellar population (e.g. Kim et al. 2005; Lee et al. 2005).”

“After excluding TWA 12, 17, 18, 19, 22, and 24 as TWA members, I characterize the final TWA membership with probability plots (so as to be immune to the effects of outliers; Lutz & Upgren 1980): dTWA = 49± 3 (s.e.m.)± 12 (1σ) pc, αTWA = 174 ◦.8± 12◦.3 (1σ), and δTWA = –37 ◦.1± 7◦.5 (1σ).”

“Mamajek & Feigelson (2001) and Lawson & Crause (2005) have suggested that TWA 14-19 are probably members of the more distant (∼120 pc; de Zeeuw et al. 1999) and older (∼16 Myr; Mamajek et al. 2002) Lower Centaurus Crux OB association (LCC).”

“This fact, combined with the finding that most of the stars in the MF01 convergent point analysis are not pre-MS (Song et al. 2002), suggests that their convergent point and dynamical age for the TWA (8.3 Myr) are not valid.”

“The small, young stellar groups associated with η Cha, ǫ Cha, and β Pic show many of these same symptoms (Mamajek et al. 1999, 2000; Ortega et al. 2002; Jilinski et al. 2005), although the η and ǫ Cha clusters appear to be more strongly bound than the TWA and β Pic groups.”

“ANALYSIS Although a trigonometric parallax is not yet available for 2M1207, one can exploit the star’s putative membership in the TW Hya association to derive a distance using the cluster parallax (or “moving-cluster”) method (e.g. Atanasijevic 1971; de Bruijne 1999a).”

“Cloudlets analogous to those on the periphery of Vel OB2 (Kim et al. 2005) and Ori OB1 (Lee et al. 2005) may be the evolutionary predecessors of small, unbound, ∼10 Myr-old associations like TWA.”

“I corrected the DENIS position for the 2MASS-DENIS offset found by Cabrera-Lavers & Garzón (2003), since the 2MASS positional errors are much smaller than DENIS’s (2MASS is tied to the ICRS via the Tycho-2 catalog to an accuracy of ∼80 mas; Cutri et al. 2003).”

“In addition, based on the study of Mamajek (2005) and parallax data from Weinberger et al. (2013), stars TWA 14, TWA 15A, TWA 15B, TWA 17, TWA 18, TWA 19A, TWA 19B, and TWA 24 are likely members of the Lower Centaurus-Crux subgroup of the Scorpius-Centaurus OB association and thus may be subject to…”

“The moving cluster distances and probabilities were calculated following Mamajek (2005) using the de Bruijne et al. (2001) convergent point solution with Hipparcos, Tycho-2, or UCAC2 proper motions (in order of preference).”

“Another limitation is that some young groups appear to have little net expansion (Mamajek (2005); Makarov (2007)), so that a time when the members were closest together is illdefined.”

“TWA 19 AB is a young star that was defined as a candidate member of TWA by Mamajek (2005), but later listed as a likely contaminant from LCC by Gagné et al. (2017a) based on a preliminary version of BANYAN Σ.”

“For our kinematic parallax calculations we followed Mamajek (2005) and used updated space motions for US, UCL and LCC tabulated in Chen et al. (2011)8.”