BANYAN. IX. The Initial Mass Function and Planetary-mass Object Space Density of the TW HYA Association

Abstract

A determination of the initial mass function (IMF) of the current, incomplete census of the 10 Myrold TW Hya association (TWA) is presented. This census is built from a literature compilation supplemented with new spectra and 17 new radial velocities from on-going membership surveys, as well as a re-analysis of Hipparcos data that confirmed HR 4334 (A2 Vn) as a member. Though the dominant uncertainty in the IMF remains census incompleteness, a detailed statistical treatment is carried out to make the IMF determination independent of binning, while accounting for small number statistics. The currently known high-likelihood members are fitted by a log-normal distribution with a central mass of 0.21−0.06 M and a characteristic width of 0.8 +0.2 −0.1 dex in the 12 M Jup -2 M range, whereas a Salpeter power law with α = 2.2 +1.1 −0.5 best describes the IMF slope in the 0.1-2 M range. This characteristic width is higher than other young associations, which may be due to incompleteness in the current census of low-mass TWA stars. A tentative overpopulation of isolated planetary-mass members similar to 2MASS J11472421-2040204 and 2MASS J11193254-1137466 is identified: this indicates that there might be as many as 10 +13 −5 similar members of TWA with hot-start modeldependent masses estimated at ∼ 5-7 M Jup , most of which would be too faint to be detected in 2MASS . Our new radial velocity measurements corroborate the membership of 2MASS J11472421-2040204, and secure TWA 28 (M8.5 γ), TWA 29 (M9.5 γ) and TWA 33 (M4.5 e) as members. The discovery of 2MASS J09553336-0208403, a young L7-type interloper unrelated to TWA, is also presented.

The Young L Dwarf 2MASS J11193254−1137466 Is a Planetary-mass Binary

“…A higher-resolution J-band spectrum (K16) shows weakened K I absorption lines having equivalent widths consistent with vl-g classification for L7 dwarfs (Allers & Liu 2013;Gagné et al 2017). We therefore adopt L7 vl-g as the unresolved spectral type.…”

“…Faherty et al (2016) analyzed moving group membership using four different tools and found >90% probabilities for TWA in three cases; LACEwING (Riedel et al 2017) found a contrasting probability of 16%. Gagné et al (2017) used the full BANYAN II analysis (including photometry) to calculate a 97% probability of membership. Liu et al (2016) found that objects lacking parallaxes which have full BANYAN II probabilities 80% tend to have memberships confirmed by subsequent parallax measurements for the well-established moving groups, including TWA.…”

The TWA 3 Young Triple System: Orbits, Disks, Evolution

“…TWA 3 is a visual binary with a 2015 position angle of 207°and a projected separation of 1 55 (Tokovinin et al 2015), corresponding to ∼50 au at a distance of 34±4 pc (Mamajek 2005). Although different distance estimates are available in the literature, we have chosen to adopt Mamajek's estimate given the consistency with Gagné et al (2017). The TW Hya association members demonstrate a wide range of distances, from ∼30 to 50pc, consistent with the elongated shape of the region as discussed in Kellogg et al (2016).…”

Nearby Young, Active, Late-type Dwarfs in Gaia's First Data Release

“…This intense coronal emission is the external manifestation of the strong internal magnetic dynamos and, hence, strong surface magnetic fields that are characteristic of young, late-type stars. If the mass functions of NYMGs resemble those of the field, however, then the presently known membership of these nearby groups is significantly lacking in stars of spectral type later than K (e.g., Gagné et al 2017). This deficiency is due, in part, to a lack of sensitive X-ray measurements over sufficiently large regions of sky, such as would be necessary to detect nearby young stars with spectral types later than early M at distances beyond a few tens of pc (Rodriguez et al 2013).…”