The Greater Taurus–Auriga Ecosystem. I. There is a Distributed Older Population

Abstract

The census of Taurus-Auriga has been assembled over seven decades and inherited the biases and incompleteness of the input studies. The unusual shape of its inferred initial mass function (IMF) and the existence of isolated diskbearing stars suggest that additional (likely disk-free) members remain to be discovered. We therefore have begun a global reassessment of the census of Taurus-Auriga that exploits new data and better definitions of youth and kinematic membership. As a first step, we reconsider the membership of all disk-free candidate members from the literature with spectral type F0, a < < 3 50 5 40 h m h m , and d  < <  14 34 . We combine data from the literature with Keck/HIRES and UH88/SNIFS spectra to test the membership of these candidates using the positions in the Hertzsprung-Russel diagram, proper motions, radial velocities, Hα, lithium, and surface gravity. We find 218 confirmed or likely Taurus members, 160 confirmed or likely interlopers, and only 18 that lack sufficient evidence to draw firm conclusions. A significant fraction of these stars (81/218 = 37%) are not included in the most recent canonical member lists. There are few additional members to the immediate vicinity of the molecular clouds, preserving the IMFs that have been deemed anomalous in past work. Many of the likely Taurus members are instead distributed broadly across the search area. When combined with the known disk hosts, our updated census reveals two regimes: a high-density population with a high disk fraction (indicative of youth) that broadly traces the molecular clouds, and a low-density population with low disk fraction (hence likely older) that most likely represents previous generations of star formation.

A Survey For Planetary-mass Brown Dwarfs in the Taurus and Perseus Star-forming Regions*

“…We also reject a few stars from the latter that do not appear to be members. Kraus et al (2017) Kraus et al (2017) compiled diagnostics of membership in the Taurus star-forming region for 396 candidate members that had been identified in previous surveys. They selected candidates that lacked evidence of disks in earlier studies 4 and that were within a large area extending well beyond the Taurus clouds that was defined by right ascensions (α) of 3 h 50 m -5 h 40 m and declinations (δ) of 14…”

“…Some candidates lacked sufficient data for definitive assessments of their membership. Kraus et al (2017) concluded that 218 of the 396 candidates were confirmed or likely members, 82 of which were absent from the compilation of members in Luhman et al (2017). Most of those 82 stars are older (10-30 Myr) and more widely distributed than the previously known members (∼ 1 Myr), so Kraus et al (2017) proposed that they represent an earlier generation of star formation that is related to the Taurus cloud complex.…”

“…A recent compilation of members was presented by Luhman et al (2017). Since that study, Kraus et al (2017) and Best et al (2017) have proposed additional members of Taurus. In this section, we examine those candidates to determine whether to add them to the list from Luhman et al (2017).…”

“…The Taurus and Perseus star-forming regions are two of the most appealing targets for a brown dwarf survey because of their proximity (d = 140 and 300 pc; Torres et al 2012;Schlafly et al 2014, references therein) and the relatively large sizes of their stellar populations (N∼400 and 800, Kenyon et al 2008;Bally et al 2008). Luhman et al (2016Luhman et al ( , 2017 have provided recent summaries of previous surveys for members of Taurus and the two richest clusters in Perseus, IC 348 and NGC 1333 (see also Kraus et al (2017) for Taurus). The census of each region contains several dozen objects with spectral types indicative of brown dwarfs (>M6), most of which were identified with optical and infrared (IR) photometry.…”

Orbiting a binary

“…Together with several other targets of the same survey, HD 284149 has been proposed to be part of the so-called Taurus-Ext association (Luhman et al 2017;Kraus et al 2017;Daemgen et al 2015), a group of stars with similar space position and kinematics of the Taurus star forming region but distinctly older ages. A dedicated age estimate was then performed for HD 284149 by Bonavita et al (2014), using several youth indicators, leading to an adopted age of 25 +25 −10 Myrs.…”