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New Low-Mass Members of Chamaeleon I and $\Epsilon $

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New Low-Mass Members of Chamaeleon I and $\Epsilon $ Astronomy & Astrophysics manuscript no. chamaeleon ©ESO 2021 February 11, 2021 New low-mass members of Chamaeleon I and Cha K. Kubiak1 , K. Mužic´1, I. Sousa1, V. Almendros-Abad1, R. Köhler, and A. Scholz2 1 CENTRA, Faculdade de Ciências, Universidade de Lisboa, Ed. C8, Campo Grande, P-1749-016 Lisboa, Portugal e-mail: [email protected] 2 SUPA, School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews KY169SS, UK Received; accepted ABSTRACT Aims. The goal of this paper is to increase the membership list of the Chamaeleon star forming region and the Cha moving group, in particular for low-mass stars and substellar objects. We extended the search region significantly beyond the dark clouds. Methods. Our sample has been selected based on proper motions and colours obtained from Gaia and 2MASS. We present and discuss the optical spectroscopic follow-up of 18 low-mass stellar objects in Cha I and Cha. We characterize the properties of objects by deriving their physical parameters, both from spectroscopy and photometry. Results. We add three more low-mass members to the list of Cha I, and increase the census of known Cha members by more than 40%, confirming spectroscopically 13 new members and relying on X-ray emission as youth indicator for 2 more. In most cases the best-fitting spectral template is from objects in the TW Hya association, indicating that Cha has a similar age. The first estimate of the slope of the initial mass function in Cha down to the sub-stellar regime is consistent with that of other young clusters. We estimate our IMF to be complete down to ≈ 0:03 M . The IMF can be represented by two power laws: for M < 0.5 M α = 0:42±0:11 and for M > 0.5 M α = 1:44 ± 0:12. Conclusions. We find similarities between Cha and the southernmost part of Lower Centaurus Crux (LCC A0), both lying at similar distances and sharing the same proper motions. This suggests that Cha and LCC A0 may have been born during the same star formation event Key words. stars:low-mass – stars:pre-main sequence – brown dwarfs – open clusters and associations individual: Cha young moving group 1. Introduction One of the primary objectives of the survey presented in this paper is the extension of previous censuses of low mass stars and The Chamaeleon molecular cloud complex is one of the nearest substellar objects in Cha I, Cha II and Cha. To achieve this, sites of star formation and dominates the dust extinction maps we use data from Gaia DR2 (Gaia Collaboration et al. 2018). of the region (Luhman et al. 2008). The complex and its sur- Gaia data has already been used in combination with near- and roundings are abundant with pre-main-sequence stars of vari- mid-infrared surveys to search for new low-mass stars and brown ous ages. The large population of young stellar objects (YSOs) dwarfs in the solar neighbourhood (e.g. Esplin & Luhman 2019, is associated with the Cha I (Luhman 2004a, 2007), Cha II Cánovas et al. 2019). (Spezzi et al. 2008) and Cha III clouds (Fig. 1). Because the We confirmed the young age of candidate low-mass objects Chamaeleon cloud complex is nearby (Cha I d ≈ 190 pc, Cha II in Cha I, Cha II and Cha by performing optical spectroscopy. d ≈ 210 pc, Dzib et al. 2018, Zucker et al. 2020) and well- Analyzing low-resolution optical spectra is not only a powerful isolated from other young stellar populations, it has been a popu- tool to confirm youth of low mass stellar objects, but also use- lar target for studies of low-mass star formation. Cha I harbours ful for investigating their physical properties by comparing with a rich population of ∼250 known YSOs with ages of ∼2 Myr, spectra of standard stars with well-defined properties and tem- grouped in two sub-clusters (Luhman 2007; Luhman & Muench plates. For nearby star-forming regions and associations, such 2008; Mužic´ et al. 2011; Sacco et al. 2017). The Chamaeleon II observations can be performed with modest size telescopes. We dark cloud, at a distance of 210 pc (Zucker et al. 2020), has performed spectroscopic follow-up using the FLOYDS instru- arXiv:2102.05589v1 [astro-ph.SR] 10 Feb 2021 modest star formation activity with a population of 51 confirmed ment installed at the 2-m Faulkes South telescope, which is part members aged 2 – 4 Myr (Spezzi et al. 2008). No young stellar of the Las Cumbres Observatory Global Network (LCOGT). objects are known in the dark cloud Cha III. In this paper, we first describe the photometric datasets used Between the clouds (in projection), we find a more nearby for the analysis (Sect. 2) and introduce the known members (d ∼ 100pc) and somewhat older (5 – 10 Myr) group of YSOs, (Sect. 2.4), followed by the selection of candidates from density known as the Chamaeleontis Association ( Cha, Feigel- maps, proper motions, and colour-magnitude diagrams (Sect. 3). son et al. 2003; Luhman et al. 2008; Dzib et al. 2018). Once In Sect. 4, we present the analysis of the optical spectra. We thought to be the result of ejection from the Chamaeleon com- then characterize the stellar parameters, spatial distribution, and plex (Sterzik & Durisen 1995), many of these isolated, older pre- spectral energy distributions of the confirmed members and sum- main-sequence stars are now believed to form a distinct nearby marize their properties. In the Appendix, we present the spectral young co-moving group of Cha (Murphy et al. 2013). template fits to FLOYDS spectra obtained in this study. Article number, page 1 of 18 A&A proofs: manuscript no. chamaeleon 2. Datasets Fig. 1, noting that some of these objects may as well be members of Cha. As we will discuss in more details later, they share the The region analysed in this work is located in the coordinate ◦ ◦ ◦ ◦ same proper motion and parallax space, as well as the location range 140 < α < 210 , and −84 < δ < −73 . This large in the barycentric Galactic Cartesian coordinate frame. For this area, as shown in Fig. 1, encompasses the Chamaeleon clouds, reason also, we do not use Goldman et al. candidates for the new and the Cha moving group. The catalogues described in this member selection, we rather rely on Cha members, most of section were cross-matched using TOPCAT (Taylor 2005), with 00 which have additionally been confirmed by spectroscopy. a matching radius of 1 . For our analysis, we take into account the following lists of members: 2.1. Gaia DR2 1. Cha I: A list of 256 spectroscopically confirmed members The Gaia (Gaia Collaboration et al. 2016) Data Release 2 has been compiled using the works from Luhman (2007); (DR2; Gaia Collaboration et al. 2018) catalog of the region was Luhman & Muench (2008); Mužic´ et al. (2011); Sacco et al. queried through VizieR (Ochsenbein et al. 2000). The catalogue (2017). was restricted to parallaxes between 3.3 and 20 mas (equiva- 2. Cha II: A list of 51 spectroscopically confirmed members lent to distances 50-300 pc), in order to reduce its size, but at published in Spezzi et al. (2008). the same time include the distances of interest, with a gener- 3. Cha: A comprehensive list of 35 members has been com- ous margin. Furthermore, we excluded the objects with only piled by Murphy et al. (2013), using both kinematic and one good observation, as reported by the catalogue keyword spectroscopic information. astrometric_n_good_obs_al. The previously confirmed members, along with the LCC (A0) candidates are shown in Fig. 1. 2.2. 2MASS The near-infrared photometry was obtained from the Two Mi- 3. Candidate selection cron All Sky Survey (2MASS; Skrutskie et al. 2006), queried through the Infrared Science Archive (IRSA) interface1. We kept 3.1. On-sky density maps only the photometric points with the quality flag (ph_qual) flag To examine the large region shown in Fig 1, we first plot stellar values of A, B, or C for any of the three 2MASS bands (J, H, surface density maps limiting each map with a range of distances and K ). S obtained from Gaia. The kernel density estimations (KDE) of the positions in the 2MASS J−band, were plotted for the dis- 2.3. AllWISE tance range of 50 pc to 300 pc, with steps of 10 pc. The KDEs were obtained using a Gaussian kernel with a bandwidths of 250, The mid-infared photometry (bands W1, W2, W3, and W4, cen- 250 and 310, for 3-dimensional (RA, Dec, parallax) for regions A tred at 3:6µm, 4:6µm, 11.6µm, and 22:1µm, was obtained from to C, respectively2. This allows us to visually identify stellar sur- the AllWISE project, making use of data from the Wide-field In- face overdensities associated with the groups of interest in which frared Survey Explorer cryogenic survey (WISE; Wright et al. we aim to select new candidates. After the visual inspection,we 2010). We have filtered out the photometry for the bands with decide to focus on the following cubes where we investigated the signal-to-noise ratio below 5 (keywords w1snr, w2snr, w3snr most prominent stellar overdensities associated with regions we and w4snr). Furthermore, the following flag values have been decided to focus on the most prominent overdensities associated accepted for W1 and W2 photometry: cc_ f lags = 0, ext_ f lg = with the regions: 0 - 3, ph_qual = A or B, and w1 f lg = w2 f lg = 0. 1. Region A (associated with the Cha I cloud): ◦ ◦ 2.4.
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