The Local Environment of the Fu-Orionis-Like Objects Ar 6a and 6b

Abstract

We present new 12 CO J = 3-2 and HCN J = 3-2 molecular line maps of the region surrounding the young star AR 6 using the 15 m James Clerk Maxwell Telescope (JCMT). AR 6 was previously found to be a double source with both components exhibiting several characteristics of FU Orionis (FUor) eruptive variable stars. The aims of this investigation were to determine if the AR 6 sources are associated with molecular outflows and if a significant reservoir of natal molecular gas and dust exists around the stars. These observations form a part of a large-scale study of the outflow and circumstellar environment characteristics of FUors and FUor-like objects to place constrains on the age and evolutionary state of sources exhibiting FUor-like tendencies. Our data indicate that AR 6, like FUor itself, does not possess a CO outflow and likewise, does not show evidence for large amounts of molecular gas in its circumstellar environment. In fact, AR 6 seems to lie in a local minimum of HCN emission. This is also found in 850 μm dust emission seen in JCMT archival data. We conclude that from the near-infrared to the sub-millimeter, AR 6 is similar to FUor in several respects. We interpret the lack of significant dust and molecular gas in the circumstellar environment of AR 6, together with the large near-infrared (NIR) thermal excess, as evidence that the sources have exhausted their natal envelopes, that they have at least small hot circumstellar disks, and that they are more evolved than Class I protostars. This, in itself, suggests that, since FUor eruptions have also been observed in stars with large dust mass envelopes (e.g., V346 Nor) and with CO outflows (e.g., L1551 IRS5), FUor events probably occur at many different stages in the early, formative phase of a star's life, and lends support to the idea that FUor outbursts are repetitive like their shorter-lived relatives occurring in EXor eruptive variables. Finally, we study the stellar environment around AR 6 using Two Micron All Sky Survey, Spitzer Infrared Array Camera, and Chandra ACIS images and show that, being part of the "Spokes" young stellar cluster, AR 6 is unlike many FUors which are typically located in more sparsely populated regions.

V1647 ORIONIS: KECK/NIRSPEC 2 μm ECHELLE OBSERVATIONS

“…However, FU Ori itself shows no evidence of having a molecular outflow either (Bally & Lada 1983;Evans et al 1994). This is also true for the FUor-like close double AR 6A and 6B in NGC 2264 (Aspin & Reipurth 2003;Moriarty-Schieven, Aspin, & Davis 2008).…”

“…The 2003-2006 outburst of the partly embedded, pre-main sequence star V1647 Orionis has provided a plethora of information on the nature of eruptive variables and the physical changes undergone by a star during such an event. V1647 Ori, the illuminating source of McNeil's Nebula (McNeil 2004) located near M 78 in the Lynds 1630 dark cloud, brightened by over 5 magnitudes in the optical (Briceño et al 2004;Reipurth & Aspin 2004a) and 3 magnitudes in the near-IR (Reipurth & Aspin 2004a;Ojha et al 2004) in the space of a few weeks. The eruption also produced an enhanced H I spectrum highlighted by a highvelocity (∼600 km s −1 ) blue-shifted Hα absorption component creating a P Cygni profile (Reipurth & Aspin 2004a;Walter et al 2004;Fedele et al 2007).…”

Spectral Energy Distribution Analysis of Class I and Class Ii Fu Orionis Stars

“…The source AR 6A has, in addition, a third companion AR 6C discovered by , with a separation of ∼ 700 AU (0.85 ′′ ). Moriarty-Schieven et al (2008) detected a molecular flow associated with AR 6A/6B. Figures 13 and 14 show the SEDs of AR 6A and AR 6B, respectively.…”

Episodic Accretion in Young Stars

“…Such reservoirs are sufficient to replenish circumstellar disks after episodes of rapid accretion. Molecular outflows were generally detected, except for the most evolved FUors (Evans et al, 1994;Moriarty-Schieven et al, 2008), with mass loss rates between a few times 10 −8 and 10 −6 M ⊙ yr −1 , without correcting for optical depth; mass loss rates likely peak at 10 −5 M ⊙ yr −1 . Dense gas traced by HCO + was also detected (McMuldroch et al, 1993(McMuldroch et al, , 1995Evans et al, 1994).…”

“…During the outburst, the near-infrared color indices of V1647 Ori shifted to bluer colors along the reddening vector, mainly due to intrinsic brightening and partly to decreasing extinction (Reipurth and Aspin, 2004a;McGehee et al, 2004;Acosta-Pulido et al, 2007). The mass accretion rate during the outburst was estimated at a few 10 −6 to 10 −5 M ⊙ yr −1 (Muzerolle et al, 2005;Aspin, 2011b;Mosoni et al, 2013). CO bandhead emission was detected along with strong H and He emission lines with P Cyg profiles, indicating a strong wind with velocities up to 600 km s −1 , together with ice absorption from H 2 O and CO and in Na I D (Reipurth and Aspin, 2004a;Vacca et al, 2004;Walter et al, 2004).…”