Photoevaporation of Two Proplyds in the Star Cluster Collinder 69 Discovered with Spitzer MIPS

In this work we present previously undiscovered tails at 24 μm, belonging to proplyd candidates in the young star cluster Collinder 69. This is the first time such tails have been detected from low-mass members of this particular star cluster.

Motivated by the possibility of finding protoplanetary disks (proplyds), such as those present in the Orion Nebula (e.g., O'dell & Wen 1994), we used the Spitzer Heritage Archive⁵ to search for proplyds in nearby star clusters. It should not be unusual to find cometary tails coming from low-mass stars in young clusters hosting massive O-Type stars, but the presence of such tails is a short-lived phenomenon, due to the rapid mass loss of the disk (e.g., Koenig et al. 2008).

The cluster Collinder 69 is located about 400 parsec away and is about 6 million years old (Kharchenko et al. 2016). The most massive member is λ Orionis A+B or Meissa, classified to have a spectral type of O8IIIf+B0.5V (Conti & Alschuler 1971). The region hosts many low-mass and brown dwarf members studied by Barrado y Navascués et al. (2007) with the Spitzer Space Telescope as part of a GTO program. Two of them are pictured in a MIPS 24 μm map in Figure 1 (next to the blue arrows). The brightest emission in Figure 1 might be an additional tail belonging to HD 36861C, but we are not able to explain the complex structure of this object. The bright point source above this emission is λ Orionis. The two tails belong to LOri043 and LOri065. LOri043 has a spectral type of M4.0 (Bayo et al. 2011) and it is a spectroscopic binary (Maxted et al. 2008).

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We obtained Gaia DR2 parallax measurements of both members (Gaia Collaboration et al. 2018), which are consistent with a distance of 400 parsec. We do not compare the parallax of these two members with the Gaia parallax of λ Orionis, because it might be affected by a large systematic error (Drimmel et al. 2019).

LOri043: In Hernández et al. (2010) the disk is classified as an evolved disk. The clearing of the inner disk in LOri043 is explained by the tidal forces of the close binary by Bayo et al. (2012). The tail, measured from LOri043 to the tip of the far end, is about 14.7 × 10³ au long at a distance of 411 parsec. LOri043 is about 0.188 parsec distant from λ Orionis.

LOri065: We estimate a spectral type of M3 (by color V–J, V–H) for this member with Pecaut & Mamajek (2013). This is roughly consistent with the photometric spectral type by Hernández et al. (2010) who classified the disk as transitional. The tail has a length of about 18.4 × 10³ au and LOri065 is 0.304 parsec distant from λ Orionis.

Both members are at a distance from the O-star typical of stars which display cometary tails, but the tails are smaller than previous discovered tails in 24 μm (compare with e.g., Koenig et al. 2008).

Dolan & Mathieu (2002) did suggest that before a supernova occurred in Collinder 69, the pre-main sequence population close to OB-stars lost their disks to the FUV radiation. This work supports a continued photoevaporation after the supernova.

We suspect that more cluster members might show disk reduction by λ Orionis in Hα or N ii images with the Hubble Space Telescope. Future observations might confirm this assumption. We encourage further research of these objects.

We would like to thank the researchers of the Disk Detective, the Backyard Worlds: Planet 9 and the Milky Way Project collaborations, as well as the Zooniverse web development team for giving us a chance to participate in state-of-the-art research. We also thank Marc Kuchner, Chris Lintott and Sarah Kendrew for giving us advice, which was significant to improve this work.

Facilities: IRSA - , Spitzer. -