A new silhouette edge-on protoplanetary disk in M20 (Trifid Nebula)?
ATel #16707; Wei-Hao Wang (Institute of Astronomy and Astrophysics, Academia Sinica)
on 13 Jul 2024; 16:34 UT
Credential Certification: Wei-Hao Wang (whwang@asiaa.sinica.edu.tw)
Subjects: Optical, Young Stellar Object
An elongated dark nebula was spotted in an optical image of M20 made with a 0.5m telescope, LATTE (Lulin-ASIAA Telescope for Transients and Education), at Lulin observatory in Taiwan. The imaging observations contain a total of 3 hr of integration with a broad-band optical white-light filter, made on July 6, 10, and 11, 2024. The center of the dark nebula has a coordinate of RA = 18:02:50.68, Dec = -23:02:38.3 (J2000.0). This dark nebula has been seen in numerous images of M20. What might be new here is that thanks to the good seeing (~ 1.2 arcsec), a faint point-like source is detected near the center of the dark nebula, but slightly offset to the northern edge. This morphology closely resembles edge-on silhouette disks with a central protostar, like those famous ones in the Orion Nebula detected by HST. A multi-band montage of this dark nebula can be found in the link http://group.asiaa.sinica.edu.tw/whwang/misc/M20_Silhouette_disk.jpg . This montage is made from public PanSTARRS grizy images and our white-light broad-band optical image. In the PanSTARRS i,z,y images, a second point-light source appears in the southern side of the dark nebula, and the dark nebula becomes less apparent toward longer wavelength. Within a radius of 10 arcsec, several tens of near-IR stars can be found from the VizieR database, but only one IRAC and MIPS 24 micron source is cataloged, right at the position reported above (within an arcsec). The source flux is 13.05 mJy at 24 micron, and has very red colors from near-IR to mid-IR (J-Ks = 2.83, [3.6]-[5.8] = 0.78, [5.8]-[24] = 5.10), indicating an YSO. The dark nebula has a semi-major axis of 2.6 arcsec. This angular size corresponds to a linear size of 3000 AU at the distance of M20 (1.26 kpc). If this were an edge on protoplanetary disk in M20, this size would make it one of the biggest disks. Although this report favors the disk scenario, the optical multi-band morphology does not rule out the possibility of an elongated dark nebula in the foreground of two sources.
multi-band montage of the silhouette disk
