Optical Spectroscopy of the Eruptive Variable ASASSN-15qi

ATel #8331; Lynne A. Hillenbrand (Caltech), Bo Reipurth, M. S. Connelley (IfA, Univ. Hawaii)
on 24 Nov 2015; 20:06 UT
Credential Certification: Bo Reipurth (reipurth@ifa.hawaii.edu)

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A high-resolution (R~34,000) spectrum of ASASSN-15qi covering ~4800-9200 A was obtained in a 1200 sec exposure with HIRES at the Keck I telescope on 2015 October 27.27 (UT).

The HIRES spectrum shows P-Cygni type profiles in the upper Paschen, H-alpha, and H-beta lines of hydrogen (v_terminal = -1100 km/s), OI 8446 A, and the CaII infrared triplet lines (v_terminal = -575 km/s). The strong outflow is also evident in the blueshifted absorption presented by NaD (terminal velocity -600 km/s), KI, and various FeII, MgI, and other lines. Notably, the alkali lines show multiple distinct absorption components. No forbidden emission is present. Besides the P-Cygni lines, the only other emission is from weak Fe I lines of multiplet 60. Interstellar DIB and NaD absorption is also evident.

There are numerous absorption lines present in the optical spectrum which do not appear to be from the outflow. The absorption spectrum is a decent match to a significantly broadened early F giant or supergiant type star. Among the FUor class, it is most similar to V1057 Cyg, though with even broader lines.

An image through the HIRES slit viewer and an R filter reveals nebulosity to the southeast of the star.

ASASSN-15qi is located near a small cloud of low extinction identified as TGU 676 (Dobashi et al. 2005, PASJ 57, SP1) also known as Dobashi 3359 (Dobashi 2011, PASJ 63, SP1). This is a few degrees southwest of the LDN 1218 cloud complex. Two HII regions are located within about an arcminute of ASASSN15qi, one is G108.3-01.0 and the other is S2-148 for which Blitz, Fich, & Stark (1982, ApJ, 49, 183) measure v(LSR) = -53.1 +/- 1.3 km/sec. The heliocentric radial velocity measured from the HIRES spectrum is -58.1 +/- 9.3 km/s which corresponds to a kinematic or dynamic v(LSR) = -47 or -50 km/s, consistent with the velocity of the nearby HII region within the large errors. Caplan et al. (2000, MNRAS, 311, 317) calculate a distance of 5.82 kpc for S2-148 using the galactic rotation curve by Brand & Blitz (1993, A&A, 275, 67). However, Choi et al. (2014, ApJ 790:A99) has determined distances to 25 star forming regions in the Perseus arm, and the nearest to A15qi is G108.47-2.81, which has a LSR velocity of -54 km/sec and a distance determined at 3.24+-0.1 kpc. Choi et al argue that the Perseus arm is considerably closer than assumed earlier. Whether or not ASASSN-15qi is associated with any of the HII regions in the area, its radial velocity indicates that it is located in the Perseus arm (Dame et al. 2001, ApJ, 547, 792). It appears that the currently best distance estimate for ASASSN-15qi is around 3.24 kpc.

Assuming this distance and pre-outburst photometry of r=16.7 and i=15.6 from iPHAS, an Av of about 5 mag is derived, along with a luminosity of ~125 Lsun. This would make the progenitor source roughly 2.5-3.0 Msun depending on whether it was somewhat young/evolved, or closer to the main sequence. The outburst amplitude was approximately 3 mag.

Despite the considerable amount of data now available on ASASSN-15qi, its nature remains unclear. While it is close in projection on the sky to a region of star formation and has a velocity consistent with HII regions in the Perseus arm, the absence of lithium in the HIRES spectrum is noteworthy (although a higher S/N spectrum is required to fully exclude that lithium is present). The lack of any infrared excess in this or in other stars in the immediate vicinity of ASASSN-15qi should also be taken into account. While the present data do not indicate that ASASSN-15qi is young, if this should be the case, then the source does not easily fit among either the FUors or the EXors young star outburst classes as they are currently understood.

The optical image and spectra discussed above can be seen at the URL http://www.astro.caltech.edu/~lah/asassn15qi/

Spectroscopy