Repeating Short-Rise-Time Outburst Events in Young Stellar Object iPTF 15afq

ATel #13321; Lynne A. Hillenbrand (California Institute of Technology)
on 1 Dec 2019; 01:57 UT
Credential Certification: Lynne Hillenbrand (lah@astro.caltech.edu)

Subjects: Optical, Young Stellar Object

As described in ATel #7428, a Class-I type young stellar object experienced a 2.5 mag brightening event, designated iPTF 15afq, taking about 50 days to go from quiescence to peak at R_PTF = 16.5 mag. The outburst spectrum was a continuum-plus-emission source, and similar to the burst phases of PTF 10nvg (V2492 Cyg) in having rare TiO emission. After fading in <6 months, iPTF 15afq presented a TiO absorption spectrum with superposed emission lines, having characteristics generally similar to those of a normally accreting T Tauri star. The source is located in the Canis Majoris (CMa) star forming region, and was later cataloged by Fischer et al. (2016, AJ 827, 96) and Sewilo et al. (2019, ApJS 240, 26) based on having the infrared spectral energy distribution of a young star plus a disk/envelope.

We report herein the repeated brightening of iPTF 15afq in 2018 and 2019, as detected by ZTF and by Gaia. We also present the results of follow-up spectroscopy obtained with Keck/HIRES. Relevant figures are located at http://www.astro.caltech.edu/~lah/ptf15afq/ .

The photometry available in DR1 of the Zwicky Transient Facility (Masci et al. 2019) and in the (Hodgkin et al. 2013) shows another brightening event in 2018, with a rise amplitude of >2.5mag on a time scale of <33 days (the peak is not measured). From r < 15.8 mag, the source subsequently declined to 19-19.5 mag in <4 months. The current 2019 outburst, designated Gaia 19fct, reaches G < 15.1 (the peak is not yet measured) with the brightening by >4 mag to date, taking <75 days.

The optical spectrum, obtained on 29 November, 2019 (UT) with HIRES (Vogt, 1994) and the W.M. Keck Observatory's Keck I Telescope, is absorption-dominated with no signature of the permitted- or forbidden-line emission that was present earlier in both the faint state and in the 2015 outburst state. Instead, the high resolution (R=37,000) spectrum is characterized purely by absorption from broadened metal lines, with widths of about 70 km/s. The spectral type is in the F0-F5 range if a dwarf/giant and F5-G0 if a supergiant. BaII at 6142 A and 6497 A, an indicator of a low surface gravity atmosphere, and LiI at 6707 A, a signature of stellar youth, are both present. There are also ``wind lines" including MgIb, NaD, and KI 7699, as well as clear P Cygni profiles in Halpha and in the CaII triplet. The terminal velocity of the blueshifted absorption in these lines is currently only about -120 km/s.

While the two previously recorded brightening events in iPTF 15afq were consistent with moderate scale and short-lived EX Lup type events, the ongoing outburst is much larger, though obviously as-yet unknown in terms of total amplitude or duration. The nature of the absorption spectrum in outburst may indicate a more dramatic accretion event such as an FU Ori outburst.

Multi-color photometric follow-up over the next several months is encouraged.