- Leloudas, Giorgos
- Dai, Lixin
- Arcavi, Iair
- Vreeswijk, Paul M
- Mockler, Brenna
- Roy, Rupak
- Malesani, Daniele B
- Schulze, Steve
- Wevers, Thomas
- Fraser, Morgan
- Ramirez-Ruiz, Enrico
- Auchettl, Katie
- Burke, Jamison
- Cannizzaro, Giacomo
- Charalampopoulos, Panos
- Chen, Ting-Wan
- Cikota, Aleksandar
- Valle, Massimo Della
- Galbany, Lluis
- Gromadzki, Mariusz
- Heintz, Kasper E
- Hiramatsu, Daichi
- Jonker, Peter G
- Kostrzewa-Rutkowska, Zuzanna
- Maguire, Kate
- Mandel, Ilya
- Onori, Francesca
- Roth, Nathaniel
- Smartt, Stephen J
- Wyrzykowski, Lukasz
- Young, Dave R
- et al.
We present light curves and spectra of the tidal disruption event (TDE)
ASASSN-18pg / AT 2018dyb spanning a period of seven months. The event shows a
plethora of strong emission lines, including the Balmer series, He II, He I and
metal lines of O III $\lambda$3760 and N III $\lambda\lambda$ 4100 and 4640.
The latter lines are consistent with originating from the Bowen fluorescence
mechanism. By analyzing literature spectra of past events, we conclude that
these lines are common in TDEs. The spectral diversity of optical TDEs is thus
larger than previously thought and includes N-rich besides H- and He-rich
events. We study how the spectral lines evolve with time, by means of their
width, relative strength and velocity offsets. The velocity width of the lines
starts at ~ 12,000 km s$^{-1}$ and decreases with time. The ratio of H$\alpha$
to H$\beta$ remains close to three, while the ratio of He II over N III
increases with time. The same is true for ASASSN-14li, which has a very similar
spectrum to AT 2018dyb but its lines are narrower by a factor of $>$2.
High-resolution spectroscopy at maximum light does not reveal any narrow
features that can be attributed to the TDE. By fitting the light curves of AT
2018dyb we estimate a mass of $4^{+5}_{-2}\times 10^6 M_{\odot}$ for the black
hole and of $0.7^{+4}_{-0.6} M_{\odot}$ for the disrupted star. The detection
of strong Bowen lines in the optical spectrum is an indirect proof for extreme
ultraviolet and (re-processed) X-ray radiation and favors an accretion origin
for the TDE optical luminosity. A model where photons escape after multiple
scatterings through a super-Eddington thick disk and its optically-thick wind,
viewed at an angle close to the disk plane, is consistent with the
observations. (Abridged)