The reddening and distance to recurrent nova V3890 Sgr from interstellar features
ATel #13069; U. Munari (INAF Padova) and F. M. Walter (Stony Brook University)
on 3 Sep 2019; 07:05 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)
Subjects: Optical, Cataclysmic Variable, Nova
The symbiotic star and recurrent nova V3890 Sgr, after the previously
recorded events of 1962 and 1990, was found by A. Pereira on 2019-08-27.87
UT to be again in outburst (spectroscopic confirmations have been reported
in ATel #13047, #13059, #13060, #13062; and detection in X-rays in ATel
#13050). We immediately began a spectroscopic follow-up with the Asiago
1.22m and SMARTS 1.5m telescopes. The SMARTS spectra were acquired with the
Chiron spectrograph at 80,000 resolving power, allowing a detailed
investigation of the interstellar absorption features from which we derive
here a measurement of the reddening affecting V3890 Sgr.
Atomic interstellar lines present twelve distinct components that we have
measured separately in five Chiron spectra (obtained nightly from 0.15 through
4.18 days after discovery), with the results averaged in the
table below (the radial velocity is heliocentric). The three strongest (and
the bluest) components are saturated in NaI doublet, but still optically
thin in KI 7699. The remaining nine are optically thin in NaI, but too weak
for a meaningful measurement in KI 7699. Using the calibration by Munari
and Zwitter (1997A&A...318..269M) of equivalent width in terms of reddening
we get
| RV | lambda/err | E.W./err | E(B-V) |
|---|
| (km/s) | (Ang) | (Ang) | |
|---|
| -7.0 | 7698.800 0.007 | 0.060 0.006 | 0.218 |
| -2.0 | 7698.928 0.015 | 0.027 0.005 | 0.100 |
| 2.1 | 7699.033 0.004 | 0.043 0.003 | 0.143 |
| 8.5 | 5890.119 0.003 | 0.097 0.002 | 0.028 |
| 19.6 | 5890.338 0.002 | 0.105 0.003 | 0.030 |
| 30.2 | 5890.545 0.001 | 0.086 0.003 | 0.024 |
| 37.9 | 5890.698 0.003 | 0.051 0.003 | 0.014 |
| 44.2 | 5890.820 0.003 | 0.069 0.002 | 0.019 |
| 53.2 | 5890.998 0.001 | 0.044 0.001 | 0.012 |
| 59.3 | 5891.118 0.001 | 0.060 0.002 | 0.017 |
| 69.4 | 5891.316 0.003 | 0.043 0.001 | 0.012 |
| 74.6 | 5891.418 0.009 | 0.013 0.002 | 0.003 |
for a total reddening of E(B-V)=0.62. At this modest reddening, there are
only a few DIBs strong enough for a meaningful measurement (those at 5780,
5797, 5850, 6379, 6614 Ang). Adopting the
calibration of DIB equivalent width in terms of E(B-V) given by Kos and Zwitter
(2013ApJ...774...72K) we obtain an average E(B-V)=0.56 from them.
These values, and their mean E(B-V)=0.59, are in good agreement with the
average B-V=0.67 observed for V3890 Sgr during its decline toward t_2 (as
derived from the ANS Collaboration photometric monitoring). The intrinsic
color of novae during these early phases is usually restricted to
B-V=0.0/+0.2 (van den Bergh and Younger, 1987A&AS...70..125V).
E(B-V)=0.59 corresponds to E(g-r)=0.70. The 3D map of interstellar
reddening by Green et al. (arXiv:1905.02734) reports E(g-r)=0.50+/-0.03 at
4.5 kpc along the line of sight to V3890 Sgr, with no data beyond that. The
situation is similar with the 3D map by Lallement et al.
(2014A&A...561A..91L). The distance to V3890 Sgr appears therefore in
excess of 4.5 kpc.
