IRAS 07572-3100 and the New Nebula in Puppis: Outburst from a Young Stellar Object or an Exotic AGB Star Event?
ATel #14416; Lynne Hillenbrand (Caltech)
on 22 Feb 2021; 08:34 UT
Credential Certification: Lynne Hillenbrand (lah@astro.caltech.edu)
Subjects: Sub-Millimeter, Far-Infra-Red, Infra-Red, Optical, Nova, Variables, Young Stellar Object
We report new photometric and spectroscopic information
regarding the previously unstudied source IRAS 07572-3100, which has been
publicized recently in both
RNAAS 4, 242
and Atel#14408 as a possible outbursting young star.
We provide an exhaustive description of knowledge regarding the source
at this URL,
including an assessment of its environment, spectral energy distribution,
multi-wavelength lightcurve information, and post-outburst optical spectrum.
At present, it is not obvious that IRAS 07572-3100 is a young star,
though there is some evidence in that direction.
The spectrum seems more consistent with an evolved, AGB type star,
yet that interpretation is not entirely satisfactory either.
As previously reported, NEOWISE data
indicate an 8 mag outburst that occurred during 2015,
with the object taking between 6 months and 1 year to rise in the mid-infrared.
However, data from ASAS (Shappee; 2014) and ATLAS (Tonry; 2018, Heinze; 2018),
both indicate that the optical peak was in late 2016, a full year
after the majority of the infrared brightening.
The optical lightcurve has a sharp cusp and steep evolution on either side
of its peak.
The optical plateau is about 3 mag (optical) below its peak.
The current brightness of IRAS 07572-3100 is estimated at
V approximately 19.5 mag (AB), and K approximately 5-6 mag (Vega).
An optical spectrum was acquired on 2021, February 3 UT (20210203)
at the W.M. Keck Observatory with ESI, the Echellette Spectrograph and Imager (Shennis; 2002).
Two 1200 second exposures were taken with the 0.5" slit to achieve spectral resolution R=8000.
The spectral range 5,000-10,000 is covered with adequate signal-to-noise in the reddest 7 spectral orders.
Spectral typing suggests an M6-M7 III.
Low gravity is indicated by the shape of the CaH region,
as well as in the shapes of the prominent TiO and VO bands, and MgH 5200.
Low gravity signatures are also shown by the relatively weak
NaI and KI lines.
Weak emission lines in the spectrum have the following properties:
Halpha W_lambda = -3.07 A
HeI 5876 W_lambda = -1.79 A
[OI] 6300 W_lambda = -1.87 A
[SII] 6717,6732 W_lambda = -1, -2.2 A
No evidence for [NII] 6548,6583
The Halpha line is resolved, with width ~140 km/s (FWHM), and some asymmetry.
The width is quite narrow for a young stellar object,
in which velocities of several hundred km/s are characteristic of rapidly accreting stars,
but it is also rather large compared to the 10-15 km/s that is typical of AGB star winds.
The forbidden-line emission strengths suggest shocked gas.
Such lines are common in young accreting stars that drive outflows.
They seem rare for lower AGB stars, but can be seen in Miras.
Finally, the spectrum shows LiI 6707 with W_lambda = 625 mA.
This is a fairly typical value for late type young stellar objects.
However, red giants can also show LiI from dredge-up processes.
At present, the nature of IRAS 07572-3100 is ambiguous.
The current lengthy plateau in the lightcurve should be monitored for changes,
with spectroscopic investigation in the event of a re-brightening or dimming.
