Supernova 2016adj in NGC 5128 - Science Thread

[madbadgalaxyman (Robert)]

I have decided to initiate a "science thread" about Supernova 2016adj in the galaxy NGC 5128, which was discovered by IIS members Greg Bock and Peter Marples in February 2016 .

The main thread for this supernova is in the "chat" forum of IIS, and therefore let's use this thread to provide some hard facts about this supernova.......
(we won't be doing any 'awestruck oohing and aahing at the exploding star' in this thread!)

(this post is a repeat of my recent "chat forum" posts on the supernova, plus a couple of emails)

Madbadgalaxyman said:

A quick summary of the situation regarding the dust-dimming and reddening of the light of SN 2016adj in NGC 5128.....
(from the astronomical telegrams)

The extremely high Visual (V-band) extinction of an estimated 2-4 mags (see: Atel 8657) in front of this supernova causes it to be a visually very unimpressive object (= "the SN is very dimmed & reddened due to dust within the dust lane"). This supernova was measured at near V-band ("visual") magnitude 15.2 or 15.7 (very discrepant measurements!) near to its discovery date, and with an observed B-V color index of 2.1 which indicates it was an extremely red object.(the supernova is now somewhat fainter than V magnitude 17 )

The dense screen of interstellar dust within the very-large-scale Warped Dust Disk of NGC 5128 both dims and greatly reddens the light of the supernova, so this makes it tough to figure out :
- what is its intrinsic (actual) luminosity ? ("energy output")
- what is its intrinsic (actual) colour ?
An accurate estimate of extinction and reddening from interstellar dust in the line-of-sight of the supernova will be essential in order to reduce the very large uncertainties in these two parameters.

While not at all a supernova specialist, I suspect that the great uncertainty regarding the intrinsic luminosity and color of this supernova introduces uncertainty as to its exact type.

So one of the main tasks of the pros will be to get an accurate estimate of the extinction. (there is also "foreground" extinction and reddening from dust within our own Galaxy, that also has to be taken into account)


then madbadgalaxyman saith:
(regarding how to figure out extinction in front of the SN)

Normally, stars that are very near in the sky to a specific object of interest, stars of known intrinsic color index (intrinsic color , for example B-V and U-B, etc, can easily be figured out from the spectrum of a star), are observed, and the reddened color indices of these stars , as observed , are used to calculate the amount of extinction in front of the object of interest. But we would require spectra of stars within NGC 5128, and very near to the SN in NG5128, to have some "intrinsic"(actual) non-reddened star colours to work with.
It's the famous "extinction curve" that you can't get around, because of the size distribution of the mainly submicron-sized dust in interstellar space.

Another possibility is that infrared observations, particularly those made in bandpasses longwards of 2-5 microns, can give you a nearly extinction-free view of an object, (e.g. galaxies, even dusty ones, are virtually transparent at 24 micrometers wavelength). Really, there is little point these days measuring apparent magnitudes at visible wavelengths, if extinction in front of an object of interest is significant.....it introduces a lot of uncertainty into estimates of the luminosity of the object of observation.

Another way to measure extinction would be to measure the observed SED (= spectrum) of an extincted emission-line object which has a well-known "standard" SED and then to compare the relative line strengths of the observed spectral lines with their theoretical or predicted or already-known line strengths.
The observed flux ratios of specific emission lines should vary predictably, depending upon the amount of extinction in front of the object of observation .
There is something called the "Balmer decrement" method using the Ha and H-beta lines, that works in this way.

madbadgalaxyman further said:

The latest report indicates that Supernova 2016adj is actually of type IIb, which indicates that it is caused by the collapse of the core of a massive star, rather than being caused by a white dwarf going 'critical' due to mass accretion, or due to some other cause.
(The initial reports gave a different type for this supernova.)

Type II supernovae spectra have hydrogen lines, while type I supernova spectra have none.

There are various subypes of Type II supernova:
type II-L, type II-P, type IIb, and type IIn.
But all type II supernovae are thought to result from the collapse of the core of a massive star.

According to information which someone quotes on the internet from Wheeler and Benetti (2007), the light curve of a type IIb supernova (somewhat confusingly) resembles the light curve of a type I supernova!

A type IIb supernova has the following
features in its spectrum:
Broad and weak H lines ; strong He I lines ;


Here is a diagram from Turatto et al., arxiv 0706.1086 , outlining the classification of supernovae:



Clear as daylight, folks?

But seriously, Turatto et al. is an excellent and readable paper on the classification of supernovae.
http://arxiv.org/pdf/0706.1086.pdf

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[madbadgalaxyman (Robert)]

Here is ATel #8890 reporting the discovery of a Light Echo around the supernova:

http://www.astronomerstelegram.org/?read=8890

[madbadgalaxyman (Robert)]

Here is the (slightly edited) text of a proposal to study the supernova in the infrared, with the Spitzer Space Telescope:
(bold highlighting was inserted by madbadgalaxyman)
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Spitzer Space Telescope - Directors Discretionary Time Proposal #12129

Light Echoes and the Progenitor of SN 2016adj in Cen A

Principal Investigator: Ben Sugerman
Institution: Goucher College

Technical Contact: Ben Sugerman, Goucher College

Co-Investigators:
Stephen Lawrence, Hofstra University
Hours Approved: 1.6

Abstract:
The Type Ib/IIb supernova (SN) 2016adj is the fifth closest SN to be discovered during the lifetime of HST. This event offers us a rich variety of rare and unique opportunities, including:

(1) identifying the progenitor;

(2) mapping the three-dimensional structure and chemical composition of the progenitor's circumstellar and the host galaxy's interstellar environments;

(3) testing models of stellar mass loss and high-mass stellar evolution.



The progenitor field of the SN has been observed from the near-UV to the mid-IR with HST and Spitzer, which will immediately allow us to accomplish the first science goal by
identifying the progenitor (or establishing its upper limits) once new image with the SN present are taken with both observatories. Preliminary analyses of early-time spectra of SN 2016adj indicate its light is being extinguished by
at least A(V)=2-4 magnitudes, meaning it is buried deep within the dust lane of Cen A. Echoes of the SN light off of this dust will allow us to produce high-resolution, three-dimensional maps of the structure and composition of the dust in and around the line-of-sight to the SN, which we will use to accomplish science goals (2)-(3) listed above. In particular, we will directly test the hypothesis that Type Ib/IIb SNe come not from very-high mass stars but from only moderately-massive stars that lost their envelopes to close binary companions.


Please note that since echoes pass through a given point in space only once, data are permanently lost for each epoch that is not observed. While we will propose for continued observations in the Cycle 13 call for proposals, most of
the science we propose cannot be achieved if the observations in this proposal are not taken before Cycle 13 begins.
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