COS Observations of the Chemical Composition of SNR LMC N132D

France et al.

Ben Folsom and Sharlene Rubio

Background on Supernovae:

-Fe56 is the most stable isotope inside a star, meaning it requires energy input for fusion or fission.

-Once enough Fe56 forms in a star's core it begins to die. If its mass exceeds the Chandrasekhar limit, it produces a supernova.

-Little is known about the mechanism driving the initial explosion of the supernova.

-What we do know is that young Supernova Remnants (SNRs) are often metal-rich, as supernovae are only known source for elements heavier than Fe. We also know that supernovae produce initial shockwaves traveling up to 30,000 km/s, which eventually slow down to between 50 and 10,000 km/s.

Chemical Makeup of a Typical Star (Not to scale)

A Typical Supernova

Within a massive, evolved star (a) the onion-layered shells of elements undergo fusion, forming an iron core (b) that reaches Chandrasekhar-mass and starts to collapse. The inner part of the core is compressed into neutrons (c), causing infalling material to bounce (d) and form an outward-propagating shock front (red). The shock starts to stall (e), but it is re-invigorated by a process that may include neutrino interaction. The surrounding material is blasted away (f), leaving only a degenerate remnant.

Supernova Remnant N132D

Supernova Remnant N132DSupernova Remnant N132D

Supernova Remnant N132DSupernova Remnant N132D

Ideal Candidate for Study:

-Minimal reddening and good spatial resolution(thanks to the relative proximity of it's home in the Large Magellanic Cloud)

-Minimal foreground extinction due to dust

Supernova Remnant N132DSupernova Remnant N132D

Prior Research:

-Oxygen-rich filaments are concentrated near the middle of the remnant.

-X-ray shell for remnant covers ~13pc

-Velocity range of ~4400 km/s for oxygen and neon filaments

-Progenitor star mass is likely at least 30 and possibly as great as 85 solar masses (depending on mixing of O-rich mantle with O-burning layers)

Supernova Remnant N132DSupernova Remnant N132D

Detection:

-Cosmic Origins Spectrograph (COS) exposure over 5 orbits (5190 and 4770 seconds on each of the available far-UV channels

-Wavelengths from 1150 to 1750 Angstroms

Full COS Spectrum in O-rich Knot

Region of interest: 1362-1418 Angstrom

Notable peak values:

Comparing Observed Velocity to Prediction from Models

Analysis

The authors conclude that the unique composition of the O-rich knot is “attributable to the degree of mixing between the stellar ejecta and the ambient presupernova medium, both interstellar and from earlier mass loss episodes of the progenitor star”.

Could there be a more specific explanation available? Assuming there was nothing out of the ordinary in the “ambient presupernova medium”, could an Oxygen abundance in the progenitor star be solely responsible for this type of knot?

Analysis

The peak heliocentric velocities of OIII, OIV and OV fall at ~185 km/s, and the velocities of O I, Si IV and He II are in a similar range (~150km/s). The authors take these observations to imply that these species are cospatial.

The observed carbon moves at much higher velocities, and is thus assumed to be located in in a different region of the remnant from the O-rich knot.

It's also worth noting that no Nitrogen ions were observed in the knot. Does this have any bearing on the CNO cycle of the star before the supernova?

Analysis

The shock model for ~130 km/s fits well with the observed line strength for O V, O IV and Si IV. However, the observed O III does not fit the model for line strength or shock velocity. The authors suggest that since the O III has a 20% offset which is identical to it's error in the dust attenuation curve, the exctinction curve to N13D may be flatter than a typical LMC curve.

Could there be any alternate explanations for this discrepancy?

Conclusions

The authors find reasonable fits using silicon/oxygen abundance ratios for progenitor stars at 40, 50, 60, 70, 85 and 100 solar masses.

The most viable candidate models are at 50 and 60 solar masses, though the error bars are left conservative.

Discussion Questions

-Since we have such a clear view of N132D, could oxygen-abundant filaments or knots also be prevalent (and as-of-yet undetected) in more distant or obscured SNRs?

-Is there just cause to assume from these findings that a significant amount of a galaxy's oxygen comes from this type of SNR? If so, how sure should we be before having SETI take a look?

-Can we safely assume that the large majority of these observed amounts of lighter-than-iron elements were part of the progenitor star's mantel? (Considering that a “typical” star will use up all fusable material before dying)

-Would a more extensive mapping of N132D's oxygen hotspots provide any further critical insight?

-Could the oxygen-rich SNR candidates in M83 (from a paper discussed earlier in the semester) be more accurately classified in light of this data on N132D?