Søgeresultater for 'V339'

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Artikle 15 fra Steve Shore

Røngtenstrålingen stiger
Densitet af ejecta
Fordelingen og mængderne af stof

Why this last stage is so important

If it weren’t a possible offense to French patriotism, I’d cite Henry V (Shakespeare) about
“we happy few” to explain how important it is to continue observations of V339 Del for as
long as possible. But instead, poetry notwithstanding, I should explain why this last stage
is so important.

The X-ray emission from the nova is continuing to increase, we have hardly yet seen the
maximum, and it is still “hard” (meaning it is stronger in the higher energy bands, around
10 keV, that indicate a temperature of > 10 MK). This is still likely from internal shocks
in the expanding ejecta but this should decrease and a soft source (< 1 MK) should appear
shortly. When it does, it will be the brightest XR nova ever observed by Swift during its
mission lifetime.

When the soft source becomes visible, the optical proxies (like the coronal lines) will
indicate the low density structure of the ejecta and allow the determination of the abundances.
That’s the other important reason for continuing low resolution observations, they are the
thermometers of the ejecta. While the individual structures in the line profiles matter,
those can be obtained from the high resolution observations (e.g. NOT, Ondrejov, etc) and
from HST. The low resolution data, instead, shows the morphology of the continuum and the
total emissivity changes — this gives the mean properties of the ejecta. We don’t have
a record of a CO nova with this precision and coverage, as I’ve said before, but it is now
at a stage where the data will be comparable in resolution to some for extragalactic
novae (for instance, in the LMC and M 31).

As an example, the main electron density diagnostic is the ratio [O III] 4959+5007 / [O III]
4363. This relates directly to the electron density and, in turn, gives an estimate of the
mass. But it requires a temperature for that number, usually chosen on energetic arguments
about cooling of the gas. BUT if you also have [N II] 6548+6583 / [N II] 5755, then you
have two independent estimates of the temperature and density that give a unique solution.
It will be important to follow the developments of [Ar III] 7135, the He I lines (especially He I 6678)
and the Balmer lines (especially H-beta since the normalization of the uncalibrated spectra
for plasma diagnostics is based on the emissivity of that line).

The other, more essential result that requires low resolution data over a broad spectral
coverage is the determination of the abundances. I think the mass problem has been solved
(personal prejudice only) but the abundances will drastically differ from one event to
another. This is the detritus of the nuclear processing so it is the fundamental, virtually
unique signature of the TNR. Only when the gas is completely transparent can standard methods
be applied, those developed for H II regions. Until one knows that there is no pumping from
the UV any flux is suspect and these density diagnostic ratios can’t be converted into
abundances for, as an example, O and N. Those two, along with C, are the overabundant
species that record the temperature and density conditions at the moment of the explosion.

There’s another reason to keep observing, although not with an extraordinary cadence.
There are so few continued programs into this stage that we don’t know if something MIGHT
happen in the later stages.

To see, for instance, if an accretion disk re-appears (as it must) requires certain lines
(e.g., He II 4686) to remain invariant while the others decline. This line is a very
good indicator of the high density, high temperature environment within the disks. And
even at a resolution of 500 you should be able to distinguish the lines from the ejecta
and disk (by the appearance of the line profile and it’s stability).

Steve roser de amatører som har meldt sig til at bidrage med data.
… de har allerede produceret en stor arv……
En af fordelene ved denne type arbejde er, at det har en begyndelse, en fornuftig afslutning,
og produktet er en permanent mine af oplysninger.

[nightskyDeltager Neutron star]

Artikel 14

About luminosity curve and dust formation

Our friend continues a steady decline, with some bumps, despite the recent flurry of reports
of dust formation. First, let me explain what the observations may be saying and then,
to illustrate what you’re seeing in the data, add a few points about the ejecta structure.

Dust, being the solid state, behaves like bricks. Radiation is absorbed with efficiency
depending on the grain composition and re-emitted locally with whatever temperature the
grain has to reach to balance the rate of absorption. This is referred to as “radiative
equilibrium”: if the temperature reaches a steady state while the irradiation is steady,
it will get as hot as it “needs to be”. The incident photons are energetic, optical and
UV. But they are diluted by distance from the emitter. So the energy density is lower
than near the central White Dwarf or even the inner ejecta. Thus the rate of absorption
is lower with increasing distance. A solid doesn’t behave like a blackbody in its spectrum,
but the emission rate depends only on temperature so the farther the grain is from the
central source the lower its temperature will be in equilibrium. This is almost independent
of the size of the grain so it could be a peanut or a planet, the energy per unit area
(flux) is all that has to balance (the book-keeping is: what comes in, goes out). Some
critical temperature must be crossed for the solid to be stable, otherwise it will evaporate
by heating (loss of atoms), that is the so-called “Debye temperature” below which the
solid (or atomic cluster) remains structurally intact.

This, for silicates and various forms of carbon (usually called “astronomical graphite”
because of laboratory analogies) is about 1500 K. It means, in a kinetic (particle) sense
that collisions with this relative velocity (the sound — or gas — speed corresponding to
this temperature is about 1 km/s) can bind (stick) and nuclear clusters remain stable.
As the cross section increases the quantity of energy absorbed increases so while the
temperature doesn’t change the luminosity does. Since the grains reach a low temperature,
they radiate in the infrared and that’s the tell-tale signature of their presence. It
isn’t only a drop in the light of the ejecta photosphere and White Dwarf. That depends
on viewing angle, how you see through this growing smog. But the infrared is transparent
so you see the cumulative radiation from the grains as an increase in the part of the spectrum
where a solid would radiate. The controversy now is whether L and M band photometry
(longward of a few microns) has increased sufficiently to signal the presence of this
absorber. Two groups seem to agree on this now but as a recent event, around Sept. 29
but this require further data. If we’re in that stage, it’s just preliminary and recall
that neither CN nor CO were detected in the nova when the NaI lines were strong.

The cross section, if dominated by direct absorption, also has certain characteristics.
Silicates (SiO complexes) are rather opaque at 10 and 12 microns (there’s a peak in the
broadband emissivity there) and rather inefficient absorbers in the UV. In contrast, the
carbon complexes are very good absorbers at around 0.20 – 0.22 microns (2000-2200A) in
the UV so they absorb where the irradiation is maximal and radiate less efficiently in
the IR because they lack the bands of the silicates. Thus, graphite (carbon) grains will
be systematically higher temperature in equilibrium than silicates. It’s likely that the
grains will be carbonaceous so they’ll be hotter than silicate grains (that are inefficient
absorbers, efficient radiators in the peak emitting range).

This all relates to where the dust will form. To date, the NOT profiles are the same as
they were, no obscuration by the grains. This may change, we’ll have to wait a bit. The
main interest now will be the process itself, if grains are there. But there’s another,
albeit slower, physical effect that we can now see.

Lines profiles

Data fra Nordic Optical Telescope, 30 sep. 2013

Since the ejecta are ionizing now, the profiles of different ions will trace out different
parts of the ejecta at the same time. In the figure above, you see this. The top is
neutral oxygen, ionized nitrogen, and twice ionized oxygen (the 5007 is a doublet with
4959, that is just barely present) and has the greatest velocity width and a unique
profiles that resembles what was seen at the start on the Balmer lines. Yes, the [O III]
lines do seem to be there. Since these are forbidden transitions, they trace low density
ionized gas and the wings suggest these are in the outer portions of the ejecta. The [N II]
is intermediate. And now the He I line profiles share the Balmer line structure, these
require a very a high excitation energy so suggest that recombination formed these. The
C II 8335 line is also now present, but there’s nothing yet at the [Fe VV] or [Fe X}
optical lines.

There’s a flight scheduled for SOFIA and we’ll keep monitoring the spectrum. Please don’t
give up now, remember that if we’re ever going to understand such a simple thing as a
nova, a lot of hard work will be preceding. The XR/Swift data to date requires about a
factor of 10 higher column density than derived from the UV Lyman alpha line, have in the
whole ISM toward the nova.

The XR turn-on was fast as far as can be known from the descriptions.

SOFIA Teleskopet (Stratospheric Observatory for Infrared Astronomy)

Nightsky2013-10-02 21:29:25

[nightskyDeltager Neutron star]

Fik en observation kassen i aften igen, men det holdt p.g.a. af en del skyer.

Ikke nogen ændringer udover at kontinuum stadig falder som forventet.

Data er ikke dark, bias, flat behandlet.

[nightskyDeltager Neutron star]

Atel 5434 bekræfter støv.

L-Bånd målinger af novaen passer med tilstedeværelsen af støv.

This flux level is consistent with a small amount of dust having formed recentlynsistent with Presence of Dust

[nightskyDeltager Neutron star]

Hejsa

En helt fantastisk animation lavet af Martin Dubs, viser hvorledes novaen har udviklet sig
når der flux kalibreret.

Som det ses ændrer styrken af brint emissionen sig ikke ret meget. Derimod er det kontinuum
der falder kraftigt. Et fantastisk smukt eksempel der viser hvorledes emissionen forsætter, mens
novaen falder i lysstyrke.

Animation der er flux kalibreret, Created by Martin Dubs.
Spectra recorded by Olivier Thizy, Jim Edlin and Francois Teyssier
Data obtained from the ARAS data base: http://www.astrosurf.com/aras/Aras_DataBase/Novae/Nova-Del-2013.htm

[nightskyDeltager Neutron star]

29. sep

Spektre fra i aften. Ikke nogen nye emissionslinjer jeg kan detektere.

Sandsynligvis er kontinuum ændret, men jeg må vente på darks for at finde ud af dette.

[nightskyDeltager Neutron star]

Hejsa

Var ude i aftes for at kigge på V339 Del (Nova Del 2013)

Umiddelbart ser det ud som om at emissionen er blevet kraftigere, men hvis man undersøger det
nærmere vil man opdage at det er kontinuum der bliver svagere for til sidst næsten at forsvinde,
ganske som Steve Shore beskriver sidst i artikel 13. Novaen begynder at ligne en
planetarisk nebula i sit spektrum.

Nedenfor 2 optagelser fra i nat. En i det visuelle domæne, gældende op til 8000Å og en
med IR longpass ~6850 Å filter, således at 2 ordre ikke kontaminer oppe i IR området.

Sidste gang jeg havde klart vejr, 10 sep. fik jeg også et spektrum af V339 Del og jeg har
lavet en sammenligning mellem de to.

Markeringer hvor jeg umiddelbart ser signifikante ændringer.

PS. Jeg kan se på http://www.aavso.org/ at der en del problemer med at få nøjagtige magnituder
ned via fotometri.
http://www.aavso.org/nova-del-2013-photometry
Måske noget at filterne ikke er helt som forventet.

Nightsky2013-09-29 19:57:20

[nightskyDeltager Neutron star]

Artikel 13

This is a lovely start to the ionized stage.
V339 Del -> Nova Del 2013

The Swift team has just announced the detection of X-ray emission from V339 Del (ATel #5429).
They give a flux that is a very small fraction of the STIS detection: in the range from
1-10 keV (corresponding to a temperature of about 10 MK), 2.3e-13 erg/s/cm^2 while the
UV(1200-3000A) gives 1.7e-8 erg/s/cm^2. This large ratio is at the start of the event but
has already been corrected for hydrogen absorption.

Interestingly, the Lyman alpha line in the UV observations seems weaker than would be
expected from the XR data, a suggestion that the ejecta are also not completely covering
the central start but are covering the region of XR emission. The nova remains very bright
in the visible and this is a real problem for the XRT on Swift that suffers from optical
leaks (it’s the nature of the detector). Your spectra are indicating the start of [O III]
4959, 5007 emission and also that He II 4686 is there. Now the He II 5411 line should
also appear (a check on the He II identification) and the disappearance of the Fe II and
other curtain lines will be a very important (and pretty) thing to watch over the next one
to two weeks.

To put this part in physical context, what’s happening is an advance, from the inside
out, of the ionization front as the WD emission strengthens. It’s always the same basic
picture, but the phenomenology accelerates now. The ionization of the heavy metal lines
removes the opacity faster than the change in density so the optical decline should also
steepen (which may be mistaken for a dust-forming event), and the highest ionization lines
from permitted transitions will have narrower profiles and come from the inner ejecta.
The outer part, and here the ionization state is a very good measure of the filling
factor (how fragmented the ejecta are governs how much of the ionizing radiation
penetrates to the outer part at this marginally thick stage); their profiles at high
resolution will be the best comparison with the [O I] and [N II] as a map of the ejecta
structure. Remember, He II is from excited states but are all permitted transitions while
the [O III] and others are low density transitions (forbidden).

To give some idea of what things look like in the UV I’m including the OS And – V339 Del
comparison. The very narrow lines that go to zero in the V339 spectrum are all interstellar
transitions (keep in mind that the resolution is about 100,000). For OS And, it is about
10000 (high resolution IUE from Dec. 1986). No extinction correction has been applied
(no interstellar dust effects have been removed) for the comparison) so you can see the
lines (e.g. He II 1640 + curtain, N III 1750, Mg II 2800, etc). The 1200A region is
particularly important for the properties of the ISM and the ejecta — this is where the
Ly-alpha profile sits (you see there seems to be emission there, and in fact there is a P
Cyg profile under the curtain on the line).

Sammenligning med Hubble data fra sidste uge.

More will be coming in the next few days, thank you all for the continuing hard work.
The changes will now be quite remarkable as the nebular spectrum appears and the
continuum disappears (this was already noted for T Aur by Clerke in her description of
the transition, and also Huggins).

http://www.nasa.gov/mission_pages/swift/spacecraft/index.html

http://arxiv.org/pdf/1211.3176.pdf
Spectroscopy of Novae – A User’s Manual by Steven N. Shore

Nightsky2013-10-02 19:53:12

[nightskyDeltager Neutron star]

Artikel 12 – Indication of dust formation?

V338 Del (Nova Del 2013) Mulig støvdannelse.

– Lavopløsningsspektra kan hjælpe med påvisning af støv.
– Måske vil vi se at molekylær dannelse ikke er et forstadie til støv kondensering.

There is an indication based on the infrared and slight changes in the optical photometry
that V338 Del may be entering a dust formation episode. If this is really happening there
are several important things to note for observations in this next week. Note that this
will be the first time since DQ Her (nova), if really starting, that this stage
will have been seen and it was impossible to follow that nova (in 1934) during the
minimum. You all have the low resolution capability to keep going — if you want to —
even through much of what could be a deep minimum (a drop of 5 or more magnitudes is not
impossible). For high resolution observations, a question is where and how the dust forms.

We know something of this from the very old observation of V705 Cas 1993 that was
observed in the UV during the start of the episode (http://adsabs.harvard.edu/abs/1994Natur.369..539S),
but that was a chance observation not covered in the optical. First, assuming the ejecta
are bipolar and inclined, the line profiles may change in a peculiar way: as the dust
formation proceeds the portion of the ejecta (the outer part) should become opaque (depending
on the geometry) and the blue part of the line will disappear. On the other hand, the
whole profile will drop, especially for the N II line and He I lines, if the ejecta are
more spherical because both parts of the line forming region will be absorbed. The UV has
now been measured, we know how much energy is available for absorption by the grains and
that emission in the infrared can be compared with that lost in the visible. If the two
balance out (everything absorbed is re-emitted) we’ll know that the ejecta are spherical
(every photon is intercepted in a spherical, completely opaque shell). On the other hand,
if there is an imbalance, that will be due to the filling factor and geometry of the ejecta.
So if this really is the start of the event, the ejecta will act as a sort of calorimeter,
registering how the energy balance proceeds.

The changes in different lines (e.g. [O I] vs. He I) indicate where in the ejecta the
dust is forming, although at this stage I have to say we don’t know much — only V705 Cas
has been observed during such an event and in the UV at low resolution. When it happened
there, the whole UV disappeared without the spectrum changing, as if a new “curtain”
dropped uniformly over the line forming region. This time, it’s anyone’s guess and your
work will be vital.

One more thing: none of the spectra showed ANY indication of molecular emission (CO, in
the IR) or CN (in the optical, your hard work). If this nova forms dust, we will have
learned something tremendous, that molecular formation is not a precursor event to dust
condensation. If so, it is in line with the idea that reactions between neutral atoms and
ions of carbon and silicon cause a sort of kinetic runaway in which the grains aggregate
like fluffballs. No matter what now happens, without your spectra we would not know that
this nova did not form the molecular seeds and that if this does condense it likely is
particle-based process instead of a thermodynamic-like phase transition (the difference
between agglomeration (kinetic) and homogeneous nucleation (like terrestrial clouds and
rain, around nuclei in a saturated vapor). With apologies for referring to my own stuff,
but this paper is an example of what I’m talking about:

http://adsabs.harvard.edu/abs/2004A%26A…417..695S ; see also
http://adsabs.harvard.edu/abs/2012BASI…40..213E
http://adsabs.harvard.edu/abs/2007M%26PS…42.1135J

Only time will now tell, but I hope you’re getting some idea from this how important your
observations have been and are.

The important thing to note is that such events have been observed in supernova ejecta in
early stages but, again, that is complicated by the very complicated ejecta structure.
Here it is simpler and since we have the optical and UV just before this event the
luminosity of the white dwarf and the continuum of the ejecta is known.

Steve
——-

V705 Cas 1993 lyskurven.

DQ Her 1934 lyskurven.

Jeg vender forhåbentlig meget hurtig tilbage med mere specifikke oplysninger om hvad vi
skal kigge efter.

[nightskyDeltager Neutron star]

Så kom der endnu et Astronomers Telegram omkring Nova Del 2013 og de data som Hubble tog i
onsdags med STIS spektrografen.

Bemærkl at amatørgruppen fra ARAS er med hele vejen. En kæmpe indsats med spektrografi
fra dem. Som tidligere nævnt er dette den bedste og mest observerede Nova til dato, takket
være det store amatør netværk.

ARAS database for Nova Del 2013
http://www.astrosurf.com/aras/Aras_DataBase/Novae/Nova-Del-2013.htm

Nightsky2013-09-24 19:06:30

[nightskyDeltager Neutron star]

Q & A

Q:
I artikel 10 om jern og jern linjer i en nova, står der at:

The abundances of the heaviest elements, e.g. Fe and higher, are so high because of internal
nucleosynthesis in the fireball of the expanding envelope of the collapsed star.

Det gav anledning til et spørgsmål hvad denne interne kernesyntese er og hvor den opstår.
Normalt tænker man jo på supernovaer, når der tales om dannelse af tunge grundstoffer.

A:
In answer to your question, it’s related to the nucleosynthesis in a supernova relative
to that in other types of explosions, so just a word on the background.

Following the collapse of a type II (core collapse) supernova and the formation event for
the neutron star, a combination of neutrino emission from the core and the bounce of the
stellar envelope on the newly formed neutron star drive a shock outward. This accelerates
moving toward lower density and the matter is ejected — it reaches the escape velocity
from the neutron star everywhere. This is essentially different than a nova explosion,
the shock here is propagating outward through infalling matter and is powering the
ejection and setting the outward velocity. The temperature is enormous behind the shock
(after it passes), high enough to produce rapid neutron capture and nucleosynthesis.
This, both neutron and proton capture, produce heavy elements of which Fe and related
elements are important products. The rapid capture of neutrons (because the background
number density is very high and the temperatures are as well, hence fast reactions)
builds up neutron rich heavy isotopes (for instance, of the rare earths like Eu).. In a
type Ia, which is not core collapse, the shock produces Fe and Co and Ni as its main
nucleosynthesis. The ejecta are hydrogen poor, unlike Type II, and the reactions are
mainly through alpha and heavier ion captures. The r-process is not as important as for
Type II, and the Ia’s are responsible for much of the Fe in the Galaxy.

The terribly hard problem that’s remained open for decades is: what’s the progenitor of
the Type Ia’s. This is the link to novae for a large part of the community. Where there’s
a white dwarf accreting, if it continues to accumulate mass (the net mass increases even
after explosions), then it may reach the Chandrasekhar mass that is the stability limit
for a degenerate star supported by electrons, and the subsequent collapse forms a neutron
star and ejects the remaining white dwarf envelope with the associated nuclear
processing. This is the physics (or scenario) behind the apparent uniqueness of Ia’s,
that they have about the same absolute brightness. If they all collapse at the same mass
“it’s obvious” they’ll release the same amount of energy. But it’s certainly not clear
that they’ll be as close to standard candles as the Ia’s seem to be.

Q:
Ovenstående svar gav så et nyt spørgsmål omkring dette “shock” skal forståes. Er det en
front som ved en eksplosion som blæser alt omkuld eller skal det opfattes som en storm,
noget der har en udstrækning, eller noget helt andet?

A:
More like a permeable (gennemtrængelig) wall, or a piston; the point of a shock is a
pressure front that moves faster than the sound speed. The matter ahead of the shock
isn’t slowed by pressure waves, it’s swept up in the front as it passes, what happens
when a high velocity car or train passes to the leaves or papers on the ground is a good
analogy. After the train passes, the papers and leaves follow at some slower speed as if
swept up, (which, for a number of reasons, they in fact are).

Q:
Den hvide dværg er sandsynligvis af CO typen. Hvis man søger på nettet for at finde ud af
hvilken type moderstjerne den hvide dværg stammer fra bliver det ikke helt nemt. Der tales
om solmasser under 4 og andre om solmasser under 9 for CO typen.

A:
Whatever the progenitor mass, it’s only coming out of scenarios.

Since we know that CO and ONe novae can also be recurrents in symbiotic stars, I think it
makes relatively little difference now.

Other than saying that the formation of the ONe core requires a higher mass star, with
all the attendant physical differences that may happen in the advanced stages, there is
little that I think can be said with the precision you’re asking. This is a question
mainly for the SN Ia controversy but we don’t know if we need a WD of one or the other
composition since that’s hidden by the nucleosynthesis in the explosion (it sort of wipes
out the memory of the original composition).

Forsat god aften – i morgen tidlig er der finaler ved Cold Hawaii.
http://new.livestream.com/friendsofcoldhawaii/KIA-Cold-Hawaii-PWA-World-Cup-2013

Nightsky2013-09-22 08:37:15

[nightskyDeltager Neutron star]

Hej Frank

Jeg venter selv på et hul mellem skyerne. Vil også prøve at få et spektre af Jacobs stjerne.

Endnu et spændende telegram om Nova Del 2013

Ongoing near-infrared observations of V339 Del (Nova Del 2013)

ATel #5404; D. P. K. Banerjee , N. M. Ashok & Vishal Joshi (1) and Nye Evans (2); (1) –
Physical Research Laboratory, Ahmedabad, India ; (2) Astrophysics Group, Keele University, U. K.
on 20 Sep 2013; 17:22 UT

Credential Certification: Dipankar P.K. Banerjee (orion@prl.res.in)

Subjects: Infra-Red, Cataclysmic Variable, Nova

We report continuing 1.08 to 2.35 micron near-infrared spectroscopy of Nova Del 2013 in the
J,H,K bands with the Mount Abu 1.2 meter telescope (+ PRL Near-Infrared NICMOS3 imager/
Spectrograph). Subsequent to our initial report on August 28.674 UT (ATel #5337), spectra of
the nova have been obtained at regular intervals. The latest spectra of September 16.5 and
19.5 UT are dominated by strong emission lines but the earlier reported strengths of many of
the lines have changed significantly. In the Sept 19.5 spectra, the most striking development
is the fairly rapid increase in the intensity of the He I lines at 2.0581 and 1.0830 micron, a
clear indication that the radiation field is hardening. Hydrogen lines still remain strong viz
H I Paschen-beta and Paschen-gamma, Brackett-gamma and Brackett 10 to 20 in the H band.
However the O I line at 1.1287 micron is by far the strongest line indicating a considerable
contribution from Lyman beta fluorescence. This line has an EW of -378 nm compared to an EW of
-135 nm of the strongest HI line seen viz. Paschen beta. The ratio of OI 1.1287 to OI 1.3164
is approximately 75. The carbon lines reported earlier (ATel #5336, #5337) are still present
but have weakened considerably in a manner consistent with their expected temporal evolution
in the Fe II class of novae (Banerjee & Ashok; BASI, 2012, 40, 243). For e.g. the intensity of
of the CI 1.175 micron line was almost equal to that of Pa beta on 28/29 August; at present it
is ~ 0.17 times the Pa beta line strength.

No molecular first overtone CO emission has been seen in the K band during the entire duration
of our observations. The unidentified 2.0890 micron feature in the K band has also been
present for some time. At the moderate R = 1000 resolution of the spectra, hints of a triple
peaked structure (central peak strongest) are being consistently seen in the HI line profiles,
especially for Br gamma at 2.1656 micron, possibly suggesting a bipolarity in the flow. The
nova has remained bright in the near-IR; we measure J = 5.64, H = 5.71, K = 5.54 on September
10.6 UT (also see ATels #5294, #5317 and #5340 for earlier IR photometric results).

Nightsky2013-09-20 19:37:51

[Frank LarsenModerator Super Nova]

Spændende. Må snart ud og lave nye spektra – ser ud til at der kommer en åbning i næste uge – bare noget p.. at jeg skal til udlandet et par dage.

[nightskyDeltager Neutron star]

Artikel 11 fra Steve Shore.

The “Helium Flash”

Spændende læsning der fortæller en del om hvad vi ser i spektret.

The last spectra from Sep. 18, now show He I 4923, 5876, 6678 (weak, on the Halpha wing)
and 7065, so the ionization is progressing in the ejecta. The [N II]5755 line seems to
have been present as early as Sep. 8 but it’s now not only quite strong, but also shows
the same profile as the other optically thin lines. The He I, in contrast, shows a strong
emission but also possibly (at low resolution) an absorption at moderate velocity. The N III
complex around 4640 has remained essentially unchanged; an indication that the UV is
still marginally thick, but if it’s not too much of a stretch it looks like He II 4686
may be present.

Her har jeg hentet en optagelses ned fra d. 18. sep. lavet af J. Edlin.
En fantastisk database med flere hundrede optagelser er tilgængelig her
http://www.astrosurf.com/aras/Aras_DataBase/Novae/Nova-Del-2013.htm
lige til at åbne i Visual Spec og lave sine egne analyser. Det bliver ikke bedre.

If you look now at the spectra you’ll see one of the effects I was discussing earlier,
something that shows up contrasting the Balmer and He I lines. The [N II] and [O I]
aren’t only forbidden, they’re also ground state transitions. The others are from excited
states which means their populations are determined by recombination and photons in the
UV that populate these levels. For example, something I should have mentioned earlier,
the Lyman series is responsible for the occupation of the Balmer line levels, Ly alpha
couples to the n=2 state of hydrogen, but Ly beta, because its upper state is n=3 — if
optically thick –, powers some (or most) of the emission on H alpha (n=3 -> n=2).

Now, again, think like a photon. If the ejecta are not spherical, these photons can leak
out both through the main Balmer lines and also from the sort of surface that isn’t a
sphere. You see that in the highest velocity parts of the Balmer line profiles that are
stronger than anything (by contrast) on the other lines that are intrinsically weaker
(and also from much less abundant species). So the peaks have the same velocities but the
relative contrast in densities between different parts of the ejecta you see more clearly
in the Balmer lines than the others. The He I (and eventually He II) form in the inner
ejecta so they have less visible “horns” since the line is weak from the outer ejecta.

These last spectra, at R ~ 1000, show the value of continuing the lower resolution work.
Don’t get frustrated that the details may not be as evident. If youhave a resolution of
~100 km/s that’s a good coverage of lines that spread over a few thousand km/s, remember
that much of the UV work was based on IUE spectra with the same (or lower) resolution!
For example, it looks now like the absorption on H delta is displaced from the line at
about -2000 km/s, as it has been in other novae at this stage. But this is so far the
only line that seems to show this (it can’t be a blend with 4076 since that’s a
doublet and has a high ionization energy, it’s something seen in shocks of high velocity
around protostellar jets, for instance, and in supernova remnants along with 6713,
6730) but here the absorption seems real.

A few other diagnostics are important, in part because they’re not yet seen. Neither [Fe VII]
6086 nor [Fe X] 6376 are present, so if there is any XR emissionn irradiating the ejecta
it is still being absorbed by so much cooler mater in the inner part that it can’t yet
ionize the regions of lower density in the periphery. The [NO III] 4363, 4959, 5007 lines
are not there yet, again a strong pointer to the still high opacity in the middle and far UV.
Yet the O I 8446 remains strong, so there is a very strong pumping still by Ly alpha of
the O I 1302 resonance line.

I hope the emphasis on small things won’t mean you’re staring to lose the big picture.
The reason for all these details is to give you an idea of how to diagnose this
particularly ill patient. Like a prescription in Hippocrates or Galen, you look at all
the symptoms before making a diagnosis. Look at which lines are visible noting the
ionization state. At this stage it will be more important than which lines in a specific
ion are there. Look at how the line profiles change with that ionization energy, this is
the tomography of the body.

steve

PS:
Der arbejdes lige nu med HST/STIS optagelserne af Nova Del 2013. Det bliver spændende om
der kommer nyt i aften eller i morgen. De bliver postet straks jeg modtager dem.

Nightsky2013-09-21 23:13:35

[nightskyDeltager Neutron star]

Artikel 10 fra Steve Shore

Iron and Iron lines in nova

Remember the original solar mixture, from the ’20s, was mainly heavy elements because
the Balmer lines are so weak. Payne (later Payne-Gaposchkin, yes, her!) demonstrated
that requiring ionization equilibrium as a function of density and temperature together
with hydrostatic and thermal balance produces a spectrum that changes appearance even
with constant abundance. Novae are strange because they pass through so many regimes of
temperature and density that, unlike a star, vary on short timescale (hence nova ejecta
NEVER resemble a stellar atmosphere and rarely a wind). The Fe isn’t a product of any
nucleosynthesis during the nova, any more than it is in a red giant compared to a main
sequence hot star. It’s an effect of the ionization and line formation. The lines are
relatively more intense because they arise from a dominant ion. For instance, was the
temperature as high as during the fireball, you’d see only He I and Balmer lines, it’s
the same ejecta you were observing a month ago but the temperature and density conditions
are very different now.

Cecilia Payne-Gaposchkin

For the temperatures reached in the thermonuclear runaway, less than 0.3-0.5 GK (a few
100 keV), you don’t obtain free neutrons (for the heaviest elements, as in r-process),
you don’t have enough time for s-process, and the explosion isn’t the result of gravitational
collapse so the energies available are far lower and you have reactions of charged particles
that run similarly to a stellar interior. To get to iron and the heaviest elements
requires continued special conditions that break out of the A < 40 region (e.g. calcium),
and that doesn’t occur.

This term “Fe II” nova is, again, just a way of saying “still optically thick and cool”.
The temperature of the ejecta drops from expansion, recombination leads to a more neutral
medium, and the radiation field is shifted to the UV and absorbed there to be re-emitted
in the visible. At the same time the excitation by collisions becomes less efficient for
the higher states, it’s linked to the kinetic (actual thermal motion) temperature of the
ambient electrons, and the lower temperature also means recombination’s are more effective
in reducing the ionization. So the combination leaves the metal lines, which are present
in two ionization stages (at least) and come from about a dozen possible species with
literally millions of possible exciting coupled transitions, dominate the spectrum. The
same sort of state change happens in a supernova expansion but at a different rate and is
more complicated because of the radioactive material from the nucleosynthesis and the
stronger shock (not to mention more matter).

The abundances of the heaviest elements, e.g. Fe and higher, are so high because of
internal nucleosynthesis in the fireball of the expanding envelope of the collapsed star.

Again, it’s important to emphasize that the processes we see for line formation in a nova
are like those in a star but in a dynamic medium so the complications result from the
interplay of velocity differences and total abundances. The heavy elements, even at 10^-5
the abundance of H and He, are still the main contributors to the opacity in any cosmic
plasma with solar abundances or the like in the temperature range below about 100 kK.

Low resolution

And for the low resolution, I would always say yes as a check on the overall behaviour of
the continuum and also for the checks against photometry…

Low resolution work is more important if it can be calibrated, the high resolution
spectra are also not going to be feasible for as long as low so the cross calibration is
very important while both can be obtained.

Nogle links:

s-process

The s-process or slow-neutron-capture-process is a nucleosynthesis process that occurs at
relatively low neutron density and intermediate temperature conditions in stars.
http://en.wikipedia.org/wiki/S-process

r-process

The r-process is a nucleosynthesis process, that occurs in core-collapse supernovae, and
is responsible for the creation of approximately half of the neutron-rich atomic nuclei
heavier than iron..
http://en.wikipedia.org/wiki/R-process

Nightsky2013-09-21 23:14:27