V339 Del, NYT 29/10 – billeder af novaens ildkugle

[nightskyDeltager Neutron star]

Frank, absorption, har fået svar. Desuden også svar på hvorfor dette støv måske er så vigtigt. Det er store spørgsmål som forsøges besvaret. Måske en mere fyldestgørende gennemgang ved næste sektionsmøde? Det er ekstremt spændende og rækker langt ud over selve novaen.

Ulrik. Den er klassificeret som en nova så vidt jeg ved. Her er lidt om hvad den var før udbruddet:
Nova is identical to the blue star USNO-B1.0 1107-0509795
(R.A.=20h23m30s.713, Decl.=+20o46’03”.97, B1=17.20, R1=17.45, B2=17.39,
R2=17.74) and ultraviolet source GALEX J202330.7+204603
(NUV=17.88+/-0.04)

Nightsky2013-08-25 02:18:45

[nightskyDeltager Neutron star]

Artikel 4 fra Steven Shore
—————————–

To this point I’ve concentrated on the optically thick stage because, well, that’s where
we are. But Francois suggested discussing the forbidden lines so first a bit of atomic
physics in a cosmic context.

Let’s concentrate on atomic lines since the molecular species (in novae) are few. The
environment is usually too hot (both in a kinetic sense and that the radiation is too
hard) for their formation and survival. Uniquely, during the opaque stage when the gas
temperature can fall below 5000 K, some radicals I’ve mentioned (e.g. CO, CN) can both
form and remain stable. But in general, most emission lines from stellar sources are
atomic. As a general statement, light is emitted when an electron (or more than one if
they’re strongly coupled) transitions from one state to another. A state is a specific
energy level that has an associated spin and orbital angular momentum — or rather a
specific symmetry. You know these from orbitals in chemistry.

If the electron distribution changes, it does so by emitting (or absorbing) a photon of
the same energy as the *difference* in the energies (to be precise, divided by Planck’s
constant). Only the ground state, the most tightly bound energy that is usually taken as
the zero point of reference, is stationary. Any excited energy level ultimately decays —
a transition to a lower state occurs in a finite time. The symmetries are the collective
result of all the electrons in the atom (or ion), they interact electrostatically because
they are charged and at different distances from the nucleus (hence from each other),
they have spins that induce a magnetic moment (they behave like dipoles and combine
according to their relative orientations (in the nuclear electrostatic field, spins are
“up” or “down”) and they also combine depending on their orbital angular momentum (for
this read the angular pattern of the collective electron “cloud”). Different approximations
have been developed to describe these couplings, nd this is the classification of each
energy level you’ll find in, say, the NIST tables

http://physics.nist.gov/PhysRefData/ASD/lines_form.html

Within a coupling scheme, not all levels can directly couple to others; certain so-called
transition rules are obeyed. For example, for hydrogen, the angular momentum must change
by one unit in any jump between levels, so there are states that cannot be connected by
what are called permitted (electron dipole) jumps. If this sounds technical, perhaps it’s
easier to think of the analogy with an antenna.

A dipole has a particular radiation pattern. The same for a so-called permitted
transition. These are the most probably jumps between tw levels, and have the highest
rate (highest transition probability); for hydrogen, the rat is about 10^8 – 10^9 per
second (implying that an excited state statistically lasts for a few nanoseconds before
decaying). These will have different intrinsic strengths depending on how the electric
dipole changes in the transition.

Any environmental disturbance, say a collision with a background charged particle, is an
impulsively varying electric field hat induces a transition without emitting a photon.
Since these occur randomly, the lifetime has a distribution and is reduced relative to
its purely radioactive decay. Thus, and the collision can also excite the electron if the
perturbing particle has sufficient energy, the excitation and de-excitation couple the
internal energy states to the background.

This is what thermal equilibrium means on the microscopic level, the populations (the
probability of the electrons being in any state) depends only on the local temperature
that determines the energy distribution of the background charged particles (and neutrals,
for that matter). For example, an absorption can occur but if before the state decays its
hit by a perturber, it de-excited without further emission and the gas is heated, this is
the absorption process and happens when the gas is dense. The photons are therefore
trapped within the medium; in a stellar or planetary atmosphere this means the spectrum
will show absorption that depends on the number of atoms along a line of sight. In a low
density gas, re-emission can occur because the level can decay freely but because the
emission pattern is not only along the line of sight there are fewer photons arrive in
your direction so the “missing” light will appear as an absorption feature.

The difference is that this scattering process doesn’t heat the gas and the process
conserves the number of photons so is coherent (hence polarized). The best example of
this is the blue of the daytime sky (although that is a molecular scattering process the
process is analogous). Both absorption and scattering occur during the first optically
thick stage of the expansion of the nova ejecta.

But there are less probable transitions, those that according to coupling rules cannot
happen by emission/absorption in a dipole mode.

These are the so-called forbidden lines because they can’t be connected by an electric
dipole transition. These normally “thermalize”, their lifetimes are so long that
collisions always (except for very low densities) provoke the decay. The rate of
collision (density dependent) compared to the decay rate (intrinsic) governs whether
these lines appear. They don’t in the laboratory except under very extreme conditions
(they have lifetimes as long as seconds or more, in air in your room the collision times
are nanoseconds) but in hot, low density regions (nebulae, or the expanded ejecta of
novae and supernovae) they appear. The O I 6300 line, seen in aurora and the upper
atmosphere of planets, is a good example. It isn’t seen in the lower regions because its
lifetime is about 180 sec.

But if the density falls below 10^5 /cm^3, then O I can emit in this line. The same holds
for higher ions and the demonstration that a region has a low density is the presence of
these highly improbable lines in the emission spectrum.

Another feature is that there are a lot of these, and from any excited state there will
frequently be other than permitted transitions possible. Once the ejecta density drops
far enough, the presence of the central white dwarf (that provided the radiation
necessary to excite the ions in the first place) guarantees they will be observed.

Think of planetary nebular, the part that’s emitting in say [O III] or [N II] is the low
density region exposed to the ultraviolet part of the central star’s spectrum that is
therefore excited by absorption and radiatively de-excited.

These lines are ideal diagnostic signatures of the physical conditions in the ejecta. If
you see them at all, the density must be low regardless of the excitation source. The
hotter (harder) the spectrum of the central star, the higher the ionization of the outer
parts of the ejecta and the stronger (relatively) the forbidden lines. This is the stage
that follows the optically thick phase of the expansion. The transitions are transparent
(no photon trapping) so you see every piece of the ejecta that radiates (is illuminated
and has a high enough column density to produce observable emission along your line of
sight). Since each piece of the ejecta has a outward velocity that depends on its
distance, and the differences are large, the different parts contribute to different
wavelength intervals around the line centre and the line profile is the projection of the
outward motion along the line of sight weighted by the amount of gas at that distance
from the central white dwarf.

Now we come to the heart of the matter, what you see in the profiles.

Take a sphere whose v
elocity is larger at its periphery than interior but whose density
is lower. The highest velocity material will produce less emission so the wings of the
profile will be fainter than the central (slower moving) part. If you have a cone (as in
the resolved HR Del 967 ejecta, the images from HST are impressive, with the emission
strongest on the boundaries, you get a different profile (one with peaks at high velocity
and a deficit in the lower radial velocity). These saddle shaped profiles are seen when
the ejecta turn transparent. Remember, each parcel of gas emits a photon in the rest
frame of the ejecta but you, as an observer, see that Doppler shifted by the projection
of that parcels outward velocity along your line of sight. In the sense, the line profile
in the “nebular” stage is actually a two dimensional projection of the three dimensional
ejecta.

Since the forbidden lines are so intrinsically weak, and the densities so low, the
comparison between line profiles of different ions of the same elect “maps” the 3D
structure of the ejecta.

As an example, think o two lines, [N II] 5755 A and [Ca V] 5303. The latter is more
ionized (requiring a higher energy) hence traces the “hottest” (most ionized gas. The N
II is, instead, barely ionized. If these two have different profiles it indicates either
different abundance distributions within the ejecta, o different excitation conditions,
or both. Comparing, say, [N II] and [O II] you can get the N/O ratio, the same for any
pair (set) of lines provided the local conditions and ionization energies are about the
same. Otherwise corrections must be applied other measurements: you need a way to
estimate what fraction of an element you don’t see because the higher ions don’t radiate
is the visible. So low resolution is needed to know what ensemble of lines is present,
and high resolution to see the individual profiles and compare them to obtain the
densities, masses of the ejecta, and some idea of what the structure is (knots,
filaments).

If you’ve survived to this state (I hope with some pleasure) you’ll see that the nebular
spectrum (the pure emission lines with both permitted and forbidden contributors) is the
only stage at which abundances can be determined unambiguously since it’s only in this
stage that you see all of the gas. For Nova Dl 2013, this will likely occur in about a
month, or at least start, for the CNO ions; for F and related metals it happens earlier
because of the absorption and excitation in the UV.

The state of the gas is given by which ions are present, and the ratios of the lines
gives densities and temperatures. That’s again because the states decay with different
rates depending on their couplings. Absorption in the UV followed by emission in the
visible (fluorescence, the same thing that happens in a kitchen bulb — the UV lines
emitted by atoms inside the tube and excited by an electric current is absorbed by an
opaque paint that re-radiates the energy in the visible). This is the origin of the heavy
metal emission lines even in the so-called iron curtain stage and fireball, the lines are
not ever self-absorbing (photon trapping). A density and temperature diagnostic comes
from the O III lines [O III]4636/([O III] 4959 + [O III] 5007), top line has a transition
rate of about 2/sec while the bottom pair have 0.02/s. As the density increases the pair
decrease relative to the 4363 whose decay goes to the upper state of the 4959,5007 pair.

So if this makes sense, which I hope, the next step is understanding why the ionization
varies in the ejecta but that’s comparatively easy.

Every ionization produces a charged pair. The higher the density the faster the matter
recombines. The lower the UV the faster recombination (lower ionization/removal
rate)hence, while the source is active the high ions are more in the inner part of the
ejecta but that zone expands as the density drops. If he central WD turns off, then the
peripheral layers recombine more slowly than the inner portions and remain more ionized.
In the ISM, after a supernova, this is a fossil H II region. In novae, it’s the state
once the X-ray source extinguishes.

—-
steve

[ulrik Planet]

Tak for oplysningen Lars !

Svært stof men spændende at følge med i. Og ikke mindst hvad flere af jer avanceret amatører kan bidrage med. Godt gået !

Ulrik

[nightskyDeltager Neutron star]

Ulrik, jeg vil lige gøre opmærksom på at jeg ikke bidrager med data som bruges efterfølgende.
Jeg kan højest gøre opmærksom på en ændring, som andre amatører med deres mere
avancerede instrumenter så kan måle på.

PT. har jeg kun min Star Analyser kørende, som kan indikationer omkring ændringer i flux på
Balmer seriens emission (mine observationer er lige bekræftet) og ændringer i kontinuum.

Forhåbentlig kommer der bedre spektrografer i gang snarest.

[ulrik Planet]

Det er forstået Lars !

Ellers god observationslyst !

Ulrik

[nightskyDeltager Neutron star]

Steven Shore har sendt endnu en artikel til os. Den forklarer lidt om hvad hvorfor eksplosion sker,
og hvad der vi ser nu.

*******************

Even at low resolution, many of you have caught the sight of narrow absorption features
at high velocity, that look like P Cygni profiles, on the metallic lines and also on He I.

As I’d mentioned earlier, the explosion is initiated on the white dwarf by the pile-up of
garbage from the companion, like a bad landfill that ignites. In fact, a silo explosion
(a grain storage facility) isn’t very different; the matter is compressed and heated to
the point where a chemical reaction starts that is fueled by the combustible material.
The thermonuclear reactions, mainly involving CNO processed by protons (hydrogen) from
the accreted material, triggers a mixing process at the interface between the accumulated
layer and the envelope of the white dwarf.

This is the part we can — so far — only model. The signature of the process should
remain in the explosion since the transformation from a flame to a shock is very fast and
unstable, leaving behind matter yet unburned and throwing off the outermost layers at
supersonic speeds. Because the expansion is above the speed of sound, pressure is
irrelevant for the structures that might be imposed and they remain preserved in the
flow. In fact, you’ve seen something analogous to this in everyday life. (the lovely
thing about hydrodynamics is that you can actually, physically, compare flows of very
different kinds when the processes are otherwise the same, a similarity notion). If
you’ve ever seen a waterfall or cascade, this will be familiar. Until the edge, the water
is flowing slower than the speed of a gravity waver (in other words, a water wave). But
at the edge it falls and decouples from any excitation, it’s in freefall and the bits
that start at a higher speed arrive ahead of those that were nearer rest at the start.
But the sheet of water preserves all of the structure imposed at the last point of
contact before the edge, the filaments and knots you follow downward that give a sense of
the speed of the fall. That’s what we see in the ejecta and that’s why these discrete
features, those now appearing, are tremendously important.

In the photographic era such lines were noted as “absorption systems” that appeared at
different stages of the light curve on the metallic lines. These were difficult to track,
often overlapping and highly subjective since the spectra were often poorly calibrated or
not at all, and the zero levels were poorly defined. All of the observers before the ’70s
clearly knew this but some were amazingly skilled at recognizing the different absorption
systems (and these were likely real, the most careful could distinguish multiple
components reliably like McLaughlin — who should be one of your heroes – and Payne-
Gaposchkin).

On the Fe-group lines, and the Balmer and neutral helium lines, these also arise from the
complex interconnections between transitions I’d mentioned earlier for the optically
thick stage. BUT the Na I lines — the D feature — is essentially different. It’s one of
very few ground state (resonance) transitions in the whole optical spectrum that isn’t a
forbidden transition (intrinsically very weak). In fact, this is one of the strongest
lines in the spectrum and also neutral. The Ca II H and K doublet is another but it’s an
ion. The K I line is in a terrible part of the spectrum, often (in many of your spectra,
for instance, inevitably!) hidden under a curtain of atmospheric water and molecular
lines and hence unusable. The Na I line is, instead, the unique tracer of the neutral
medium and the features that have now appeared on the D1, D2 components (together) are at
an intermediate velocity even with respect to the Fe II and Balmer lines. In other novae,
especially the work we’ve just finished on T Pyx (an old friend of some of you) the
velocities are intermediate but the same as we see in the later stages, more than a year
later the same feature is still showing up in other ions. This means the structures, the
density enhancements i the ejecta, are actually not moving with respect to the other gas
in velocity and expanding as a frozen-in feature, just like the waterfall. The striking
thing is that the velocities are intermediate, not the innermost of the ejecta and far
lower than the outermost (in other words, these are sort of imbedded in the ejecta and
“persistent”). Since the expansion is supersonic, they don’t “grow” spontaneously within
the ejecta – they have to be imposed on the expelled matter at the time of ejection.

Nova T Pyx

This point back to the explosion site itself, buried at the start under the mass of the
accreted layer. In T Pyx the broader narrow features (what a description, no?) dissolve
into an ensemble of filaments of widths no more than 10’s of km/s within a broader
envelope of a few hundred km/s but still far lower than the several thousand km/s of the expansion.

That these are seen in a certain stage is the result, it seems, of a recombination wave
I’d discussed earlier. But the most important feature is that being resonance lines from
a neutral species, these features trace the progress of the recombination better than any
metallic or Balmer lines. Now, in the last spectra sent by Christian Buil, you see the
two Na I feature but, if you displace to the first spectrum and use He I 876 (that then
disappeared after Aug. 15) you’ll see that He I also now has a detached feature. These
absorb at a specific position (radial position) in the ejecta and they have to be large
or we wouldn’t see the absorption.

In V705 Cas 1994 they formed as soon as the Na I emission peak strengthened. The same in
T Pyx. For V959 Mon we don’t know because it was hidden, and few other novae have been
caught at high time coverage (and also higher resolution, R > 1000) to make the evolution
clear. And taking the Ca II to Na I ratio at each component is a direct measure of the
ionization fraction (not just abundance since the Ca/Na elemental ratio doesn’t change
while Ca II/Na I will. As the wave progressed you will see different features appear on
different lines but always within the same intervals.

Now a quick word for the moment about CN and why this is so important.

One paper (!!) by Wilson and Merrill

http://adsabs.harvard.edu/abs/1935PASP…47…53W

reported this line and only in DQ Her. But they also discussed the NA I in another paper
and Payne-Gaposchkin discussed this also. The molecule, CN, is amazingly stable for a radical
(no, not a political comment). It has a high dissociation energy and can remain in stellar
atmospheres to hotter values than the Sun (> 5800 K). The same for CH and CO but we don’t
see those in the optical; they’ve been detected in the IR. The usual molecule is CO that
consumes almost all of the C w=o if that channel is saturated it means the C/O ratio is
high enough for other organics and hydrocarbons to form. The others, often quite complex,
are seen in winds from highly evolved stars. And the higher the C abundance the more is
available from which the solid phase — dust — can condense. Any isotopic anomalies
remaining from the nuclear burning will also remain locked in the dust so after a while
drifting through the Galaxy (shades of the Hitchhiker’s Guide, no?) they can be
incorporated through passage in a molecular cloud, into a star. The dust forms in a way
we don’t well understood but it is likely that molecular formation and grow
th is a signal
of the right environment for the appearance of grains. This may be purely chemical,
homogeneous condensation or nucleation”, or it may be induced (sorry, some of my own
work) but whatever the mechanism, it happens.

Therefore we can witness the dust formation process in a well constrained event and —
Holy Grail though it is — figure out what triggers the dust formation. Other molecules
have been detected in the IR, CO for example, but nothing from the cold matter in the
ground state.

In Nova del 2013, it seems that the CN has not appeared but it may yet and there’s every
reason to continue at all resolutions.

I’ll repeat, ad nauseum, that what you are producing is a legacy of depth and range
we’ve never had before. PLEASE don’t get discouraged or tired. I’m writing this as the
dawn breaks over Pisa thinking of all of you and I promise to explain more (as I’d
previously promised) about the origin of the binaries in this state.

Spektroskopi amatørerne bl.a. i på ARAS kan vist ikke få større anerkendelse.

http://adsabs.harvard.edu/abs/1935PASP…47…53W

Nightsky2013-08-26 18:17:02

[nightskyDeltager Neutron star]

Opdatering med egne observationer.

Styrken stiger stadig på Balmer linjerne. F.eks. er der en tydelig stigning fra d. 25 til 26. aug.

Sammenligning over den fulde flux fra natten mellem 14/15 til d. 26. aug. Første gang jeg nogen
sinde har fået så komplet et sæt data.

Nedenfor er skalaen ændret så vi bedre ser ændringerne. Det er ganske tydeligt at udover stigningen
i brint emission, kommer der også masser af absorptionslinjer. Denne fase med nye absorptionslinjer,
betegnes med “Iron Curtain”

På den sidste nedenfor ser vi også fint at der kommer nogle andre emissionslinjer.

I morgen kan jeg have kigget på nogle dybe optagelser af kontinuum.

Nightsky2013-08-28 21:36:21

[Frank LarsenModerator Super Nova]

Flotte optagelser Lars!! Håber vejret holder så jeg kan få taget et spektre i aften. var forhindret i går.

[nightskyDeltager Neutron star]

Mange tak Frank. Det vil være spændende hvis du får nogle spektroskopi data.

Vejret er jo fint derovre, klar himmel da jeg hoppede flyveren kl. 20.00

[nightskyDeltager Neutron star]

Steve med det seneste omkring Nova Del 2013 udviklingen. En del om de linjer vi nu ser på
vore optagelser.

Endnu engang tak til Steve.

There’s been interest in some explanation of what developments are yet to come so here
are a few notes for the next week or so.

First, a word of advice. In thinking about what your spectra are telling you, it’s best
to “think like a photon”. By that I mean think about what a photon traversing a medium,
in this case the ejecta, will encounter and what will happen. In fact, this is the origin
of the Monte Carlo method, a technique for simulating the passage of a particle through a
very complex environment, subject to a wide range of processes and a wide range of
densities and states. You couldn’t find a better description for the ejecta. Recall
that the inner and outer parts, even were this a wind, have different outward velocities.
So a photon emitted in one place sees the rest of the surrounding gas moving — on
macroscopic scales — at different velocities and therefore differently Doppler shifted.
So if a photon is emitted in the outer parts, where the density is low, it most probably
escapes. If, instead, it’s emitted in the inner part, where the density is higher, it will
quite literally bounce around in both space and frequency (absorbed in a line center,
emitted in a line wing, encountering another atom in the line core, perhaps, and being
re-emitted there, etc). So in the initial stages, where the photons are actually from
the hot gas itself, the thinning of the outer regions is like the expansion of a wind and
the photosphere (an intrinsic one) moves inward. You see this in some of the film version
of the spectral sequences some of you have produced (especially for H-alpha). At first
the P Cyg absorption seems to move inward as the outer layers become optically thin, and
then the absorption disappears on that line (leaving a sort of dent) as even the
approaching material becomes transparent. The higher Balmer lines, on the other hand,
have a smaller emission/absorption ratio (the emission is formed further in) and the
absorption is progressively stronger. At the same time, you see with increasing clarity
and strength the structure of the whole ejecta, the various emission peaks, which signal
the thinning of the material at the highest distances and velocities.

But don’t forget the poor remaining white dwarf. It’s now in the supersoft phase,
although we don’t yet see that, burning the residual material from the explosion in a
source that reaches several 100,000’s K (of order 0.05-0.1 keV). The nuclear source is
deep, not at the surface, and has a photosphere of its own that depends on the newly
established structure of the envelope of the WD. This is inside the ejecta, at this
stage (as of 1 Sept) we don’t yet see that directly.

But we see another, important effect: the ionization produced by this source is gradually
advancing outward in the ejecta from its base as the ejecta thin and the photosphere
moves inward. This is the so-called “lifting of the Iron curtain” that’s happening in
the UV and the cause of the decline in the optical. Progressively more of the photons
can escape in the UV without being degraded through optical or IR transitions and the
continuum temperature increases as the two oppositely directed “fronts” approach. The
individual transitions from the ground state of neutral and low ions are in the UV and
some of them remain opaque although the continuum is increasing sufficiently to power
emission lines in the optical. Oxygen, in the form of O I, is the best example. The
[OI]6364 and 6300 lines are connected to the O I 1302, 1304 resonance lines. The latter
are still thick, so the photons knock around and finally emerge through “open channels”,
e.g. 8446 and the two forbidden lines. Their presence indicates the density is finally
low enough at the photospheric depth that the emission from forbidden lines is no longer
collisionally suppressed. The transition is abrupt in the optical, hence the term “flash”
used by the early observers, because when the right optical depth is hit, the transition
is almost instantaneous since the emission becomes local. The [O I] line widths, you
will have noticed, are lower than the wings of the Balmer lines so this is from the inner
parts. The O I 8446 was visible for a longer time. In the UV, we would see absorption
at O I 1302,1304 but that will gradually give way to P Cyg and then emission.

Something else to remember is that different elements ionize at different energies.
Oxygen, for instance, is slightly more bound than H, so the Balmer lines will be strong
when the O is still completely neutral. Once the O (and N) starts ionizing, they also
contribute recombination lines that can’t decay to the ground state directly because of
the blockage of the UV channels so they emerge where they can, at the exits marked “6300”
and “6364” and so on. The same for the C I and C II, and the N II lines. We are not yet
at the point where the N III 4640 lines appear but they will in due course.

The Fe II lines are now turning completely into emission as the peak moves toward Fe III
and higher and the UV lines turn transparent. The Fe-curtain will, once the ionization
reaches Fe^+3, disappear since that ion (Fe IV) has very few transitions in the part of
the spectrum where the UV is strongest. All of this is powering the decline of the light
curve and is what “the founders” didn’t suspect: the changes in the UV from the light
curve are timed to appearances of specific ions and transitions because the continuum
temperature continually changes, moving toward stronger UV and even XR, while the optical
is a passive responding medium. When the Lyman series turns transparent, and becomes
recombination dominated, the P Cyg profile disappears. The same for the He I lines, they
will reappear along with He II and other higher ions as the opacity in the UV drops.
Once the two fronts meet, that’s the nebular stage: the moment when the spectrum turns to
emission, we see completely through it, and the line profiles all look basically the
same. I say “basically” because density and structural differences leave their signature
on individual lines depending on their transition probabilities (forbidden or permitted,
as discussed a while back).

The nebular stage is a complicated period and very sensitive to the specifics of the
explosion. If the ejecta are spherical and smooth, all profiles will be basically the
same but differ in width because of their “weighted depth of line formation” (in other
words, recombination line strengths depend on density so the inner part always
contributes more, but it also depends on where in the ejecta a specific ion appears).
All of this changes quantitatively for nonspherical explosions, but not qualitatively.
The strength and velocities are those we see projected along a line of sight through the
expanding medium.

I apologize if this is staring to get heavy, it’s not intended. You have here a problem
of photons (motorcycles) weaving their way through traffic (cars, trucks) whose speeds
depend on where they are in the lane of traffic. If the ejecta are spherical the only
escape is along the direction of the flow. If aspherical, there’s a way out and free
escape by swerving to the side. This is something we’re just starting to deal with in
detail, and it’s your work that will illuminate it even more clearly for this
prototypical nova.

And as a last comment, one on the intensities/fluxes. In the next weeks, as the ejecta
change ionization and approach the sate of freeze-out (when the recombination’s are
independent of the WD illumination and depend only on
the rate of expansion), we will see
how structured the ejecta really are, the density and ionization stratification, and the
abundance inhomogenities. The absolute fluxes are the key, they tell you how much energy
is in each transition and therefore the number of radiating atoms. It seems, for
instance, that a few days ago H-alpha alone accounted for almost 8000 L_sun if the
distance is 5 kpc (less as1/D^2 depending on the distance). From this we’ll have a
first estimate of the ejecta mass, one of the key unknowns in any explosion and the
pointer to the conditions at the outburst. The other is that there is structure here in
the ejecta, you’ve already seen that in emission and absorption, and as different ions
appear that will link to the central engine.

steve

Nightsky2013-09-08 18:49:08

[nightskyDeltager Neutron star]

Steve har kigget på data fra NOT teleskopet og her er nogle spektra. Jeg har spurgt lidt
ind til hvorledes man kunne sige noget om hvordan materialet som er kastet ud (ejecta) er formet.
Kugleformet, ej kugleformet osv.

Han har kørt en model igennem computeren og fået resultatet nedenfor frem og det sammenholder
han så med ee faktiske data fra NOT teleskopet. I mine “amatør” øjne ser det jo ud som om ?????????

Modellen

Sammenligning med data fra NOT d. 29. aug.

Jeg har også rodet noget med kalibrering af data, og som eksempel har jeg dette fra NOT

…the first shows the reduction of the standard star, the second is the application
of the calibration to Nova Del 2013.

Lige nu hænger jeg stadig på med det alleryderste af neglene og tror jeg forstår hvad der skrives.
Men for den da jeg savner en astronomiforening hvor man bl.a. kunne tage lidt tungere emner op.

************************************
Se lige denne bemærkning, simpelthen imponerende:

Steve har han kigget på de data amatør astronomerne har samlet, specielt ARAS gruppen.

“I just did my first pass calibration and I’m checking those in the
database. This is really a spectacular pass ahead for the
group, something that will revolutionize the whole damned field!”

Amatør spektroskopi har virkelig en fremtid og kan bidrage signifikant med ny viden.
De data som er samlet ind matcher fuldt ud professionelle data eller mere.

En skam de super gode spektrografer som jeg ved enkelte danske amatører har IKKE bliver brugt.
Her kunne de virkelig bidrage med data.

Nightsky2013-09-03 01:24:06

[Frank LarsenModerator Super Nova]

Lars,

Spændende.

Tak for opsangen – min chef og min datter er desværre ikke enig i at jeg skal droppe søvnen ;(

MEN jeg forventer at du sidder klistret fast på min højre hånd fredag og lørdag nat i weekenden.
Jeg har en og KUN en dagsorden her til MAF starparty. Spektroskopi og fotometri på NovaDEL 2013.
Du er hermed udvalgt til at hjælpe mig.

Jeg har brugt de sidste mange aftener til at sikre mig at udstyret spiller. Optagelsen i sidste uge var hastet igennem efter at spektrografen har ligget adskilt.
Håber jeg kan få hentet LXD75 hos Lars M i morgen så jeg kan få sat to kikkerter op på den inden afrejse – ville helst have en EQ6 som tracker noget bedre.
Skal også liige have renset akromaten som bliver hoved instrument.

[nightskyDeltager Neutron star]

Selv tak Frank.

Desværre kommer jeg ikke – en eller olding runder et halvt århundrede om fredagen.

[nightskyDeltager Neutron star]

Lidt Q & A med Steven Shore – Nogle spørgsmål fra jeg selv og et par andre DK amatører

Now to your questions.

Q: We have looked at your model for the ejecta shape and the line profiles from
NOT you compare with the model. At first it’s looks like a reasonable match, but when looked
at in more detail, maybe there’s more to it. On both wings on the emission line there’s
absorption features that’s not in the model. These absorption lines, should they be considered
when evaluating the model?

A: The models are Monte Carlo simulations of the geometry and emissivity. The
description is in Paper II on T Pyx and Paper I on Mon 2012

http://arxiv.org/abs/1211.3453
http://arxiv.org/pdf/1211.3453v1 (side 6)

http://arxiv.org/abs/1303.0404
http://arxiv.org/pdf/1303.0404v1 (side 10-11)

They’re not purely qualitative but because they’re generic profiles expected for specific
emissivities they’re not to be interpreted without reference to the ionization state. In
this case it’s simple because the individual line profiles are optically thin. I’m
enclosing the comparison with O I in two lines, just as an indication.

The parameters are:

vmax (km/s); (density exponent); )(line exponent);
theta_out,theta_in,dr,prolateness,inclination

In this case, for a change, I used an inclination of 45 degrees to the line of sight. The
angles and some other parameters are different from the first pass model because this is
an illustration of some of the inherent uncertainties in this kind of model. You can’t
derive TOO much quantitatively. The “image” corresponding to these line profiles is also
produced by the code and I’m enclosing that for your amusement.

It’s for precisely the parameters listed.

Den omtalte IDL kode:
; IDL Version 3.6 (vms alpha)
; Journal File for PALADN::SSHORE
; Working directory: DKB500:[SSHORE.GHRSIDL]
; Date: Sun Jan 20 23:41:04 2002

plot,x,b*wt,psym=10,charsiz=1.5
!xtitle=’!4D!8v!3 (km/s)!3 (MBM40 !U12!D!NCO)
plot,x,b*wt,psym=10,charsiz=1.5
set_plot,’ps
plot,x,b*wt,psym=10,charsiz=1.5
device,/close
$laser idl.ps
$ftp avatar.kennesaw.edu
$laser idl.ps
$rename idl.ps mbm40-pdf-apj.ps
diff= small(10:109,10:135)-ss(10:109,10:135)
x=findgen(101)*0.01-0.5
d=diff
i=sort(d)
bins,d(i),d(i)*0+1,x,x*0+0.01,b,ws,wt

Q: The bipolar shape of the ejecta in your model looks very much what I/we would
expect from looking/observing planetary nebulas. When we started discussing details, we
didn’t have any ideas about how/why this shape occurs.

Eksempel på formen:

V445 Puppis

A: As William of Baskerville says in “The Name of the Rose”, “If I knew the answer
to that, Adso, I’d be teaching theology in Paris”. I don’t know. This is something that
has been a deep concern of mine now for about two years; I have some ideas but they’re
still to be worked through and remain conjectures (related to material transport to the
WD poles). It’s not a magnetic effect nor is it likely (for this case) to be the disk
alone. But it’s characteristic of virtually all nova ejecta, although some are more
spherical than others.

The planetary nebula case (ikke det samme som Nova ejecta) is more varied and nuanced.
Here we’re seeing an explosion, there it’s a wind that has timescales far longer and can
be continually structured in outflow (e.g. planetaries, LBVs, AGB stars).

Q: When reading your information about the WD and how the material transfers from
the accretion star to a spot on the surface on the WD and then ignite, we would expect an
explosion/burn on one side of the WD and not bipolar or spherical.

A: This is a beautiful point. Yes, you would expect that IF the accretion weren’t
from a disk. That’s why this is a different case. We don’t know the ignition conditions
well enough, not how the flame proceeds in a WD that’s differentially rotating.

Q: So why not just an explosion on the surface that ejects material in one
direction away from the WD as the WD rotates? Why sould we also look for a spherical
shape?

A: The layer is clearly mixed before the explosion, but premixing is a diverse
process. Some models use diffusion, slow and steady, others invoke shear mixing and
turbulence, it also depends how deep the mixing and spin-up go. The models we have done
to date in 2D and 3D are not there yet.

Q: Can you further comment on this from your paper?:
The catch here is an implicit assumption regarding the geometry of the ejecta, that the
central source is completely covered not relative to the observer but in the expanding
frame.

A: It’s that the ejecta are one dimensional in any sight line. The observer may
see the ejecta to the side but the material in the outer ejecta “looking toward” the
central object sees all of the garbage intervening along that line of sight. Think of
being in a fog. Looking up it’s clear, looking ahead it’s opaque. The ejecta look ahead.

Q: I’m I correct in assuming that this work will give us better “standard candles”
that will change our assumptions on distances to these objects?

A: Yes, at least I hope so, but that’s not likely to be true for the CO novae,
they’re likely from a broader range of WD masses than the ONe. This is only a sort of
secondary standard candle but it’s important for understanding the gamma-ray production
mechanisms and the evolution of the WD in the post-ejection stage.

I hope this helps!!

steve

Nightsky2013-09-08 22:04:08

[nightskyDeltager Neutron star]

Artikel 7 fra Steven Shore om Nova Del 2013

First, we’re nearly at the stage, t_3, where the optical spectrum usually goes through
another transition. The emission lines should strengthen, the continuum should quickly
fade, and emission lines of moderately ionized species should appear. That’s the standard
statement, that this timescale defines the nova event.

But as we discussed earlier, the timing of these events is tied to the structure of the
ejecta and the evolution of the underlying WD. In these spectra, for instance (And
Christian’s are also showing much of this) there’s a new feature. Look at the Ca II lines
(those around 8500A). There’s virtually identical structure on these lines, it’s not
atmospheric water absorption as demonstrated by the [O I] and Ca II 3933. These tiny
features, throughout the line profile, symmetric about zero, are signs of the ejecta
structure and the signal that these transitions are optically thin. The lines from
similar ions, or similar ionization/excitation conditions, should be the same and you see
the same structure on a forbidden line ([O I]) as the permitted (Ca II), from a neutral
and from an ion. The ejecta geometry, if we use a bipolar model, seems to fit a rather
high inclination but it’s also showing another effect. Notice in the second set of
profiles that the O I 8446 extends to higher velocity in the wings (like H delta) than
[O I]6300. The O I is connected to the ground transition O I 1302 in its lower state, the
upper state is fluorescent with Lyman beta, hence it looks like H-delta and the higher
Balmer lines that are weighted toward the inner part of the ejecta. The forbidden line
bleeds off the photons from O I 1302 so it’s a different profile, more like the Ca II
which are excited state transitions only. There are three of there, one of which is
nearly coincident with O I.

As the shorter wavelengths become more transparent, the profiles will become more nearly
the same. The next moment is when the UV starts to ionize the Fe and the curtain lifts,
when the [N II] 5755, 6548,6583 lines appear, and then when the [O III] 4363, 4949,5007
are excited.

The former are simple forbidden transitions, although with the same atomic configuration
as the O III. This is called “isoelectronic” in having the same state structures (recall
that N+ is the same number of electrons as O+2 but with a different nuclear charge, that
makes relatively little difference for the binding, hence the lines are near each other).
In the ejecta, since the O I 8446 line is formed by pumping, it’s intensity varies
linearly with density while the recombination lines, like Ca II (permitted and excited
states) form by recombination so the intensity varies as density-squared. To be more
precise, and I hope less technical, the formation of a line by recombination means that
electron capture takes place so the emission depends on the number of captured electrons
(one power of density) and the number of ions (the other power). Pumping depends only on
the number of ions to be pumped and the availability of photons, so it’s a different
density dependence. Now recalling that the density is lower in the periphery of the
ejecta where the velocity is highest (in this ejecta picture, but also for a wind), the
wings are weaker but extend to the point of invisibility. The [O I] is formed, instead,
by the 1302 photons being trapped and “leaking out” and that requires the inner region.
But there’s another important piece of information here, that the forbidden transitions
aren’t seen if the density is any region (for a temperature of about 10,000 K or so) is
too high so there’s an upper limit (about 1E9/cm^3) for the inner part. If we take that
to be about 1000 km/s, assuming what we know from other novae, then as a first pass guess
the mass of the ejecta is about 8E-5 solar masses (yes, you heard it first here). This
depends on the filling factor which, from the NOT observations and what you’ve seen in
the fine structure, suggests about 10% or 30% of the ejecta s filled with an aerosol of
filaments so this could be as low as 2E-5 M_sun.

This is a normal value for the ejecta and I’m assuming that the inner density is low
enough to produce the [O I].

The calculation assumes that we’re seeing this at 20 days with a velocity of 1000 km/s
for the inner part and about 3000 km/s for the outer, fiducial numbers. It doesn’t give
an abundance but it’s a start. The other is that the emission at H-alpha accounts for
almost 8000 L_sun if the nova is at 5 kpc and scales as (D/5 kpc)^2, so a lot of energy
is coming out in a single line.

It’s this last point I wanted to also mention because the ejecta are acting as a sort of
bolometer, or calorimeter. The energy now derives from the original hot gas and the
heating from the WD radiation. That will keep up until the nova turns off, when the
nuclear source collapses and the WD starts to cool. The rapidity of this stage is probed
by the direct measurement of the XRs, which will appear shortly if all is right here, and
by the appearance of very highly ionized species like Fe VII and Ca V, or even higher.
That’s still in the future but shouldn’t be very long. I haven’t heard whether the gamma
ray source is still on but it shouldn’t be, if the internal shocks are the powering
agent, but the radio should also turn on soon as the ejecta turn optically thin in the
centimeter wavelength range.

So that’s what’s to come, but the beauty of this stage is that we’re beginning the
transition when you get to see, like a tomogram of a body, the individual parts of the
inner ejecta becoming visible. I don’t know another stage, whether in stellar outflows
(like luminous blue variables) or even planetary nebulae (this is the last stage after
the superwind from the central star turns on) when you see the third dimension of the
universe so clearly.

As always, I hope these explanations are clear. If — and please always keep this in
mind — if you have any questions or comments I’m delighted to hear them.

steve

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