Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userDo novae have optically thick winds during outburst with large deviations from spherical symmetry?
435
0
0.0
(
0
)
Authors
Michael Friedjung
Ask ChatGPT about the research
No Arabic abstract
The evidence for the presence of optically thick winds, produced by classical novae after optical maximum, has been challenged in recent papers. In addition, signs of orbital phase dependent photometric variations, sometimes seen quite early in the development of nova outbursts, are hard to interpret in the framework of optically thick envelopes and especially winds. A general discussion for belief in the presence of optically thick winds with increasing ejection velocities during the early stages of novae after their explosion, must be given. This has to be done in order to clarify ideas about novae as well as to contribute in particular to the understanding of the behaviour of novae V1500 Cyg and V1493 Aql showing phase dependent variations during very early decline after the outburst. Possible ways of overcoming the apparent contradiction of phase dependent variations through the production of deviations from spherical symmetry of the winds, are looked at and order of magnitude estimates are made for different theoretical scenarios, which might produce such deviations. It is found that large deviations from spherical symmetry of the optically thick winds in early phases after the explosion can easily explain the problem of variations. In particular, the presence of a magnetic field might have had a non-negligible effect on the wind of V1500 Cyg, while at the present there is not enough information available concerning V1493 Aql. Optically thick winds/envelopes are almost certainly present in the early stages after optical maximum of a nova, while it is difficult to make pure Hubble flow models fit the observations of those stages. New more detailed observational and theoretical work, in particular including the effects of magnetic fields on the winds, is needed.
rate research
Read More
Here we compute detailed model spectra of recently published optically thick one-dimensional radial baundary layer (BL) models in cataclysmic variables and compare them with observed soft X-ray/extreme ultraviolet (EUV) spectra of dwarf novae in outburst. Every considered BL model is divided into a number of rings, and for each ring, a structure model along the vertical direction is computed using the stellar-atmosphere method. The ring spectra are then combined into a BL spectrum taking Doppler broadening and limb darkening into account. Two sets of model BL spectra are computed, the first of them consists of BL models with fixed white dwarf (WD) mass (1 M_sun) and various relative WD angular velocities (0.2, 0.4, 0.6 and 0.8 break-up velocities), while the other deals with a fixed relative angular velocity (0.8 break-up velocity) and various WD masses (0.8, 1, and 1.2 M_sun). The model spectra show broad absorption features because of blending of numerous absorption lines, and emission-like features at spectral regions with only a few strong absorption lines. The model spectra are very similar to observed soft X-ray/EUV spectra of SS Cyg and U Gem in outburst. The observed SS Cyg spectrum could be fitted by BL model spectra with WD masses 0.8 - 1 M_sun and relative angular velocities 0.6 - 0.8 break up velocities. These BL models also reproduce the observed ratio of BL luminosity and disk luminosity. The difference between the observed and the BL model spectra is similar to a hot optically thin plasma spectrum and could be associated with the spectrum of outflowing plasma with a mass loss rate compatible with the BL mass accretion rate. The suggested method of computing BL spectra seems very promising and can be applied to other BL models for comparison with EUV spectra of dwarf novae in outburst.
The nova T Pyx was observed with high resolution spectroscopy (R ~ 65000) spectroscopy, beginning 1 day after discovery of the outburst and continuing through the last visibility of the star at the end of May 2011. The interstellar absorption lines of Na I, Ca II, CH, CH$^+$, and archival H I 21 cm emission line observations have been used to determine a kinematic distance. Interstellar diffuse absorption features have been used to determine the extinction independent of previous assumptions. Sample Fe-peak line profiles show the optical depth and radial velocity evolution of the discrete components. We propose a distance to T Pyx $geq$4.5kpc, with a strict lower limit of 3.5 kpc (the previously accepted distance). We derive an extinction, E(B-V)$approx0.5pm$0.1, that is higher than previous estimates. The first observation, Apr. 15, displayed He I, He II, C III, and N III emission lines and a maximum velocity on P Cyg profiles of the Balmer and He I lines of $approx$2500 km s$^{-1}$ characteristic of the fireball stage. These ions were undetectable in the second spectrum, Apr. 23, and we use the recombination time to estimate the mass of the ejecta, $10^{-5}f$M$_odot$ for a filling factor $f$. Numerous absorption line systems were detected on the Balmer, Fe-peak, Ca II, and Na I lines, mirrored in broader emission line components, that showed an accelerated displacement in velocity. We also show that the time sequence of these absorptions, which are common to all lines and arise only in the ejecta, can be described by recombination front moving outward in the expanding gas without either a stellar wind or circumstellar collisions.
Recent observation of some luminous transient sources with low color temperatures suggests that the emission is dominated by optically thick winds driven by super-Eddington accretion. We present a general analytical theory of the dynamics of radiation pressure-driven, optically thick winds. Unlike the classical adiabatic stellar wind solution whose dynamics are solely determined by the sonic radius, here the loss of the radiation pressure due to photon diffusion also plays an important role. We identify two high mass loss rate regimes ($dot{M} > L_{rm Edd,}/c^2$). In the large total luminosity regime the solution resembles an adiabatic wind solution. Both the radiative luminosity, $L$, and the kinetic luminosity, $L_k$, are super-Eddington with $L < L_k$ and $L propto L_k^{1/3}$. In the lower total luminosity regime most of the energy is carried out by the radiation with $L_k < L approx L_{rm Edd,}$. In a third, low mass loss regime ($dot{M} < L_{rm Edd,}/c^2$), the wind becomes optically thin early on and, unless gas pressure is important at this stage, the solution is very different from the adiabatic one. The results are independent from the energy generation mechanism at the foot of the wind, therefore they are applicable to a wide range of mass ejection systems, from black hole accretion, to planetary nebulae, and to classical novae.
We present the first polarimetric observations of a Type I superluminous supernova (SLSN). LSQ14mo was observed with VLT/FORS2 at five different epochs in the V band, with the observations starting before maximum light and spanning 26 days in the rest frame (z=0.256). During this period, we do not detect any statistically significant evolution (< 2$sigma$) in the Stokes parameters. The average values we obtain, corrected for interstellar polarisation in the Galaxy, are Q = -0.01% ($pm$ 0.15%) and U = - 0.50% ($pm$ 0.14%). This low polarisation can be entirely due to interstellar polarisation in the SN host galaxy. We conclude that, at least during the period of observations and at the optical depths probed, the photosphere of LSQ14mo does not present significant asymmetries, unlike most lower-luminosity hydrogen-poor SNe Ib/c. Alternatively, it is possible that we may have observed LSQ14mo from a special viewing angle. Supporting spectroscopy and photometry confirm that LSQ14mo is a typical SLSN I. Further studies of the polarisation of Type I SLSNe are required to determine whether the low levels of polarisation are a characteristic of the entire class and to also study the implications for the proposed explosion models.
We consider the consequences of appreciable line optical depth for the profile shape of X-ray emission lines formed in stellar winds. The hot gas is thought to arise in distributed wind shocks, and the line formation is predominantly via collisional excitation followed by radiative decay. Such lines are often modelled as optically thin, but the theory has difficulty matching resolved X-ray line profiles. We suggest that for strong lines of abundant metals, newly created photons may undergo resonance scattering, modifying the emergent profile. Using Sobolev theory in a spherically symmetric wind, we show that thick-line resonance scattering leads to emission profiles that still have blueshifted centroids like the thin lines, but which are considerably less asymmetric in appearance. We focus on winds in the constant-expansion domain, and derive an analytic form for the profile shape in the limit of large line and photoabsorptive optical depths. Our theory is applied to published {it Chandra} observations of the O star $zeta$ Pup.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
