3,053 research outputs found
Non-Equilibrium Ionization Model for Stellar Cluster Winds and its Application
We have developed a self-consistent physical model for super stellar cluster
winds based on combining a 1-D steady-state adiabatic wind solution and a
non-equilibrium ionization calculation. Comparing with the case of collisional
ionization equilibrium, we find that the non-equilibrium ionization effect is
significant in the regime of a high ratio of energy to mass input rate and
manifests in a stronger soft X-ray flux in the inner region of the star
cluster. Implementing the model in X-ray data analysis softwares (e.g., XSPEC)
directly facilitates comparisons with X-ray observations. Physical quantities
such as the mass and energy input rates of stellar winds can be estimated by
fitting observed X-ray spectra. The fitted parameters may then be compared with
independent measurements from other wavelengths. Applying our model to the star
cluster NGC 3603, we find that the wind accounts for no more than 50% of the
total "diffuse" emission, and the derived mass input rate and terminal velocity
are comparable to other empirical estimates. The remaining emission most likely
originate from numerous low-mass pre-main-sequence stellar objects.Comment: 29 pages, 17 figures. accepted by MNRA
M31* and its circumnuclear environment
We present a multiwavelength investigation of the circumnuclear environment
of M31. Based on Chandra/ACIS data, we tightly constrain the X-ray luminosity
of M31*, the central supermassive black hole of the galaxy, to be L (0.3-7
keV)<= 1.2x10^{36}erg/s, approximately 10^{-10} of the Eddington luminosity.
From the diffuse X-ray emission, we characterize the circumnuclear hot gas
with a temperature of ~0.3 keV and a density of ~0.1 cm^{-3}. In the absence of
an active SMBH and recent star formation, the most likely heating source for
the hot gas is Type Ia SNe. The presence of cooler, dusty gas residing in a
nuclear spiral has long been known in terms of optical line emission and
extinction. We further reveal the infrared emission of the nuclear spiral and
evaluate the relative importance of various possible ionizing sources. We show
evidence for interaction between the nuclear spiral and the hot gas, probably
via thermal evaporation. This mechanism lends natural understandings to 1) the
inactivity of M31*, in spite of a probably continuous supply of gas from outer
disk regions, and 2) the launch of a bulge outflow of hot gas, primarily
mass-loaded from the circumnuclear regions. One particular prediction of such a
scenario is the presence of gas with intermediate temperatures arising from the
conductive interfaces. The FUSE observations do show strong OVI1032
and 1038 absorption lines against the bulge starlight, but the effective OVI
column density (~4x10^{14} cm^{-2}), may be attributed to foreground gas
located in the bulge and/or the highly inclined disk of M31. Our study strongly
argues that stellar feedback, particularly in the form of energy release from
SNe Ia, may play an important role in regulating the evolution of SMBHs and the
interstellar medium in galactic bulges.Comment: Submitted to MNRAS, 33 pages, 9 figures. Comments welcom
A magnetohydrodynamic model for multi-wavelength flares from Sagittarius~A (I): model and the near-infrared and X-ray flares
Flares from the supermassive black hole in our Galaxy, Sagittarius~A
(Sgr A), are routinely observed over the last decade or so. Despite
numerous observational and theoretical efforts, the nature of such flares still
remains poorly understood, although a few phenomenological scenarios have been
proposed. In this work, we develop the Yuan et al. (2009) scenario into a
magnetohydrodynamic (MHD) model for Sgr A flares. This model is
analogous with the theory of solar flares and coronal mass ejection in solar
physics. In the model, magnetic field loops emerge from the accretion flow onto
Sgr A and are twisted to form flux ropes because of shear and
turbulence. The magnetic energy is also accumulated in this process until a
threshold is reached. This then results in a catastrophic evolution of a flux
rope with the help of magnetic reconnection in the current sheet. In this
catastrophic process, the magnetic energy is partially converted into the
energy of non-thermal electrons. We have quantitatively calculated the
dynamical evolution of the height, size, and velocity of the flux rope, as well
as the magnetic field in the flare regions, and the energy distribution of
relativistic electrons in this process. We further calculate the synchrotron
radiation from these electrons and compare the obtained light curves with the
observed ones. We find that the model can reasonably explain the main
observations of near-infrared (NIR) and X-ray flares including their light
curves and spectra. It can also potentially explain the frequency-dependent
time delay seen in radio flare light curves.Comment: 17 pages, 13 figures, accepted by MNRA
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