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Register a new userX-rays from the Power Sources of the Cepheus A Star-Forming Region
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We report an observation of X-ray emission from the exciting region of Cepheus A with the Chandra/ACIS instrument. What had been an unresolved X-ray source comprising the putative power sources is now resolved into at least 3 point-like sources, each with similar X-ray properties and differing radio and submillimeter properties. The sources are HW9, HW3c, and a new source that is undetected at other wavelengths h10. They each have inferred X-ray luminosities >= 10^31 erg s^-1 with hard spectra, T >= 10^7 K, and high low-energy absorption equivalent to tens to as much as a hundred magnitudes of visual absorption. The star usually assumed to be the most massive and energetic, HW2, is not detected with an upper limit about 7 times lower than the detections. The X-rays may arise via thermal bremsstrahlung in diffuse emission regions associated with a gyrosynchrotron source for the radio emission, or they could arise from powerful stellar winds. We also analyzed the Spitzer/IRAC mid-IR observation from this star-formation region and present the X-ray results and mid-IR classifications of the nearby stars. HH 168 is not as underluminous in X-rays as previously reported.
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The Cepheus B (CepB) molecular cloud and a portion of the nearby CepOB3b OB association, one of the most active regions of star formation within 1 kpc, have been observed with the IRAC detector on board the Spitzer Space Telescope. The goals are to study protoplanetary disk evolution and processes of sequential triggered star formation in the region. Out of ~400 pre-main sequence (PMS) stars selected with an earlier Chandra X-ray Observatory observation, 95% are identified with mid-infrared sources and most of these are classified as diskless or disk-bearing stars. The discovery of the additional >200 IR-excess low-mass members gives a combined Chandra+Spitzer PMS sample complete down to 0.5 Mo outside of the cloud, and somewhat above 1 Mo in the cloud. Analyses of the nearly disk-unbiased combined Chandra+Spitzer selected stellar sample give several results. Our major finding is a spatio-temporal gradient of young stars from the hot molecular core towards the primary ionizing O star HD 217086. This strongly supports the radiation driven implosion (RDI) model of triggered star formation in the region. The empirical estimate for the shock velocity of 1 km/s is very similar to theoretical models of RDI in shocked molecular clouds...ABRIDGED... Other results include: 1. agreement of the disk fractions, their mass dependency, and fractions of transition disks with other clusters; 2. confirmation of the youthfulness of the embedded CepB cluster; 3. confirmation of the effect of suppression of time-integrated X-ray emission in disk-bearing versus diskless systems.
We present linear and circular polarization observations of the water masers in 4 distinct regions spread over 1x2 arcseconds around the HW2 high-mass young stellar object in the Cepheus A star-forming region. We find magnetic fields between 100-500 mG in the central maser region, which has been argued to trace a circumstellar disk. The masers further from HW2 have field strengths between 30-100 mG. In all cases the magnetic field pressure is found to be similar to the dynamic pressure, indicating that the magnetic field is capable of controlling the outflow dynamics around HW2. In addition to several water maser complexes observed before, we also detect a new maser filament, ~1 arcsec (~690 AU) East of HW2, which we interpret as a shocked region between the HW2 outflow and the surrounding medium. We detect a linear polarization gradient along the filament as well as a reversal of the magnetic field direction. This is thought to mark the transition between the magnetic field associated with the outflow and that found in the surrounding molecular cloud. In addition to the magnetic field we determine several other physical properties of the maser region, including density and temperatures as well as the maser beaming angles.
Massive young star clusters contain dozens or hundreds of massive stars that inject mechanical energy in the form of winds and supernova explosions, producing an outflow which expands into their surrounding medium, shocking it and forming structures called superbubbles. The regions of shocked material can have temperatures in excess of 10$^6$ K, and emit mainly in thermal X-rays (soft and hard). This X-ray emission is strongly affected by the action of thermal conduction, as well as by the metallicity of the material injected by the massive stars. We present three-dimensional numerical simulations exploring these two effects, metallicity of the stellar winds and supernova explosions, as well as thermal conduction.
VLBI multi-epoch water maser observations are a powerful tool to study the dense, warm shocked gas very close to massive protostars. The very high-angular resolution of these observations allow us to measure the proper motions of the masers in a few weeks, and together with the radial velocity, to determine their full kinematics. In this paper we present a summary of the main observational results obtained toward the massive star-forming regions of Cepheus A and W75N, among them: (i) the identification of different centers of high-mass star formation activity at scales of 100 AU; (ii) the discovery of new phenomena associated with the early stages of high-mass protostellar evolution (e.g., isotropic gas ejections); and (iii) the identification of the simultaneous presence of a wide-angle outflow and a highly collimated jet in the massive object Cep A HW2, similar to what is observed in some low-mass protostars. Some of the implications of these results in the study of high-mass star formation are discussed.
We report our 110 ks Chandra observations of the supernova remnant (SNR) 0104-72.3 in the Small Magellanic Cloud (SMC). The X-ray morphology shows two prominent lobes along the northwest-southeast direction and a soft faint arc in the east. Previous low resolution X-ray images attributed the unresolved emission from the southeastern lobe to a Be/X-ray star. Our high resolution Chandra data clearly shows that this emission is diffuse, shock-heated plasma, with negligible X-ray emission from the Be star. The eastern arc is positionally coincident with a filament seen in optical and infrared observations. Its X-ray spectrum is well fit by plasma of normal SMC abundances, suggesting that it is from shocked ambient gas. The X-ray spectra of the lobes show overabundant Fe, which is interpreted as emission from the reverse-shocked Fe-rich ejecta. The overall spectral characteristics of the lobes and the arc are similar to those of Type Ia SNRs, and we propose that SNR 0104-72.3 is the first case for a robust candidate Type Ia SNR in the SMC. On the other hand, the remnant appears to be interacting with dense clouds toward the east and to be associated with a nearby star-forming region. These features are unusual for a standard Type Ia SNR. Our results suggest an intriguing possibility that the progenitor of SNR 0104-72.3 might have been a white dwarf of a relatively young population.
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