Rm Bol Research Articles

Recent star formation in the inner Galactic bulge seen by ISOGAL

We present 5--38 $\mu$m spectroscopic observations of a sample of 68 ISOGAL sources with unknown natures, taken with the Spitzer Infrared Spectrograph. Based on the characteristics and the slope of their spectra we classified the sources as young or late-type evolved objects. These sources were selected to test selection criteria based on the ISOGAL [7]--[15] color and the spatial extent parameter $\sigma_{\rm 15}$. We revised these criteria until they reliably distinguished between young and late-type evolved objects and then applied them to all ISOGAL sources in the central molecular zone (CMZ), resulting in the selection of 485 sources believed to be young. Furthermore, we added 656 Midcourse Space Experiment (MSX) sources to the CMZ sample that fulfilled $F_{\rm E}/F_{\rm D} > 2$ with $F{\rm D}$ and $F_{\rm E}$ being the flux densities in the D (15 $\mu$m) and E (21 $\mu$m) bands. After obtaining $\frac{L_{\rm bol}}{F_{\rm 15}}$ conversion factors, we calculated the bolometric luminosity, $L_{\rm bol}$, values for the CMZ sample and subsequently the masses of the sources. Applying a Kroupa initial mass function, we derived the total mass in young objects that has been formed over the last 1 Myr, resulting in an average star formation rate of 0.08 solar masses per year for the CMZ.

A Large X-Ray Flare from a Single Weak-Lined T Tauri Star TWA-7 Detected with MAXI GSC

Abstract We present a large X-ray flare from a nearby weak-lined T Tauri star TWA-7 detected with the Gas Slit Camera (GSC) on the Monitor of All-sky X-ray Image (MAXI). The GSC captured X-ray flaring from TWA-7 with a flux of 3 $\times$ 10 $^{-9}\ $ erg cm $^{-2}\ $ s $^{-1}$ in 2–20 keV band during the scan transit starting at 2010-09-07 18:24:30 (UT). The estimated X-ray luminosity at the scan in the energy band is 3 $\times$ 10 $^{32}\ $ erg s $^{-1}$ , indicating that the event is among the largest X-ray flares from T Tauri stars. Since MAXI GSC monitors a target only during a scan transit of about a minute per 92 min orbital cycle, the luminosity at the flare peak might have been higher than that detected. At the scan transit, we observed a high X-ray-to-bolometric luminosity ratio, log $\ L_{\rm X}/L_{\rm bol}$$=$$-$ 0.1 $^{+0.2}_{-0.3}$ ; i.e., the X-ray luminosity is comparable to the bolometric luminosity. Since TWA-7 has neither an accreting disk nor a binary companion, the observed event implies that none of those are essential to generate such big flares in T Tauri stars.

Skewed Distributions and Opposite Velocity Gradients of Submillimeter Molecular Lines in Low-Mass Protostellar Envelopes

Abstract We have made mapping observations of L 1551 IRS 5, L 1551 NE, L 723, and L 43 and single-point observations of IRAS 16293$-$2422 in the submillimeter CS ($J$$=$ 7–6) and HCN ($J$$=$ 4–3) lines with Atacama Submillimeter Telescope Experiment (ASTE). Using the present data and our previous ASTE observations of L 483 and B 335 together, we found a definite correlation between source bolometric luminosities and total integrated intensities of the submillimeter lines ($I_{\rm CS}$$\propto$$L_{\rm bol}^{0.92}$). The combined ASTE $+$ Submillimeter Array CS (7–6) image of L 1551 IRS 5 exhibits an extended ($\sim$ 2000 AU) component tracing the associated reflection nebula at west and southwest, as well as a compact ($\lesssim$ 500 AU) component centered on the protostellar position. The peaks of the CS and HCN emissions in L 1551 NE are not located at the protostellar position but offset ($\sim$ 1400 AU) toward the associated reflection nebula at west. By statistical analyses, we confirmed the velocity gradients of the CS (7–6) emission that are opposite to those of the millimeter lines along the outflow direction, which we reported in our early paper. The magnitudes of the submillimeter velocity gradients are estimated to be (9.7$\ \pm\ $1.7) $\times$ 10$^{-3}\ $km s$^{-1}\ $arcsec$^{-1}\ $ in L 1551 IRS 5 and (7.6$\ \pm\ $2.4) $\times$ 10$^{-3}\ $km s$^{-1}\ $arcsec$^{-1}\ $ in L 483. We suggest that the “skewed” submillimeter molecular emissions toward the associated reflection nebulae at a few thousand AU scale trace the warm ($\gtrsim$ 40 K) walls of the envelope cavities, excavated by the associated outflows and directly irradiated by the central protostars. The opposite velocity gradients along the outflow direction likely reflect the dispersing gas motion at the wall of the cavity in envelopes perpendicular to the outflow.

A HOT MOLECULAR OUTFLOW DRIVEN BY THE IONIZED JET ASSOCIATED WITH IRAS 16562–3959

We report molecular line observations in the CO J=3$\rightarrow$2, 6$\rightarrow$5 and 7$\rightarrow$6 transitions, made using the Atacama Pathfinder Experiment Telescope (APEX), toward the massive and dense core IRAS 16562$-$3959. This core harbors a string of radio sources thought to be powered by a central collimated jet of ionized gas. The molecular observations show the presence of high velocity gas exhibiting a quadrupolar morphology, most likely produced by the presence of two collimated outflows. The southeast-northwest molecular outflow is aligned with the string of radio continuum sources, suggesting it is driven by the jet. We find that the excitation temperature of the gas in the SE-NW outflow is high, with values of 145 and 120 K for the blueshifted and redshifted lobes, respectively. This outflow has a total mass of 1.92 \Msun, a total momentum of $\sim 89$ \Msun \kms and an averaged momentum rate of $\sim 3.0\times10^{-2}$ \Msun \kms yr$^{-1}$, values characteristics of flows driven by young massive stellar objects with high luminosities ($L_{\rm bol} \sim 2\times10^4$ \Lsun). Complementary data taken with the Atacama Submillimeter Telescope Experiment (ASTE) in high density and shock tracers support the picture that IRAS 16562$-$3959 is an accreting young massive star associated with an ionized jet, which is the energy source of a molecular outflow.

Fundamental Plane of FSRQs from SDSS DR5 Quasar Catalogue

In this work, we explore a sample of 362 flat-spectrum radio quasars (FSRQs) to investigate the jet formation. We find that the fundamental plane for our FSRQs can be expressed as \(L_{\rm 5~GHz}\propto M_{\rm bh}^{-0.19}L_{\rm 2~keV}^{1.08}\). We also find that the 5 GHz luminosities are tightly related to both black hole mass and Eddington ratio, which is established as \(L_{\rm 5~GHz}\propto M_{\rm bh}^{0.67}(L_{\rm bol}/ L_{\rm EDD})^{1.32}\).

Luminosities and mass-loss rates of SMC and LMC AGB stars and red supergiants

(Abridged) Dust radiative transfer models are presented for 101 carbon stars and 86 oxygen-rich evolved stars in the Magellanic Clouds for which 5-35 \mum\ {\it Spitzer} IRS spectra are available. The spectra are complemented with available optical and infrared photometry to construct the spectral energy distribution. A minimisation procedure is used to fit luminosity, mass-loss rate and dust temperature at the inner radius. Different effective temperatures and dust content are also considered. Periods from the literature and from new OGLE-III data are compiled and derived. The O-rich stars are classified in foreground objects, AGB stars and Red Super Giants. For the O-rich stars silicates based on laboratory optical constants are compared to "astronomical silicates". Overall, the grain type by Volk & Kwok (1988) fit the data best. However, the fit based on laboratory optical constants for the grains can be improved by abandoning the small-particle limit. The influence of grain size, core-mantle grains and porosity are explored. Relations between mass-loss rates and luminosity and pulsation period are presented and compared to the predictions of evolutionary models, those by Vassiliadis & Wood (1993) and their adopted mass-loss recipe, and those based on a Reimers mass-loss law with a scaling of a factor of five. The Vassiliadis & Wood models describe the data better, although there are also some deficiencies, in particular to the maximum adopted mass-loss rate. The OGLE-III data reveal an O-rich star in the SMC with a period of 1749 days. Its absolute magnitude of $M_{\rm bol}= -8.0$ makes it a good candidate for a super-AGB star.

Spectral Transitions of an Ultraluminous X-Ray Source, NGC 2403 Source 3

Abstract A Suzaku observation of an ultraluminous X-ray source, NGC 2403 Source 3, performed on 2006 March 16–17, is reported. The Suzaku XIS spectrum of Source 3 was described as multi-color black-body-like emission from an optically thick accretion disk. The innermost disk temperature and radius were measured to be $T_{\rm in}$$=$ 1.08$_{-0.03}^{+0.02}$keV and $R_{\rm in}$$=$ 122.1$_{-6.8}^{+7.7} \ \alpha^{1/2}$km, respectively, where $\alpha$$=$ (cos 60$^\circ /$cos $i$) with $i$ being the disk inclination. Its bolometric luminosity was estimated to be $L_{\rm bol}$$=$ 1.82 $\times$ 10$^{39}\alpha $ergs$^{-1}$. Archival Chandra and XMM-Newton data of the source were analyzed. In almost all observations, the source showed multi-color black-body-like X-ray spectra with parameters similar to those of the Suzaku observation. In only one Chandra observation, however, Source 3 exhibited a power-law-like spectrum, with a photon index of $\Gamma$$=$ 2.37$\pm$0.08, when it was fainter by $\sim$15% than in the Suzaku observation. This behavior is explained in terms of a transition between a slim-disk state and “very high” states, both found in Galactic black hole binaries, when their luminosity apporached the Eddington limit. These results are utilized to argue that ultraluminous X-ray sources generally have significantly higher masses than ordinary stellar-mass black holes.

Discovery of a Bright Transient Ultraluminous X-Ray Source, Suzaku J1305–4931 in NGC4945

Abstract An X-ray source, Suzaku J1305$-$4931, was discovered in the south-west arm of a nearby Seyfert II galaxy, NGC4945, at 0.5-10keV flux of $2.2 \times 10^{-12}$ergcm$^{-2}$s$^{-1}$ during a Suzaku observation conducted on 2006 January 15-17. It was undetectable in a shorter observation on 2005 August 22-23, with an upper limit of $1.7 \times 10^{-14}$ergcm$^{-2}$s$^{-1}$. At a distance of 3.7Mpc, the bolometric luminosity of the source became $L_{\rm bol} = 4.4 \times 10^{39} \alpha$ergs$^{-1}$, where $\alpha = ({\rm\cos} 60^\circ \!/ \cos i)$ and $i$ is the disk inclination. The time-averaged X-ray spectrum of the source is described by a multi-color disk model, with an innermost disk temperature of $T_{\rm in} = 1.69_{-0.05}^{+0.06}$keV. It varied by a factor of $2$ in intensity, following a clear correlation of $L_{\rm bol} \propto T_{\rm in}^4$. The innermost disk radius is inferred to stay constant at $R_{\rm in} = 79_{-3.9}^{+4.0} \alpha^{1/2}$ km, suggesting the presence of a standard accretion disk. Relating $R_{\rm in}$ with the last stable orbit around a non-rotating black hole yields a rather low black-hole mass, $\sim 9 \alpha^{1/2}$ solar masses, which would imply that the source is shining at a considerable super-Eddington luminosity. These results can be better interpreted by invoking sub-Eddington emission from a rapidly spinning black hole with 20-130 solar masses.

The Unified Model of Active Galactic Nuclei. I. Non–Hidden Broad‐Line Region Seyfert 2 and Narrow‐Line Seyfert 1 Galaxies

The unified model of Seyfert galaxies suggests that there are hidden broad-line regions (HBLRs) in Seyfert 2 galaxies (S2s). However, there is increasing evidence for the appearance of a subclass of S2s lacking of HBLR (non-HBLR S2s). An interesting issue arises as to relations of non-HBLR S2s with other types of Seyfert galaxies and whether or not they can be included in the unified model. We assemble two sub-samples consisting of 42 non-HBLR S2s and 44 narrow-line Seyfert 1s (NLS1s) with redshift $z\le 0.05$ from published literatures to explore this issue. We compare black hole masses in the galactic centers, accretion rates, infrared color ratio ($f_{60 \mu \rm m}/f_{25 \mu \rm m}$) as a potential indicator of the dusty torus orientation, \oiii $\lambda 5007$, radio and far infrared luminosities. We find that non-HBLR S2s and NLS1s have: 1) similar distributions of the black hole masses ($10^6-3\times 10^7\sunm$) and the Eddington ratios ($L_{\rm Bol}/L_{\rm Edd}\sim 1$); 2) significantly different distributions of $f_{60 \mu \rm m}/f_{25 \mu \rm m}$ ratios; 3) similar distributions of bulge magnitudes and luminosities of [O {\sc iii}], radio, far infrared emission. The similarities and differences can be understood naturally if they are intrinsically same but non-HBLR S2s are viewed at larger angles of observer's sight than NLS1s. We thus suggest that non-HBLR S2s only have narrower broad line regions and they are the counterparts of NLS1s viewed at high inclination angles. The absence of the polarized emission line in non-HBLR S2s is caused by the less massive black holes and high accretion rate similar to NLS1s. The implications of the unification scheme of non-HBLR S2s and NLS1s are discussed.

Discovery of an energetic bipolar molecular outflow towards IRAS 16547-4247

Aims. We present molecular line observations of the molecular core associated with IRAS 16547-4247, which have allowed us to determine its physical and kinematical properties at angular resolutions of ~18´´. Methods. The observations were made using the Atacama Pathfinder Experiment Telescope in the $J = 3 \rightarrow 2$ transitions of 12 CO and 13 CO, $J = 4 \rightarrow 3$ transitions of HCO + and H 13 CO + , and $J = 7 \rightarrow 6$ transition of CS. Results. Our observations reveal the presence of a collimated bipolar outflow with lobes ~0.7 pc in extent and aligned with the thermal jet located at the center of the core. The morphology and velocity structure of the flow are well described by a biconical outflow that is inclined from the line of sight by an angle of 84°, has a semi-opening angle of 14°, and in which the gas moves outwards with a constant total velocity, with respect to the cone apex, of ~120 km s -1 . The outflow is massive and energetic (flow mass ~110 $M_{\odot}$; mass outflow rate ~$2\times10^{-2}$ $M_{\odot}$ yr -1 ; momentum ~$2\times10^3$ $M_{\odot}$ km s -1 and kinetic energy ~$9\times 10^{47}$ erg), and has a dynamical time scale of $6\times10^3$ yr. These parameters are consistent with the flow being driven by a young massive stellar object with $L_{\rm bol} \sim 6\times10^4$ $L_{\odot}$.

A possible bias on the estimate of Lbol/Ledd in AGN as a function of luminosity and redshift

Context. The BH mass (and the related Eddington ratio, l = $L_{\rm bol}/L_{\rm edd}$) in broad line AGN is usually evaluated by combining estimates (often indirect) of the BLR radius and of the FWHM of the broad lines, under the assumption that the BLR clouds are in Keplerian motion around the BH. Such an evaluation depends on the geometry of the BLR. There are two major options for the BLR configuration: spherically symmetric or “flattened”. In the latter case the inclination to the line of sight becomes a relevant parameter. Aims. This paper is devoted to evaluate the bias on the estimate of the Eddington ratio when a spherical geometry is assumed (more generally when inclination effects are ignored), while the actual configuration is “flattened”, as some evidence suggests. This is done as a function of luminosity and redshift, on the basis of recent results which show the existence of a correlation between the fraction of obscured AGN and these two parameters up to at least $z=2.5$ (date at larger redshifts being insufficient). Methods. The assumed BLR velocity field is akin to the “generalized thick disk” proposed by Collin et al. (2006, A&A, 456, 75). Assuming an isotropic orientation in the sky, the mean value of the bias is calculated as a function of luminosity and redshift. Results. It is demonstrated that, on average, the Eddington ratio obtained assuming a spherical geometry is underestimated for high luminosities, and overestimated for low luminosities. This bias converges for all luminosities at z about 2.7, while nothing can be said on this bias at larger redshifts due to the lack of data. The effects of the bias, averaged over the luminosity function of broad line AGN, have been calculated. The results imply that the bias associated with the a-sphericity of the BLR make even worse the discrepancy between the observations and the predictions of evolutionary models.

A new strongly X-ray flaring M 9 dwarf in the solar neighborhood

We report on the discovery of a very low mass (VLM) star in the solar neighborhood, originally identified as an optical counterpart of a flaring X-ray source detected in the ROSAT All-Sky survey. Optical spectroscopy and infrared photometry consistently reveal a spectral type of $\rm M\,9\, \pm\, 0.5$ and a distance of ≈ $11\,\pm\,2$ pc. The optical counterpart of 1RXS J115928.5-524717 shows a large proper motion of $1.08\,\pm\,0.06\arcsec$/year. 1RXS J115928.5-524717 is the fourth object among the VLM stars displaying a huge X-ray flare, reaching the unprecedent value of $L_X/L_{\rm bol}\simeq 0.1$.

Near–infrared and ISOCAM observations of the Chamaeleon II dark cloud

An infrared study including ISOCAM images at 6.75 and 14.3 $\,\mu \rm m$ of a large portion (28´ $\times$ 26´) of the Chamaeleon II dark cloud and sub-arcsec resolution JHK s images of the central (4.9´ $\times$ 4.9´) area is presented. Combining the ISOCAM observations with J and K s photometry obtained with DENIS, we have found 12 young stars, of which 8 are previously identified sources. Of the new candidate YSOs, ISO-ChaII13 shows a clear mid–IR excess with an infrared luminosity of ~0.02 $L_\odot$ and a stellar luminosity ≥ 0.01 $L_\odot$. This last value of luminosity corresponds to that expected for a sub–stellar object with an age between 1–10 Myr. The analysis of the $J-H$/$H-K$ diagram of the sources detected in the central part of the dark cloud has allowed us to identify 10 very faint sources not found by ISOCAM with possible near-IR excesses that may be embedded young brown dwarfs. Finally, the center of the dark cloud is characterized by the presence of the Class I YSO IRAS 12553–7651 (ISO-ChaII28) with $L_{{\rm bol}}=$ 1.49 $L_\odot$, and associated with a faint nebulosity observed at 2.2 $\,\mu \rm m$.

ISO-LWS two-colour diagram of young stellar objects

We present a [60--100] vs. [100--170] $\mu$m two colour diagram for a sample of 61 young stellar objects (YSOs) observed with the Long Wavelength Spectrometer (LWS) on-board the Infrared Space Observatory (ISO). The sample consists of 17 Class 0 sources, 15 Class I, 9 Bright Class I ($L_{\rm bol}>10^4 L_\odot$), 20 Class II (14 Herbig Ae/Be stars and 6 T Tauri stars). We find that each class occupies a well defined region in our diagram with colour temperatures increasing from Class 0 to Class II. Therefore the [60--100] vs. [100--170] two colours diagram is a powerful and simple tool to derive from future (e.g. with the Herschel Space Observatory) photometric surveys the evolutionary status of YSOs. The advantage over other tools already developed is that photometry at other wavelengths is not required: three flux measurements are enough to derive the evolutionary status of a source. As an example we use the colours of the YSO IRAS 18148$-$0440 to classify it as Class I. The main limitation of this work is the low spatial resolution of the LWS which, for some objects, causes a high uncertainty in the measured fluxes due to the background emission or to the source confusion inside the LWS beam.

HST, VLT, and NTT imaging search for wide companions to bona-fide and candidate brown dwarfs in the Cha I dark cloud

We present results from a deep imaging search for companions around the young bona-fide and candidate brown dwarfs Cha H α 1 to 12 in the Cha I dark cloud, performed with HST WFPC2 ( R , I , H α ), VLT FORS1 ( VRI ), and NTT SofI ( JHK s ). We find 16 faint companion candidates around five primaries with separations between 1.5$^{\prime \prime}$ and 7$^{\prime \prime}$ and magnitudes in R & I from 19 to 25 mag, i.e. up to 8 mag fainter than the primaries. While most of these companion candidates are probably unrelated background objects, there is one promising candidate, namely $1.5^{\prime \prime}$ SW off the M6-dwarf Cha H α 5. This candidate is 3.8 to 4.7 mag fainter than the primary and its colors are consistent with an early- to mid-L spectral type. Assuming the same distance (140 pc) and absorption ( A I = 0.47 mag) as towards the primary, the companion candidate has $\log$ ($L_{\rm bol}/L_{\odot}) = -3.0 \pm 0.3$. At the age of the primary (1 to 5 Myrs), the faint object would have a mass of 3 to 15 Jupiter masses according to Burrows et al. ([CITE]) and Chabrier & Baraffe ([CITE]) models. The probability for this companion candidate to be an unrelated fore- or background object is ≤ $ 0.7\%$, its colors are marginally consistent with a strongly reddened background K giant. One other companion candidate has infrared colors consistent with an early T-dwarf. In addition, we present indications for Cha H α 2 being a close (~$ 0.2^{\prime \prime}$) binary with both components very close to the sub-stellar limit. Our detection limits are such that we should have detected all companions above ∼ 1 M jup with separations $\ge $ $2^{\prime \prime}$ ( ≥ $ 320$ AU) and all above ~$ 5$ M jup at ≥ $ 0.35^{\prime \prime}$ ( ≥ $ 50$ AU).

XMM-Newton observation of a distant X-ray selected cluster of galaxies at $\vec{z}=\mathsf{1.26}$ with possible cluster interaction

We report on the XMM-Newton observation of RX J1053.7+5735, one of the most distant $(z = 1.26)$ selected clusters of galaxies, which also shows an unusual double-lobed X-ray morphology, indicative of possible cluster-cluster interaction. The cluster was discovered during our ROSAT deep pointings in the direction of the Lockman Hole. The XMM-Newton observations were performed with the European Photon Imaging Camera (EPIC) during the performance verification phase. Total effective exposure time was ~100 ksec. The best fit temperature based on a simultaneous fit of spectra from all EPIC cameras (pn+MOS) is $4.9^{+1.5}_{-0.9}$ keV. Metallicity is poorly constrained even using the joint fit of all spectra, with an upper limit on the iron abundance of 0.62 solar. Using the best fit model parameters, we derived an unabsorbed (0.2–10) keV flux of $f_{0.2-10} = 3.0 \times 10^{-14}$ erg cm -2 s -1 , corresponding to a bolometric luminosity of $L_{\rm bol} = 3.4 \times 10^{44}~ h_{50}^{-2}$ erg s -1 . Despite the fact that it was observed at fairly large off-axis angle, the temperature errors are much smaller compared with those of typical measurements based on ASCA or Beppo-Sax observations of high- z ($z > 0.6$) clusters, demonstrating the power of the XMM for determining the X-ray temperature for high- z clusters. The measured temperature and luminosity show that one can easily reach the intrinsically X-ray faint and cool cluster regime comparable with those of $z \sim 0.4$ clusters observed by past satellites. The new cluster temperature and L bol we have measured for RX J1053.7+5735 is consistent with a weak/no evolution of the L bol –$T\rm _x$ relation out to $z \sim 1.3$, which lends support to a low $\Omega_{\rm m}$ universe, although more data-points of $z > 1$ clusters are required for a more definitive statement. The caution has to be also exercised in interpreting the result, because of the uncertainty associated with the dynamical status of this cluster.

The X-Ray Spectrum of the Vela Pulsar Resolved with the [ITAL]Chandra X-Ray Observatory[/ITAL

We report the results of the spectral analysis of two observations of the Vela pulsar with the Chandra X-ray observatory. The spectrum of the pulsar does not show statistically significant spectral lines in the observed 0.25-8.0 keV band. Similar to middle-aged pulsars with detected thermal emission, the spectrum consists of two distinct components. The softer component can be modeled as a magnetic hydrogen atmosphere spectrum - for the pulsar magnetic field $B=3\times 10^{12}$ G and neutron star mass $M=1.4 M_\odot$ and radius $R^\infty =13$ km, we obtain $\tef^\infty =0.68\pm 0.03$ MK, $L_{\rm bol}^\infty = (2.6\pm 0.2)\times 10^{32}$ erg s$^{-1}$, $d=210\pm 20$ pc (the effective temperature, bolometric luminosity, and radius are as measured by a distant observer). The effective temperature is lower than that predicted by standard neutron star cooling models. A standard blackbody fit gives $T^\infty =1.49\pm 0.04$ MK, $L_{\rm bol}^\infty=(1.5\pm 0.4)\times 10^{32} d_{250}^2$ erg s$^{-1}$ ($d_{250}$ is the distance in units of 250 pc); the blackbody temperature corresponds to a radius, $R^\infty =(2.1\pm 0.2) d_{250}$ km, much smaller than realistic neutron star radii. The harder component can be modeled as a power-law spectrum, with parameters depending on the model adopted for the soft component - $\gamma=1.5\pm 0.3$, $L_x=(1.5\pm 0.4)\times 10^{31} d_{250}^2$ erg s$^{-1}$ and $\gamma=2.7\pm 0.4$, $L_x=(4.2\pm 0.6)\times 10^{31} d_{250}^2$ erg s$^{-1}$ for the hydrogen atmosphere and blackbody soft component, respectively ($\gamma$ is the photon index, $L_x$ is the luminosity in the 0.2--8 keV band). The extrapolation of the power-law component of the former fit towards lower energies matches the optical flux at $\gamma\simeq 1.35$--1.45.

The Effect of Radiative Cooling on Scaling Laws of X-Ray Groups and Clusters

We have performed cosmological simulations in a $\Lambda$CDM cosmology with and without radiative cooling, in order to study the effect of cooling on the cluster scaling relations. Our simulations consist of 4.1 million particles each of gas and dark matter within a box-size of 100 $h^{-1}$ Mpc and the run with cooling is the largest of its kind to have been evolved to $z=0$. Our cluster catalogues both consist of over 400 objects and are complete in mass down to $\sim 10^{13} h^{-1} {\rm M_{\odot}}$. We contrast the emission-weighted temperature-mass ($T_{\rm ew}-M$) and bolometric luminosity-temperature ($L_{\rm bol}-T_{\rm ew}$) relations for the simulations at $z=0$. We find that radiative cooling \emph{increases} the temperature of intracluster gas and \emph{decreases} its total luminosity, in agreement with the results of Pearce et al. Furthermore, the temperature dependence of these effects flattens the slope of the $T_{\rm ew}-M$ relation and steepens the slope of the $L_{\rm bol}-T_{\rm ew}$ relation. Inclusion of radiative cooling in the simulations is sufficient to reproduce the observed X-ray scaling relations without requiring excessive non-gravitational energy injection.

Stars resembling the Sun

This review is primarily directed to the question whether photometric solar analogues remain such when subjected to detailed spectroscopic analyses and interpreted with the help of internal stucture models. In other words, whether the physical parameters: mass, chemical composition, age (determining effective temperature and luminosity), chromospheric activity, equatorial rotation, lithium abundance, velocity fields etc., we derive from the spectral analysis of a photometric solar analogue, are really close to those of the Sun. We start from 109 photometric solar analogues extracted from different authors. The stars selected had to satisfy three conditions: i) their colour index $(B-V)$ must be contained in the interval: $\Delta (B-V) = 0.59$ –0.69, ii) they must possess a trigonometric parallax, iii) they must have undergone a high resolution detailed spectroscopic analysis. First, this review presents photometric and spectrophotometric researches on solar analogues and recalls the pionneering work on these stars by the late Johannes Hardorp. After a brief discussion on low and high resolution spectroscopic researches, a comparison is made between effective temperatures as obtained, directly, from detailed spectral analyses and those obtained, indirectly, from different photometric relations. An interesting point in this review is the discussion on the tantalilizing value of the $(B-V)_\odot$ of the Sun, and the presentation of a new reliable value of this index. A short restatement of the kinematic properties of the sample of solar analogues is also made. And, finally, the observational $(\log T_{\rm eff}, M_{\rm bol})$ diagram, obtained with 99 of the initially presented 109 analogues, is compared to a theoretical $(\log T_{\rm eff}, M_{\rm bol})$ diagram. This latter has been constructed with a grid of internal structure models for which, (very important for this investigation), the Sun was used as gauge. In analysing the position, with respect to the Sun, of each star we hoped to find a certain number of stars tightly neighbouring the Sun in mass, chemical composition and state of evolution. The surprising result is that the stars occupy in this HR Diagram a rather extended region around the Sun, many of them seem more evolved and older than the Sun, and only 4 of the evolved stars seem younger. The age of some stars in the sample is also discussed in terms of chromospheric activity and Li-content. Our conclusion is much the same as that contained in previous papers we have written on the subject: in spite of a much larger number of stars, we have not been able to nominate a single star of the sample for a “perfect good solar twin”. Another aim in beginning, 25 years ago, this search for solar analogues, was to have ready a bunch of stars resembling the Sun and analysed spectroscopically in detail, in order that, when planets hunters of solar type stars, finally would have found such a specimen, we would have been able to immediately compare the physical parameters of this star to those of the Sun. We have been lucky enough: one of the good solar analogues we present herewith, is 51 Pegasi (HD 217014) which, according to the very recent observations by Mayor and Queloz (1995), has a planet orbiting around it. And what is more: two other stars possessing planets: 47 Ursae Majoris (HD 95128) and 70 Virginis (HD 117176), have just been discovered by Marcy and Butler (187 $^{\rm th}$ Meeting of the AAS, January 1996). One of them, 47 Ursae Majoris, is also included in the list of photometric solar analogues. The other star, 70 Virginis, has only been included after the “Planets News”, because the colour index $(B-V)$ of this star is slightly higher than the prescribted limit of the selection, ( $B-V = 0.71$ , instead, 0.69). It would have been a pity to leave the third ” planet star out of the competition.

Circumstellar envelopes and Asymptotic Giant Branch stars

Circumstellar envelopes and Asymptotic Giant Branch stars