On the formation of cold fronts in massive mergers

Using adiabatic hydrodynamical simulations, we follow the evolution of two symmetric cold fronts forming in the remnant of a violent   z = 0.3  massive cluster merger. Because the fronts develop after the first passage of the two gas cores of the merging subclusters, and because they soon move ahead of their associated dark matter cores, both the structure and the location of our simulated cold fronts may correspond to a stage that is later than that of most cold fronts observed so far. The cold fronts are preceded by a roughly spherical shock that originates in the centre of the cluster and disappears in the outer regions after 1.6 Gyr. The cold fronts last longer, until   z ∼ 0  . We follow the spatial evolution of the gas of the subcluster cores, and find that a fraction of this gas is liberated in the intracluster medium after core passage, but mainly at apocentre, and that it does not fall back onto the cluster centre. Conversely, we trace back the low-temperature gas constituting the fronts and find that it is initially associated with the two dense cores of the merging clusters. In addition, we find some evidence for discontinuity of the gas velocity field across the edge of the forming cold fronts, suggesting the presence of a contact discontinuity there. In the light of other recent work, we then speculate on the physical mechanism resulting in the cold fronts. We suggest that sloshing induced by strongly varying ram pressure along the subcluster's orbit and/or spatial segregation between the dark matter and gas components of the cores of the subclusters results in strong tidal forces on the gas, and that these forces could be responsible for the deposition of part of the cold dense gas in the surrounding hot intracluster medium. This deposited gas then expands, cools down further, and constitutes the cold fronts.     

高温刺激对刺参浮游期幼体存活率及Hsp70表达量的影响

设计不同温度(21.5,26,28和30℃),对刺参(Apostichopus japonicus)囊胚期、原肠期和耳状幼体期幼体进行热激处理,热激45 min后,将幼体在21.5℃条件下继续培养至耳状幼体后期,计算刺参幼体的存活率,并用蛋白杂交方法测定幼体Hsp70的表达量。结果表明,高温刺激对幼体的存活有显著性影响,各时期幼体的存活率均随热激温度的升高而降低,原肠期的存活率要高于囊胚期和耳状幼体时期。原肠期幼体Hsp70表达量要比囊胚期和耳状幼体时期高,这表明在发育过程中Hsp70表达量与幼体存活率具有相关性。     

X-rays from massive OB stars: thermal emission from radiative shocks

Chandra grating spectra of a sample of 15 massive OB stars were analysed under the basic assumption that the X-ray emission is produced in an ensemble of shocks formed in the winds driven by these objects. Shocks develop either as a result of radiation-driven instabilities or due to confinement of the wind by a relatively strong magnetic field, and since they are radiative, a simple model of their X-ray emission was developed that allows a direct comparison with observations. According to our model, the shock structures (clumps, complete or fractional shells) eventually become 'cold' clouds in the X-ray sky of the star. As a result, it is expected that for large covering factors of the hot clumps, there is a high probability for X-ray absorption by the 'cold' clouds, resulting in blueshifted spectral lines. Our analysis has revealed that such a correlation indeed exists for the considered sample of OB stars. As to the temperature characteristics of the X-ray emission plasma, the studied OB stars fall in two groups: (i) one with plasma temperature limited to ∼0.1–0.4 keV and (ii) the other with X-rays produced in plasmas at considerably higher temperatures. We argue that the two groups correspond to different mechanisms for the origin of X-rays: in radiation-driven instability shocks and in magnetically confined wind shocks, respectively.     

Formation of HD molecules in merging dark matter haloes

HD molecules can be an important cooling agent of the primordial gas behind the shock waves originated through merging of the dark matter haloes at epochs when the first luminous objects formed. We study the necessary conditions for the HD cooling to switch on in the low-temperature range   T < 200 K  . We show that these conditions are fulfilled in merging haloes with total (dark matter and baryon) mass in excess of   M _cr∼ 10⁷[(1 + z )/20]⁻² M_⊙  . Haloes with masses   M > M _cr  may be the sites of low-mass star formation.     

Martian shock and magnetic pile-up boundary positions and shapes determined from the Phobos 2 and Mars Global Surveyor data sets

A great many Martian bow shock and magnetic pile-up boundary crossings have been identified in the Phobos 2 and Mars Global Surveyor, MGS, data. From these observations the positions and shapes of the bow shock and magnetic pile-up boundary, MPB, have been derived and modelled, using curve-fitting techniques. The models thus derived separately from the Phobos 2 and MGS data sets do not differ drastically, despite the different time and space data coverages. The purpose of the paper is therefore to show the results obtained from the mixing of the Phobos 2 and MGS data bases and to compare the derived bow shock and MPB models with the ones obtained previously. The underlying objective was to see whether it was possible to determine improved bow shock and MPB models or not. The answer is definitely yes, and particularly for the MPB, thanks to the complementary nature of the observations. The boundaries crossed close to the subsolar direction or mostly far downstream by Phobos 2 indeed allow a better coverage of the Martian space environment to be considered. Nevertheless, in order to reduce the domination of the overabundant MGS data set and/or the crossings that are close to Mars (x>−4 R_M, i.e. x>−13 562 km) weighting factors have been introduced.