Compact massive object

However, according to the latest estimates, the fraction of our Galaxy s dark halo that could be explained by baryonic matter (low-luminosity stars and non-luminous compact, massive objects) cannot exceed 20%. These estimates are based on the effect such objects would have on the hght from stars in the Magellanic Clouds. It is concluded that the halo of our Galaxy, and probably that of other spirals of this type, is not principally made up of ordinary, atomic matter. 

Note that the baryonic density of 2% to 5% is of the same order of magnitude as the galactic density indicated by rotation curves (viz. 5%). It is thus perfectly reasonable to suggest that a large part of galactic matter is located in compact massive objects (CMOs) assumed to swarm around the bright galactic disk. Will these compact massive objects remain forever hidden ... 

To everyone s surprise, no event lasting less than two weeks has ever been observed. The mean mass inferred is half the solar mass, which rules out brown and red dwarfs but favours white dwarfs. However, too many of these remnants from intermediate-mass stars (1-8 Mq at birth) would contradict the traditional tenets of astrophysics. Indeed, it would imply a frenzied spate of nucleosynthesis during the formation of the galactic halo. The nature of the compact massive objects thus remains a mystery. 

Consequently, we are forced to admit that compact massive objects are not the main component of the dark halo of our Galaxy, or any of its kind. This in turn implies that the Sun and stars are lost amongst a halo of darkness, in the middle of a haze of neutralinos, hypothetical particles predicted by so-called supersymmetric particle theories. 

Points two and four seem incompatible, usually the massive objects are slower than light ones for a given density. Thus good quantity to look at is not mass but compactness C (see Bonazzola in Barone et al., Eds., 2000). Compactness can be defined as ... 

Primordial black holes (PBH) cannot be entirely ruled out at present, provided that they formed early enough not to have been baryonic during nucleosynthesis. One can say that either the universe is not closed by black holes near 1015g, or Hawking radiation doesn t happen, or both (Ap02, Sect. 12.7). Planet-to-star masses that would gravitationally lens stars behind them (called MACHOs for MAssive Compact Halo Objects) are excluded, and so are (a) blacks holes of 105 6 solar masses, which would mess up galactic... 

Three most popular DM candidates could contribute to the explanation of the above wealth of observations. Historically, faint stars/planetary objects constituted of baryonic matter were invoked first, with masses smaller than 0.1 solar mass (this is the mass limit minimally needed for nuclear burning and the subsequent electromagnetic radiation). The search for massive compact halo objects (MACHOs) was initiated in the early 1990s based on the so-called microlensing effect — a temporary variation of the brightness of a star when a MACHO crosses the line of sight between the star and the observer. This effect is sensitive to all kind of dark matter, baryonic or nonbaryonic. The very conservative combined conclusion from these observations and some theoretical considerations is that at most 20% of the galactic halo can be made up of stellar remnants (Alcock et al. 2000). 

Black hole A compact object produced by the collapse of a massive object such as a star or star cluster from self-gravity. The object collapses to such a small size that not even light can escape from it. 

In a moment of dark fancy, we might imagine the halo of the Milky Way run through with sombre, massive and compact objects, black stars and inky clouds. Just what is needed to excite the curiosity of astronomers. Some have aheady dreamt up a vast celestial cemetery where stellar corpses and aborted stars gradually accumulate. 

The time scale of such an event is about a few days to a few month depending on the mass of the deflectors. Currently a fair number of such events have been recorded and constraints on the content of our halo with low massive compact objects have been put. 

Below we focus on the observed association of GRBs with an energetic subclass of core-collapse supernovae, type Ibc SNe, which with each new finding provides an increasing evidence that the GRB phenomenon is related to the evolution of most massive stars and formation of relativistic compact objects (neutron stars and black holes). 

In this short paper at first I shall try to show links between studies of cosmic rays (CRs) and studies of close-by compact objects (in particular neutron stars - NSs). The reason for an existence of such relation is obvious both phenomena (CRs and NSs) have the same origin - supernova (SN) explosions. Then I discuss in more details the population synthesis of close-by NSs. The analysis of the population of these sources makes us to conclude that the solar neighbourhood (by that I mean a region about few hundred of parsecs around the Sun) in enriched with young NSs. It is a natural consequence of the existence of the Gould Belt - local structure formed by massive stars. 

The massive star forming regions are statistically more distant than the low mass star forming regions, leading to limited spatial resolutirai and a more difficult recognition of small compact objects. 

As long as the core left behind the supernova explosion is less than about three solar masses, the collapse will be stopped by the pressure of degenerate neutrons. Thus a neutron star is formed which is a very compact object with a radius of only 10 km. Pulsars are rapidly rotating neutron stars. A famous example of a supernova remnant is the Crab nebula. The shell of this nebula expands at velocities of up to 1500 km/s. The filaments contain anomalously high abundances of helium and other more massive chemical elements. The Crab Pulsar rotates 30 times a second (see Fig. 8.9). The Crab Pulsar (PSR B0531-I-21) is a relatively young neutron star. The star is the central star in the Crab Nebula, a remnant of the supernova SN 1054 and it was observed e.g. by ancient Chinese astronomers. 

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