898 research outputs found
Capture of field stars by giant interstellar clouds: the formation of moving stellar groups
In the solar neighbourhood, there are moving groups of stars with similar
ages and others of stars with heterogeneous ages as the field stars. To explain
these facts, we have constructed a simple model of three phases. Phase A: a
giant interstellar cloud is uniformly accelerated (or decelerated) with respect
to the field stars during a relatively short period of time (10 Myr) and the
cloud's mass is uniformly increased; phase B: the acceleration (or
deceleration) and mass accretion of the cloud cease. The star formation spreads
throughout the cloud, giving origin to stellar groups of similar ages; and
phase C: the cloud loses all its gaseous component at a constant rate and in
parallel is uniformly decelerated (or accelerated) until reaching the initial
velocity of phase A (case 1) or the velocity of the gas cloud remains constant
(case 2). Both cases give equivalent results. The system equations for the star
motions governed by a time-dependent gravitational potential of the giant cloud
and referred to a coordinate system co-moving with the cloud have been solved
analytically. We have assumed a homogeneous spheroidal cloud of fixed
semi-major axis a=300 pc and of an initial density of 7 at cm^{-3}, with a
density increment of 100 per cent and a cloud's velocity variation of 30 km
s^{-1}, from the beginning to the end of Phase A. The result is that about 4
per cent of the field stars that are passing within the volume of the cloud at
the beginning of phase A is captured. The Sun itself could have been captured
by the same cloud that originated the moving groups of the solar neighbourhood.Comment: 13 pages, 15 figure
Magnetic resonance in iron-doped magnesium oxide
A linewidth analysis of the Âœ â -Âœ transition of Fe(^3+) in Magnesium Oxide crystals has been made using electron spin resonance techniques at x-band and at room and nitrogen temperatures. The values of linewidth at 6=0 (i.e. magnetic field along fourfold axis of the crystal) lay between 7 and 9 Gauss for specimens containing from 140 to 11900 p.p.m. Fe. The experimental linewidths measured for a range of samples show that the linewidth is concentration independent, as opposed to the (concentration)(^Âœ) dependence expected from Van Vleck's second moment theory. The discrepancy is explained on the grounds of exchange narrowing interaction between the ferric ions and the idea is supported by the values of the ratio of the fourth to second moments obtained by numerical integration of the absorption3+derivative line. The linewidth data indicates that the Fe(^3+) enters the lattice substitutionally, occupying Mg sites, at concentrations of up to11,900 p.p.m. Fe. Preliminary spin-lattice relaxation time measurements have also been made using the pulse-saturation method at 35.5 GHz over the temperature range 4.2 - 27 K. Experimental evidence is presented for a fast relaxing process taking place in all the samples studied; this is explained in terms of spin exchange-interaction (Fe(^3+) âFe(^3+) by correlation with the proposed linewidth model. It also shows angular and concentration dependence and it is suggested that cross-relaxation between different parts of the spin system may account for variations observed. As regards the concentration dependence it is tentatively proposed that the spin-lattice relaxation time obeys a (concentration) (^-1) law and comparison is made with results published for other ions in the same host lattice. Using 9 GHz results available in the literature it is shown that Mattuck and Strandberg's (resonance frequency)(^-2) dependence for T(_1) holds for Fe(^3+) in magnesia; furthermore there is approximate agreement of the experimental points with T(^-1)(direct) and T(^-7) (Raman) lines for the temperature variation which suggests that the slower relaxation process proceeds as for a single ion relaxing to the lattice
L1521E: A Starless Core in the Early Evolutionary Stage ?
We have studied the physical and chemical properties of a quiescent starless
core L1521E with various molecular lines. It is found that there exists a
compact dense core traced by the H^13CO^+, HN^13C, CCS, and HC_3N lines; their
distributions have a single peak at the same position. The core radius is as
small as 0.031 pc, whereas the H_2 density at the peak position is as high as
(1.3-5.6)times10^5 cm^-3. Although the density is high enough to excite the
inversion transitions of NH_3, these lines are found to be very faint in
L1521E. The distributions of NH_3 and CCS seem to be different from those of
well-studied starless cores, L1498 and L1544, where the distribution of CCS
shows a shell-like structure while that of NH_3 is concentrated at the center
of the core. Abundances of carbon-chain molecules are higher in L1521E than the
other dark cloud cores, and especially those of sulfur-bearing molecules C_nS
are comparable to the cyanopolyyne peak of TMC-1. Our results suggest that
L1521E would be in a very early stage of physical and chemical evolution.Comment: 10 pages, 3 EPS figures, uses aaspp4.sty and epsf.sty, AAS LaTeX
macros v4.0, The Astrophysical Journal, in pres
Molecular Line Observations of Carbon-Chain-Producing Regions L1495B and L1521B
We present the first comprehensive study on physical and chemical properties
of quiescent starless cores L1495B and L1521B, which are known to be rich in
carbon-chain molecules like the cyanopolyyne peak of TMC-1 and L1521E. We have
detected radio spectral lines of various carbon-chain molecules such as CCS,
CS, CH, HCN, and HCN. On the other hand, the NH
lines are weak and the NH lines are not detected. According to our
mapping observations of the HCN, CCS, and CS lines, the dense cores
in L1495B and L1521B are compact with the radius of 0.063 and 0.044 pc,
respectively, and have a simple elliptical structure. The distributions of CCS
seem to be different from those of well-studied starless cores, L1498 and
L1544, where the distribution of CCS shows a shell-like structure. Since the
HCO, HNC, and CS lines are detected in L1495B and
L1521B, the densities of these cores are high enough to excite the NH and
NH lines. Therefore, the abundances of NH and NH
relative to carbon-chain molecules are apparently deficient, as observed in
L1521E. We found that longer carbon-chain molecules such as HCN and
CH are more abundant in TMC-1 than L1495B and L1521B, while those of
sulfur-bearing molecules such as CS, CCS, and CS are comparable.
Both distributions and abundances of the observed molecules of L1495B and
L1521B are quite similar to those of L1521E, strongly suggesting that L1495B
and L1521B is in a very early stage of physical and chemical evolution.Comment: 19 pages, 6 figures, accepted to The Astrophysical Journa
Detection of HC11N in the Cold Dust Cloud TMC-1
Two consecutive rotational transitions of the long cyanopolyyne HC11N,
J=39-38, and J=38-37, have been detected in the cold dust cloud TMC-1 at the
frequencies expected from recent laboratory measurements by Travers et al.
(1996), and at about the expected intensities. The astronomical lines have a
mean radial velocity of 5.8(1) km/s, in good agreement with the shorter
cyanopolyynes HC7N and HC9N observed in this very sharp-lined source [5.82(5)
and 5.83(5) km/s, respectively]. The column density of HC11N is calculated to
be 2.8x10^(11) cm^(-2). The abundance of the cyanopolyynes decreases smoothly
with length to HC11N, the decrement from one to the next being about 6 for the
longer carbon chains.Comment: plain tex 10 pages plus 3 ps fig file
The nature of the dense core population in the Pipe Nebula: A survey of NH3, CCS, and HC5N molecular line emission
Recent extinction studies of the Pipe Nebula (d=130 pc) reveal many cores
spanning a range in mass from 0.2 to 20.4 Msun. These dense cores were
identified via their high extinction and comprise a starless population in a
very early stage of development. Here we present a survey of NH3 (1,1), NH3
(2,2), CCS (2_1,1_0), and HC5N (9,8) emission toward 46 of these cores. An
atlas of the 2MASS extinction maps is also presented. In total, we detect 63%
of the cores in NH3 (1,1) 22% in NH3 (2,2), 28% in CCS, and 9% in HC5N
emission. We find the cores are associated with dense gas (~10^4 cm-3) with 9.5
< T_k < 17 K. Compared to C18O, we find the NH3 linewidths are systematically
narrower, implying that the NH3 is tracing the dense component of the gas and
that these cores are relatively quiescent. We find no correlation between core
linewidth and size. The derived properties of the Pipe cores are similar to
cores within other low-mass star-forming regions: the only differences are that
the Pipe cores have weaker NH3 emision and most show no current star formation
as evidenced by the lack of embedded infrared sources. Such weak NH3 emission
could arise due to low column densities and abundances or reduced excitation
due to relatively low core volume densities. Either alternative implies that
the cores are relatively young. Thus, the Pipe cores represent an excellent
sample of dense cores in which to study the initial conditions for star
formation and the earliest stages of core formation and evolution.Comment: 35 pages, 10 figures (excluding the appendix). For the complete
appendix contact [email protected]. Accepted for publication in ApJ
The high-velocity clouds and the Magellanic Clouds
From an analysis of the sky and velocity distributions of the high-velocity
clouds (HVCs) we show that the majority of the HVCs has a common origin. We
conclude that the HVCs surround the Galaxy, forming a metacloud of 300 kpc in
size and with a mass of 3 10^9 M_sun, and that they are the product of a
powerful ``superwind'' (about 10^58 ergs), which occurred in the Magellanic
Clouds about 570 Myr ago as a consequence of the interaction of the Large and
Small Magellanic Clouds. The HVCs might be magnetic bubbles of semi-ionized
gas, blown from the Magellanic Clouds around 570 Myr ago, that circulate
largely through the halo of the Galaxy as a stream or flow of gas.Comment: 28 pages with 23 figure
Triggered Star Formation by Massive Stars
We present our diagnosis of the role that massive stars play in the formation
of low- and intermediate-mass stars in OB associations (the Lambda Ori region,
Ori OB1, and Lac OB1 associations). We find that the classical T Tauri stars
and Herbig Ae/Be stars tend to line up between luminous O stars and
bright-rimmed or comet-shaped clouds; the closer to a cloud the progressively
younger they are. Our positional and chronological study lends support to the
validity of the radiation-driven implosion mechanism, where the Lyman continuum
photons from a luminous O star create expanding ionization fronts to evaporate
and compress nearby clouds into bright-rimmed or comet-shaped clouds. Implosive
pressure then causes dense clumps to collapse, prompting the formation of
low-mass stars on the cloud surface (i.e., the bright rim) and
intermediate-mass stars somewhat deeper in the cloud. These stars are a
signpost of current star formation; no young stars are seen leading the
ionization fronts further into the cloud. Young stars in bright-rimmed or
comet-shaped clouds are likely to have been formed by triggering, which would
result in an age spread of several megayears between the member stars or star
groups formed in the sequence.Comment: 2007, ApJ, 657, 88
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