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181.
Patrick Petitjean 《Astrophysics and Space Science》2002,281(1-2):467-470
I describe briefly the status of an ongoing mini-survey for molecular hydrogen in high-redshift Damped Lyman-α systems using
UVES at the VLT. H2 is detected in about 30% of the cases. When H2 is not detected the molecular fraction f = 2N(H2)/(2N(H2)+N(HI) is smaller than 10-5. Therefore, most of the DLA systems arise in warm (T > 3000 K) and diffuse neutral gas embedded in a strong UV flux. The very recent detection of HD molecules in a Damped Lyman-α
system at z
abs = 2.337 demonstrates the possibility to discuss the high redshift chemistry.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
182.
Using the Very Large Array (VLA), we observed all three of the 6-cm lines of the 2 Π1/2 , J = 1/2 state of OH with sub-arcsecond resolution (∼0.4 arcsec) in W49A. While the spatial distribution and the range in velocities of the 6-cm lines are similar to those of the ground-state (18-cm) OH lines, a large fraction of the total emission in all three 6-cm lines has large linewidths (∼5–10 km s−1 ) and is spatially extended, very unlike typical ground-state OH masers, which typically are point-like at VLA resolutions and have linewidths ≤1 km s−1 . We find brightness temperatures of 5900, 4700 and ≥730 K for the 4660, 4750 and 4765-MHz lines, respectively. We conclude that these are indeed maser lines. However, the gains are ∼0.3, again very unlike the 18-cm lines, which have gains ≥104 . We compare the excited-state OH emission with that from other molecules observed with comparable angular resolution to estimate physical conditions in the regions emitting the peculiar, low-gain maser lines. We also comment on the relationship with the 18-cm masers. 相似文献
183.
A new technique of Fourier analysis, DFT/CLEAN, has been adapted for the study of solar time series. The technique was developed
by Roberts and his collaborators (1987, 1994) to address the limitations of other techniques of Fourier analysis and the shortcomings
of many astronomical time series. The utility of the technique is illustrated with several applications to solar time series. 相似文献
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186.
Patrick O'Sullivan 《The Professional geographer》1980,32(3):355-360
It is hypothesized that modern suburbia provides for greater mobility than cover in military operations. This ready penetrability gives the advantage to the attack rather than the defense. The strategic implication of this denial of current and traditional military thinking is that a Soviet attack along the Börde of Germany might progress more rapidly than presently envisaged and nullify the present NATO intention of winning time for a political solution to any conflict. 相似文献
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The influence of winter on methane (CH4) stored in pore water and emitted through snow was investigated in a temperate poor fen in New Hampshire over two winters. Methane accumulated beneath ice layers (1 cm) deposited by freezing rain, resulting in snow-pore air mixing ratios as high as 140 ppmv during the first winter and 600 ppmv during the second. An early winter snow crust of 300 kg m?3 caused no discontinuity in a linear mixing ratio profile and therefore was not observed to retard snowpack emissions. Methane concentration-depth profiles in pore water steepened and concentrations increased by as much as 400 μM at the 10 and 20 cm depths as the ice cover formed. This suggests that the peat-ice cover plays an important part in CH4 build-up in pore water by limiting the transport of gases between the peat and the atmosphere. Pore water concentrations gradually declined through late winter. The seasonality of dissolved CH4 in pore water over two winters and one summer showed an average annual amplitude of 1.3 gCH4m?2 (25–75cm depth range), with a winter maximum of 4.7gCH4m?2. Emissions during the winter with average snowfall accounted for a larger percentage (9.2% in 1993–1994) of total annual emission than the winter with below-average snowfall and warmer air temperature (2% in 1994–1995). Emissions averaged 56 and 26mg m?2 day?1 during the first and second winter (December, January and February), respectively. 相似文献
190.
Models of the collapse of a protostellar cloud and the formation of the solar nebula reveal that the size of the nebula produced will be the larger of (where J, M, and cs are the total angular momentum, total mass, and sound speed of the protosetellar material; G is the gravitational constant; k is a number of order unity; and v is the effective viscosity in the nebula). From this result it can be deduced that low-mass nebulas are produced if P ≡ (RV/RCF)2 ? 1; “massive” nebulas result if P ? 1. Gravitational instabilities are expected to be important for the evolution of P ? 1 nebulas. The value of J distinguishes most current models of the solar nebula, since P ∝ J?4. Analytic expressions for the surface density, nebular mass flux, and photospheric temperature distributions during the formation stage are presented for some simple models that illustrate the general properties of growing protostellar disks. It does not yet seem possible to rule out either P ? 1 or P < 1 for the solar nebula, but observed or possible heterogeneities in composition and angular-momentum orientation favor P < 1 models. 相似文献