121 research outputs found
Characterization of fluids involved in the Gneiss-Charnockite transformation in Southern Kerala (India)
The characterization of fluids involved in the gneiss-charnockite transformation in southern Kerala are discussed. Using a variety of techniques, including microthermometry, Raman laser probe analysis, and mass spectrometry, it was concluded that the CO2-rich, N2-bearing metamorphic fluids in these rocks were internally-derived rather than having been introduced by CO2-streaming
Kabbaldurga-type charnockitization: A local phenomenon in the granulite to amphibolite grade transition zone
In the deeply eroded Precambrian crust of South India and Sri Lanka, a series of spectacular exposures shows progressive development of coarse-grained charnockite through dehydration of amphibolite grade gneisses in different arrested stages. At Kabbaldurga, charnockitization of Archaean grey biotite-hornblende gneisses occurred about 2.5 Ga ago and evidently was induced by the influx of external carbonic fluids along a system of ductile shears and the foliation planes. The results of oxygen isotope thermometry and of geothermobarometry in adjacent areas indicate a P-T regime of 700 to 750 C and 5 to 7 kb. The decrease of water activity in the fluid infiltrated zones caused an almost complete breakdown of hornblende and biotite and the new growth of hypersthene. Detailed petrographic and geochemical studies revealed marked changes in mineralogy and chemistry from granodioritic to granitic which document the metasomatic nature of the process
Petrogenesis of Tertiary Alkaline Magmas in the Siebengebirge, Germany
Basanites from the Tertiary Siebengebirge area of Germany (part of the Central European Volcanic Province; CEVP) have high Mg# (>0·60), moderate to high Cr (>300 ppm) and Ni (>200 ppm) contents and strong light rare earth element enrichment, but systematic depletion in Rb and K relative to trace elements of similar compatibility in anhydrous mantle. Rare earth element melting models can explain the petrogenesis of these basanites in terms of partial melting of a spinel peridotite source containing residual amphibole. It is inferred that amphibole, indicated by the relative K and Rb depletion and the melting model, was precipitated in the spinel peridotite lithospheric mantle beneath the Siebengebirge, by metasomatic fluids or melts from a rising mantle diapir or plume. Alkali basalts and more differentiated rocks have lower Mg# and lower abundances of Ni and Cr, and have undergone fractionation of mainly olivine, clinopyroxene, Fe-Ti oxides, amphibole and plagioclase. Most of the basanites and alkali basalts approach the Sr-Nd-Pb isotope compositions inferred for the European Asthenospheric Reservoir component. Trace element constraints (i.e. low Nb/U and Ce/Pb ratios) and the Sr-Nd-Pb isotope composition of the differentiated rocks indicate that assimilation of lower crustal material has modified the composition of the primary mantle-derived magmas. High 207Pb/204Pb ratios in the differentiated lavas point to assimilation of ancient lower crustal components having high U/Pb and Th/Pb ratios. Relatively shallow melting of inferred amphibole-bearing spinel peridotite sources may suggest an origin from the metasomatized part of the thermal boundary layer. Application of new thermobarometric equations for the basaltic magmas indicates relatively normal mantle potential temperatures (1300-1400°C); thus the inferred mantle ‘baby plume' or ‘hot finger' is not thermally anomalou
Physical Conditions and Star Formation Activity in the Intragroup Medium of Stephan's Quintet
New multi-band observations of the famous compact group of galaxies Stephan's
Quintet (SQ) are presented and analyzed. These include far infrared (FIR)
images at 60 and 100 (ISOPHOT C-100 camera), radio continuum
images at 1.4 GHz (VLA B-array) and 4.86 GHz (VLA C-array), and long-slit
optical spectrographs (Palomar telescope). With these new data, we aim
to learn more about the X-ray/radio ridge in the middle of the intragroup
medium (IGM) and the IGM starburst SQ-A, both are likely to be caused by the
high speed collision ( km s) between the intruder galaxy NGC
7318b ( km s) and the IGM ( km s).Comment: 31 pages text, 17 figures. Accepted by ApJ. A PS file including all
figures can be found in
http://spider.ipac.caltech.edu/staff/cxu/preprints/sq/apj_sq.ps.g
A multi-scale study of infrared and radio emission from Scd galaxy M33
We investigate the energy sources of the infrared (IR) emission and their
relation to the radio continuum emission at various spatial scales within the
Scd galaxy M33. We use the wavelet transform to analyze IR data at the Spitzer
wavelengths of 24, 70, and 160m, as well as recent radio continuum data at
3.6cm and 20cm. An H map serves as a tracer of the star forming regions
and as an indicator of the thermal radio emission. We find that the dominant
scale of the 70m emission is larger than that of the 24m emission,
while the 160m emission shows a smooth wavelet spectrum. The radio and
H maps are well correlated with all 3 MIPS maps, although their
correlations with the 160m map are weaker. After subtracting the bright
HII regions, the 24 and 70m maps show weaker correlations with the 20cm
map than with the 3.6cm map at most scales. We also find a strong correlation
between the 3.6cm and H emission at all scales. Comparing the results
with and without the bright HII regions, we conclude that the IR emission is
influenced by young, massive stars increasingly with decreasing wavelength from
160 to 24m. The radio-IR correlations indicate that the warm dust-thermal
radio correlation is stronger than the cold dust-nonthermal radio correlation
at scales smaller than 4kpc. A perfect 3.6cm-H correlation implies that
extinction has no significant effect on H emitting structures.Comment: 15 pages, 10 figures, accepted for publication in the Astronomy and
Astrophysics Journa
Molecular gas in the Andromeda galaxy
We present a new 12CO(J=1-0)-line survey of the Andromeda galaxy, M31,
covering the bright disk with the highest resolution to date (85 pc along the
major axis), observed On-the-Fly (in italics) with the IRAM 30-m telescope. We
discuss the distribution of the CO emission and compare it with the
distributions of HI and emission from cold dust traced at 175mum. Our main
results are: 1. Most of the CO emission comes from the radial range R=3-16 kpc,
but peaks near R=10 kpc. The emission is con- centrated in narrow, arm-like
filaments defining two spiral arms with pitch angles of 7d-8d. The average
arm-interarm brightness ratio along the western arms reaches 20 compared to 4
for HI. 2. For a constant conversion factor Xco, the molecular fraction of the
neutral gas is enhanced in the arms and decreases radially. The apparent
gas-to-dust ratios N(HI)/I175 and (N(HI)+2N(H2))/I175 increase by a factor of
20 between the centre and R=14 kpc, whereas the ratio 2N(H2)/I175 only
increases by a factor of 4. Implications of these gradients are discussed. In
the range R=8-14 kpc total gas and cold dust are well correlated; molecular gas
is better correlated with cold dust than atomic gas.Comment: 21 pages, 16 figures. Accepted for publication in A&
Relating dust, gas and the rate of star formation in M31
We derive distributions of dust temperature and dust opacity across M31 at
45" resolution using the Spitzer data. With the opacity map and a standard dust
model we de-redden the Ha emission yielding the first de-reddened Ha map of
M31. We compare the emissions from dust, Ha, HI and H2 by means of radial
distributions, pixel-to-pixel correlations and wavelet cross-correlations. The
dust temperature steeply decreases from 30K near the center to 15K at large
radii. The mean dust optical depth at the Ha wavelength along the line of sight
is about 0.7. The radial decrease of the dust-to-gas ratio is similar to that
of the oxygen abundance. On scales<2kpc, cold dust emission is best correlated
with that of neutral gas and warm dust emission with that of ionized gas. Ha
emission is slightly better correlated with emission at 70um than at 24um. In
the area 6kpc<R< 17kpc, the total SFR is ~0.3Msun/yr. The Kennicutt-Schmidt law
between SFR and total gas has a power-law index of 1.30+-0.05 in the radial
range of R=7-11kpc increasing by about 0.3 for R=11-13kpc. The lack of H2 in
the central region could be related to the lack of HI and the low opacity/high
temperature of the dust. Since neither SFR nor SFE is well correlated with the
surface density of H2 or total gas, other factors than gas density must play an
important role in the formation of massive stars in M31. The molecular
depletion time scale of 1.1 Gyr indicates that M31 is about three times less
efficient in forming young massive stars than M33.Comment: 22 pages accepted for publication in A&
An analysis of the FIR/RADIO Continuum Correlation in the Small Magellanic Cloud
The local correlation between far-infrared (FIR) emission and radio-continuum
(RC) emission for the Small Magellanic Cloud (SMC) is investigated over scales
from 3 kpc to 0.01 kpc. Here, we report good FIR/RC correlation down to ~15 pc.
The reciprocal slope of the FIR/RC emission correlation (RC/FIR) in the SMC is
shown to be greatest in the most active star forming regions with a power law
slope of ~1.14 indicating that the RC emission increases faster than the FIR
emission. The slope of the other regions and the SMC are much flatter and in
the range of 0.63-0.85. The slopes tend to follow the thermal fractions of the
regions which range from 0.5 to 0.95. The thermal fraction of the RC emission
alone can provide the expected FIR/RC correlation. The results are consistent
with a common source for ultraviolet (UV) photons heating dust and Cosmic Ray
electrons (CRe-s) diffusing away from the star forming regions. Since the CRe-s
appear to escape the SMC so readily, the results here may not provide support
for coupling between the local gas density and the magnetic field intensity.Comment: 19 pages, 7 Figure
Molecular gas in NGC6946
We present imaging of molecular gas emission in the star-forming spiral
galaxy NGC6946. Our CO(1-0) and CO(3-2) images, made at 22" resolution with the
IRAM 30-m and the Heinrich Hertz 10-m radio telescopes, are the most extensive
CO observations of this galaxy and are among the most extensive observations of
molecular gas in any spiral galaxy. The molecular component in NGC6946 is
unusually massive, with a ratio of molecular to atomic Hydrogen of 0.57. A star
formation efficiency image for NGC6946 ranges by over two orders of magnitude
with highest values found in the northeastern spiral arm, and anticorrelates
with the 6cm polarized emission image, which traces the regular part of the
magnetic field. We analyse the ISM in NGC6946's disk by making 1-D and 2-D
comparisons of images made in several wavebands. A point-by-point correlation
technique finds that the molecular gas is closely associated with the
7micron-emitting dust. The high correlation found between the MIR emission and
the radio continuum at 6cm cannot be due to dust heating and gas ionization in
star-forming regions because the thermal radio emission is less correlated with
the MIR than the nonthermal emission. A coupling of magnetic fields to gas
clouds is proposed as a possible scenario.Comment: A&A accepted, 23 pages, 11 figures. Version with high resolution
figures available at: http://cfa-www.harvard.edu/~wwalsh/sp.htm
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