663 research outputs found
Formation of Stellar Clusters and the Importance of Thermodynamics for Fragmentation
We discuss results from numerical simulations of star cluster formation in
the turbulent interstellar medium (ISM). The thermodynamic behavior of the
star-forming gas plays a crucial role in fragmentation and determines the
stellar mass function as well as the dynamic properties of the nascent stellar
cluster. This holds for star formation in molecular clouds in the solar
neighborhood as well as for the formation of the very first stars in the early
universe. The thermodynamic state of the ISM is a result of the balance between
heating and cooling processes, which in turn are determined by atomic and
molecular physics and by chemical abundances. Features in the effective
equation of state of the gas, such as a transition from a cooling to a heating
regime, define a characteristic mass scale for fragmentation and so set the
peak of the initial mass function of stars (IMF). As it is based on fundamental
physical quantities and constants, this is an attractive approach to explain
the apparent universality of the IMF in the solar neighborhood as well as the
transition from purely primordial high-mass star formation to the more normal
low-mass mode observed today.Comment: 10 pages, invited review, to appear in Dynamical Evolution of Dense
Stellar Systems, Proceed. of the IAU Symp. 246 (Capri, Sept. 2007), eds.
E.Vesperini, M. Giersz, and A. Sill
The CN Isotopic Ratios In Comets
Our aim is to determine the isotopic ratios (12)C/(13)C and (14)N/(15)N in a variety of comets and link these measurements to the formation and evolution of the solar system. The (12)C/(13)C and (14)N/(15)N isotopic ratios are measured for the CN radical by means of high-resolution optical spectra of the R branch of the B-X (0, 0) violet band. 23 comets from different dynamical classes have been observed, sometimes at various heliocentric and nucleocentric distances, in order to estimate possible variations of the isotopic ratios in parent molecules. The (12)C/(13)C and (14)N/(15)N isotopic ratios in CN are remarkably constant (average values of, respectively, 91.0 +/- 3.6 and 147.8 +/- 5.7) within our measurement errors, for all comets whatever their origin or heliocentric distance. While the carbon isotopic ratio does agree with the terrestrial value (89), the nitrogen ratio is a factor of two lower than the terrestrial value (272), indicating a fractionation in the early solar system, or in the protosolar nebula, common to all the comets of our sample. This points towards a common origin of the comets independently of their birthplaces, and a relationship between HCN and CN.NSFAstronom
Molecular ions in L1544. I. Kinematics
We have mapped the dense dark core L1544 in H13CO+(1-0), DCO+(2-1),
DCO+(3-2), N2H+(1-0), NTH+(3-2), N2D+(2-1), N2D+(3-2), C18O(1-0), and C17O(1-0)
using the IRAM 30-m telescope. We have obtained supplementary observations of
HC18O+(1-0), HC17O+(1-0), and D13CO+(2-1). Many of the observed maps show a
general correlation with the distribution of dust continuum emission in
contrast to C18O(1-0) and C17O(1-0) which give clear evidence for depletion of
CO at positions close to the continuum peak. In particular N2D+(2-1) and (3-2)
and to a lesser extent N2H+(1-0) appear to be excellent tracers of the dust
continuum. We find that the tracers of high density gas (in particular N2D+)
show a velocity gradient along the minor axis of the L1544 core and that there
is evidence for larger linewidths close to the dust emission peak. We interpret
this using the model of the L1544 proposed by Ciolek & Basu (2000) and by
comparing the observed velocities with those expected on the basis of their
model. The results show reasonable agreement between observations and model in
that the velocity gradient along the minor axis and the line broadening toward
the center of L1544 are predicted by the model. This is evidence in favour of
the idea that amipolar diffusion across field lines is one of the basic
processes leading to gravitational collapse. However, line widths are
significantly narrower than observed and are better reproduced by the Myers &
Zweibel (2001) model which considers the quasistatic vertical contraction of a
layer due to dissipation of its Alfvenic turbulence, indicating the importance
of this process for cores in the verge of forming a star.Comment: 24 pages, 9 figures, to be published in Ap
Molecular ions in L1544. II. The ionization degree
The maps presented in Paper I are here used to infer the variation of the column densities of HCO+, DCO+, N2H+, and N2D+ as a function of distance from the dust peak. These results are interpreted with the aid of a crude chemical model which predicts the abundances of these species as a function of radius in a spherically symmetric model with radial density distribution inferred from the observations of dust emission at millimeter wavelengths and dust absorption in the infrared. Our main observational finding is that the N(N2D+)/N(N2H+) column density ratio is of order 0.2 towards the L1544 dust peak as compared to N(DCO+)/N(HCO+) = 0.04. We conclude that this result as well as the general finding that N2H+ and N2D+ correlate well with the dust is caused by CO being depleted to a much higher degree than molecular nitrogen in the high density core of L1544. Depletion also favors deuterium enhancement and thus N2D+, which traces the dense and highly CO-depleted core nucleus, is much more enhanced than DCO+. Our models do not uniquely define the chemistry in the high density depleted nucleus of L1544 but they do suggest that the ionization degree is a few times 10^{-9} and that the ambipolar diffusion time scale is locally similar to the free fall time. It seems likely that the lower limit which one obtains to ionization degree by summing all observable molecular ions is not a great underestimate of the true ionization degree. We predict that atomic oxygen is abundant in the dense core and, if so, H3O+ may be the main ion in the central highly depleted region of the core
The 16OH/18OH and OD/OH isotope ratios in comet C/2002 T7 (LINEAR)
The 16OH/18OH and OD/OH isotope ratios are measured in the Oort-Cloud comet
C/2002 T7 (LINEAR) through ground-based observations of the OH ultraviolet
bands at 3063 A (0,0) and 3121 A (1,1) secured with the Very Large Telescope
(VLT) feeding the Ultraviolet-Visual Echelle Spectrograph (UVES). From the
16OH/18OH ratio, we find 16O/18O = 425 +/- 55, equal within the uncertainties
to the terrestrial value and to the ratio measured in other comets, although
marginally smaller. We also estimate OD/OH from which we derive D/H = 2.5 +/-
0.7 10-4 in water. This value is compatible with the water D/H ratios evaluated
in other comets and marginally higher than the terrestrial value.Comment: Accepted for publication in A&A Letter
Deep Impact : High Resolution Optical Spectroscopy with the ESO VLT and the Keck 1 telescope
We report on observations of comet 9P/Tempel 1 carried out before, during,
and after the NASA DEEP IMPACT event (UT July 4), with the optical
spectrometers UVES and HIRES mounted on the telescopes Kueyen of the ESO VLT
(Chile) and Keck 1 on Mauna Kea (Hawaii), respectively. A total observing time
of about 60 hours, distributed over 15 nights around the impact date, allowed
us (i) to find a periodic variation of 1.709 +/- 0.009 day in the CN and NH
flux, explained by the presence of two major active regions; (ii) to derive a
lifetime > ~ 5 x 10^4 s for the parent of the CN radical from a simple modeling
of the CN light curve after the impact; (iii) to follow the gas and dust
spatial profiles evolution during the 4 hours following the impact and derive
the projected velocities (400 m/s and 150 m/s respectively); (iv) to show that
the material released by the impact has the same carbon and nitrogen isotopic
composition as the surface material (12C/13C = 95 +/- 15 and 14N/15N = 145 +/-
20).Comment: Accepted for publication in ApJ Letter
A hydroalcoholic gel-based disinfection system for deteriogenic fungi on the contemporary mixed media artwork Poesia by Alessandro Kokocinski
The disinfection of deteriogenic microorganisms and the removal of induced chromatic alterations in artworks are still open challenges in the field of conservation. For this purpose, a new alcoholic hydrogel was tested to remove an extensive fungal attack from a multimaterial collage by the artist Alessandro Kokocinski and to mitigate chromatic changes caused by the contamination of its poster paper and plywood support layers. A Gellan gum-based hydrogel was used, which was modified by adding a high concentration of alcohol (66.7% ethanol), to give the system an effective disinfecting agent in addition to the detergent capacity of the gel for water-sensitive works of art. It was successfully tested on samples mimicking the complex stratigraphy of the artwork under study. To create replica mock-ups, the artwork materials and stratigraphy were investigated through diagnostic and laboratory techniques such as multispectral imaging, X-ray fluorescence spectroscopy, Fourier transform infrared spectroscopy, and pyrolysis coupled with gas-chromatography-mass spectrometry. The treatment was shown to have a disinfecting effect on the test samples and did not alter their structure, allowing us to apply the method to the artwork. Here, the hydrogel successfully removed and inhibited fungal proliferation in addition to mitigating the color changes caused by fungi
The dust temperature distribution in prestellar cores
We have computed the dust temperature distribution to be expected in a
pre-protostellar core in the phase prior to the onset of gravitational
instability. We have done this in the approximation that the heating of the
dust grains is solely due to the attenuated external radiation field and that
the core is optically thin to its own radiation. This permits us to consider
non spherically symmetric geometries. We predict the intensity distributions of
our model cores at millimeter and sub-millimeter wavelengths and compare with
observations of the well studied object L1544. We have also developed an
analytical approximation for the temperature at the center of spherically
symmetric cores and we compare this with the numerical calculations. Our
results show (in agreement with Evans et al. 2001) that the temperatures in the
nuclei of cores of high visual extinction (> 30 magnitudes) are reduced to
values of below ~8 K or roughly half of the surface temperature. This has the
consequence that maps at wavelengths shortward of 1.3 mm see predominantly the
low density exterior of pre-protostellar cores. It is extremely difficult to
deduce the true density distribution from such maps alone. We have computed the
intensity distribution expected on the basis of the models of Ciolek & Basu
(2000) and compared with the observations of L1544. The agreement is good with
a preference for higher inclinations (37 degrees instead of 16) than that
adopted by Ciolek & Basu (2000). We find that a simple extension of the
analytic approximation allows a reasonably accurate calculation of the dust
temperature as a function of radius in cores with density distributions
approximating those expected for Bonnor-Ebert spheres and suggest that this may
be a useful tool for future calculations of the gas temperature in such cores.Comment: 14 latex pages, 10 ps figures, A&A accepte
Molecular ions in L1544. II. The ionization degree
The maps presented in Paper I are here used to infer the variation of the
column densities of HCO+, DCO+, N2H+, and N2D+ as a function of distance from
the dust peak. These results are interpreted with the aid of a crude chemical
model which predicts the abundances of these species as a function of radius in
a spherically symmetric model with radial density distribution inferred from
the observations of dust emission at millimeter wavelengths and dust absorption
in the infrared. Our main observational finding is that the N(N2D+)/N(N2H+)
column density ratio is of order 0.2 towards the L1544 dust peak as compared to
N(DCO+)/N(HCO+) = 0.04. We conclude that this result as well as the general
finding that N2H+ and N2D+ correlate well with the dust is caused by CO being
depleted to a much higher degree than molecular nitrogen in the high density
core of L1544. Depletion also favors deuterium enhancement and thus N2D+, which
traces the dense and highly CO-depleted core nucleus, is much more enhanced
than DCO+. Our models do not uniquely define the chemistry in the high density
depleted nucleus of L1544 but they do suggest that the ionization degree is a
few times 10^{-9} and that the ambipolar diffusion time scale is locally
similar to the free fall time. It seems likely that the lower limit which one
obtains to ionization degree by summing all observable molecular ions is not a
great underestimate of the true ionization degree. We predict that atomic
oxygen is abundant in the dense core and, if so, H3O+ may be the main ion in
the central highly depleted region of the core.Comment: 31 pages, 8 figures, to be published in Ap
Large excess of heavy nitrogen in both hydrogen cyanide and cyanogen from comet 17P/Holmes
From millimeter and optical observations of the Jupiter-family comet
17P/Holmes performed soon after its huge outburst of October 24, 2007, we
derive 14 N/15N = 139 +/- 26 in HCN, and 14N/15N = 165 +/- 40 in CN,
establishing that HCN has the same non-terrestrial isotopic composition as CN.
The same conclusion is obtained for the long-period comet C/1995 O1 (Hale-Bopp)
after a reanalysis of previously published measurements. These results are
compatible with HCN being the prime parent of CN in cometary atmospheres. The
15N excess relative to the Earth atmospheric value indicates that N-bearing
volatiles in the solar nebula underwent important N isotopic fractionation at
some stage of Solar System formation. HCN molecules never isotopically
equilibrated with the main nitrogen reservoir in the solar nebula before being
incorporated in Oort-cloud and Kuiper-belt comets. The 12C/13C ratios in HCN
and CN are measured to be consistent with the terrestrial value.Comment: Accepted for publication in the Astrophysical Journal (Letters) 4
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