1,019 research outputs found
Termologia del lago di Bracciano: onde interne
In precedenti lavori, sono stati 1101'ta.t.i alcuni cont.ributi ) alla conoseenza·dei moti liberi e forzati del lago (li Bmceiano,lll'eso nel suo insipnlP, eonsitlemto cioè comebaeino oSf'illant.e nella SWl interezza
Ulteriori esperienze geofisiche sul lago di Caldouazzo. Morfologia ed origine delle sesse. Dati microtermologici. Progressivo insabbiamento elei piccoli lagbi*.
The limnographic, barographic and tliermological recordings,taken in a week of October of the years 1906, 1967 and 1971 onthe Lake of Caldonazzo, are reviewed. The properties of free uninodal,binodal etc. oscillations of the Lake have been examined, once again withthe result that they coincide with the values previously elaborated theoretically:the amplitudes of uninodals in the North (Lido di San Cristoforo)are about twice as large as those measured in the extreme South (Lido diCaldonazzo); the behaviour of uninodals in the Northern part of the Lake(besides the uninode) in conformity with the theory, etc.The simultaneous function of a microbarograph, mounted near theLido of San Cristoforo, proved that the main, if not exclusive, reason ofthe seiches lies in the interaction between the atmosphere and the hydrosphere.The passage of even the slightest pressure variations above theLake, with rhythms of 10m, 5m, . . . cause an immediate response of theLake by free uninodal, binodal. . . oscillations, and the amplitude of wateroscillations is all the larger the more the period of perturbing causes approachesthe period of the corresponding free oscillation, in other words,the more it approaches resonance.Observations on microvariations of temperature have been made aswell which may occur on very short periods (in the order of 1 -2 sec) andgive evidence of a swift brownian motion of liquid molecules. Thismust have a considerable influence on the Lake's microbiology. The increaseof the period of uninodals, against the period of batliymetric mea
The puzzle of metallicity and multiple stellar populations in the Globular Clusters in Fornax
We examine the photometric data for Fornax clusters, focussing our attention
on their horizontal branch color distribution and, when available, on the RR
Lyr variables fraction and period distribution. Based on our understanding of
the HB morphology in terms of varying helium content in the context of multiple
stellar generations, we show that clusters F2, F3 and F5 must contain
substantial fractions of second generation stars (~54-65%). On the basis of a
simple chemical evolution model we show that the helium distribution in these
clusters can be reproduced by models with cluster initial masses ranging from
values equal to ~4 to ~10 times larger than the current masses. Models with a
very short second generation star formation episode can also reproduce the
observed helium distribution but require larger initial masses up to about
twenty times the current mass. While the lower limit of this range of possible
initial GC masses is consistent with those suggested by the observations of the
low metallicity field stars, we also discuss the possibility that the
metallicity scale of field stars (based on CaII triplet spectroscopy) and the
metallicities derived for the clusters in Fornax may not be consistent with
each other. The reproduction of the HB morphology in F2,F3,F5 requires two
interesting hypotheses: 1) the first generation HB stars lie all at "red"
colours. According to this interpretation, the low metallicity stars in the
field of Fornax, populating the HB at colours bluer than the blue side
((V-I)o<=0.3 or (B-V)o<=0.2) of the RR Lyrs, should be second generation stars
born in the clusters;a preliminary analysis of available colour surveys of
Fornax field provides a fraction ~20% of blue HB stars, in the low metallicity
range; 2) the mass loss from individual second generation red giants is a few
percent of a solar mass larger than the mass loss from first generation stars.Comment: 14 pages, 8 figures. Accepted for publication in MNRA
Dense, Fe-rich Ejecta in Supernova Remnants DEM L238 and DEM L249: A New Class of Type Ia Supernova?
We present observations of two LMC supernova remnants (SNRs), DEM L238 and
DEM L249, with the Chandra and XMM-Newton X-ray satellites. Bright central
emission, surrounded by a faint shell, is present in both remnants. The central
emission has an entirely thermal spectrum dominated by strong Fe L-shell lines,
with the deduced Fe abundance in excess of solar and not consistent with the
LMC abundance. This Fe overabundance leads to the conclusion that DEM L238 and
DEM L249 are remnants of thermonuclear (Type Ia) explosions. The shell emission
originates in gas swept up and heated by the blast wave. A standard Sedov
analysis implies about 50 solar masses in both swept-up shells, SNR ages
between 10,000 and 15,000 yr, low (< 0.05 cm^-3) preshock densities, and
subluminous explosions with energies of 3x10^50 ergs. The central Fe-rich
supernova ejecta are close to collisional ionization equilibrium. Their
presence is unexpected, because standard Type Ia SNR models predict faint
ejecta emission with short ionization ages. Both SNRs belong to a previously
unrecognized class of Type Ia SNRs characterized by bright interior emission.
Denser than expected ejecta and/or a dense circumstellar medium around the
progenitors are required to explain the presence of Fe-rich ejecta in these
SNRs. Substantial amounts of circumstellar gas are more likely to be present in
explosions of more massive Type Ia progenitors. DEM L238, DEM L249, and similar
SNRs could be remnants of ``prompt'' Type Ia explosions with young (~100 Myr
old) progenitors.Comment: 24 pages, 8 figures, ApJ, in pres
Helium variation due to self-pollution among Globular Cluster stars: consequences on the horizontal branch morphology
It is becoming clear that `self--pollution' by the ejecta of massive asymptotic giant branch stars has an important role in the early chemical evolution of globular cluster stars, producing CNO abundance spreads which are observed also at the surface of unevolved stars. Considering that the ejecta which are CNO processed must also be helium enriched, we have modelled stellar evolution of globular cluster stars by taking into account this possible helium enhancement with respect to the primordial value. We show that the differences between the main evolutionary phases (main sequence, turn--off and red giants) are small enough that it would be very difficult to detect them observationally. However, the difference in the evolving mass may play a role in the morphology of the horizontal branch, and in particular in the formation of blue tails, in those globular clusters which show strong CNO abundance variations, such as M13 and NGC 6752
CCD Photometry of the Classic Second Parameter Globular Clusters M3 and M13
We present high-precision V, B-V color-magnitude diagrams (CMDs) for the
classic second parameter globular clusters M3 and M13 from wide-field deep CCD
photometry. The data for the two clusters were obtained during the same
photometric nights with the same instrument, allowing us to determine accurate
relative ages. Based on a differential comparison of the CMDs using the Delta
(B-V) method, an age difference of 1.7 +/- 0.7 Gyr is obtained between these
two clusters. We compare this result with our updated horizontal-branch (HB)
population models, which confirm that the observed age difference can produce
the difference in HB morphology between the clusters. This provides further
evidence that age is the dominant second parameter that influences HB
morphology.Comment: 27 pages, 12 figures, Accepted for publication in A
Hot Horizontal-Branch Stars: The Ubiquitous Nature of the "Jump" in Stromgren u, Low Gravities, and the Role of Radiative Levitation of Metals
A "jump" in the BHB distribution in the V, u-y CMD was recently detected in
the GC M13. It is morphologically best characterized as a discontinuity in u,
u-y, with stars in the range 11,500<Teff(K)<20,000 deviating systematically
from (in the sense of appearing brighter and/or hotter than) canonical ZAHBs.
We present u, y photometry of 14 GCs obtained with 3 different telescopes
(Danish, NOT, HST) and demonstrate that the u-jump is present in every GC whose
HB extends beyond 11,500K, irrespective of [Fe/H], mixing history on the RGB,
and other GC parameters. We suggest that the u-jump is a ubiquitous feature,
intrinsic to all HB stars hotter than 11,500K. We draw a parallel between the
ubiquitous nature of the u-jump and the problem of low measured gravities among
BHB stars. We note that the "logg-jump" occurs over the same temperature range
as the u-jump, and that it occurs in every metal-poor GC for which gravities
have been determined--irrespective of [Fe/H], mixing history on the RGB, or any
other GC parameters. Furthermore, the u-jump and the logg-jump are connected on
a star-by-star basis. The two are likely different manifestations of the same
physical phenomenon. We present a framework which may simultaneously account
for the u-jump and the logg-jump. Reviewing spectroscopic data for several
field BHB stars, as well as two BHB stars in the GC NGC 6752, we find evidence
that radiative levitation of heavy elements takes place at Teff>11,500 K,
dramatically enhancing their abundances in the atmospheres of BHB stars in the
"critical" temperature region. Model atmospheres taking diffusion effects into
account are badly needed, and will likely lead to better overall agreement
between canonical evolutionary theory and observations for BHB stars.Comment: ApJ, Main Journal, accepted. Contains several changes and update
Infrared Photometry of Red Supergiants in Young Clusters in the Magellanic Clouds
We present broad-band infrared photometry for 52 late-type supergiants in the
young Magellanic Clouds clusters NGC 330, NGC 1818, NGC 2004 and NGC 2100.
Standard models are seen to differ in the temperature they predict for the red
supergiant population on the order of 300K. It appears that these differences
most probably due to the calibration of the mixing-length parameter,
, in the outermost layers of the stellar envelope. Due to the
apparent model dependent nature of we do not quantitatively
compare between models. Qualitatively, we find that
decreases with increased stellar mass within standard models. We do not find
evidence for a metallicity dependence of .Comment: 11 pages, 4 figures. AJ accepte
Single and Composite Hot Subdwarf Stars in the Light of 2MASS Photometry
Utilizing the Two Micron All Sky Survey (2MASS) Second Incremental Data
Release Catalog, we have retrieved near-IR magnitudes for several hundred hot
subdwarfs (sdO and sdB stars) drawn from the "Catalogue of Spectroscopically
Identified Hot Subdwarfs" (Kilkenny, Heber, & Drilling 1988, 1992). This sample
size greatly exceeds that of previous studies of hot subdwarfs. Examining 2MASS
photometry alone or in combination with visual photometry (Johnson BV or
Stromgren uvby) available in the literature, we show that it is possible to
identify hot subdwarf stars that exhibit atypically red IR colors that can be
attributed to the presence of an unresolved late type companion. Utilizing this
large sample, we attempt for the first time to define an approximately volume
limited sample of hot subdwarfs. We discuss the considerations, biases, and
difficulties in defining such a sample.
We find that, of the hot subdwarfs in Kilkenny et al., about 40% in a
magnitude limited sample have colors that are consistent with the presence of
an unresolved late type companion. Binary stars are over-represented in a
magnitude limited sample. In an approximately volume limited sample the
fraction of composite-color binaries is about 30%.Comment: to appear in Sept 2003 AJ, 41 pages total, 12 figures, 2 tables are
truncated (full tables to appear in electronic journal or available by
request
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