1,357 research outputs found
Star formation rate in the solar neighborhood
This thesis develops a method for obtaining the star formation histories of a
mixed, resolved population through the use of color-magnitude diagrams (CMDs).
The method is applied to the derivation of the local star formation rate,
modeling observations of the Hipparcos satellite wigth synthetic CMDs computed
for different star formation histories with an updated stellar evolution
library. Parallax and photometric uncertainties are included explicitly and
corrected using the Bayesian Richardson-Lucy algorithm. We find that the solar
neighborhood star formation rate has a characteristic timescale for variation
of about 6 Gyr, with a maximum activity close to 3 Gyr ago. This suggests a
global, rather than local, star forming event. The summary and conclusions are
included here, the full thesis is available at the URL listed above.Comment: PhD thesis/Physics 2006, University of Pisa, 167 pages, complete pdf
version is available at:
http://etd.adm.unipi.it/theses/available/etd-04032006-122227
Star formation histories of dwarf galaxies from the Colour-Magnitude diagrams of their resolved stellar populations
In this tutorial paper we summarize how the star formation (SF) history of a
galactic region can be derived from the colour-magnitude diagram (CMD) of its
resolved stars. The procedures to build synthetic CMDs and to exploit them to
derive the SF histories (SFHs) are described, as well as the corresponding
uncertainties. The SFHs of resolved dwarf galaxies of all morphological types,
obtained from the application of the synthetic CMD method, are reviewed and
discussed. In short: 1) Only early-type galaxies show evidence of long
interruptions in the SF activity; late-type dwarfs present rather continuous,
or gasping, SF regimes; 2) A few early-type dwarfs have experienced only one
episode of SF activity concentrated at the earliest epochs, whilst many others
show extended or recurrent SF activity; 3) No galaxy experiencing now its first
SF episode has been found yet; 4) No frequent evidence of strong SF bursts is
found; 5) There is no significant difference in the SFH of dwarf irregulars and
blue compact dwarfs, except for the current SF rates. Implications of these
results on the galaxy formation scenarios are briefly discussed.Comment: 29 pages. Tutorial Review to appear in the special issue
"Dwarf-Galaxy Cosmology" in Advances in Astronom
A Model for Configuration Management of Open Software Systems
The article proposes a model for the configuration management of open
systems. The model aims at validation of configurations against given
specifications. An extension of decision graphs is proposed to express
specifications. The proposed model can be used by software developers to
validate their own configurations across different versions of the components,
or to validate configurations that include components by third parties. The
model can also be used by end-users to validate compatibility among different
configurations of the same application. The proposed model is first discussed
in some application scenarios and then formally defined. Moreover, a type
discipline is given to formally define validation of a configuration against a
system specificationComment: 13 page
Pre-Main sequence Turn-On as a chronometer for young clusters: NGC346 as a benchmark
We present a novel approach to derive the age of very young star clusters, by
using the Turn-On (TOn). The TOn is the point in the color-magnitude diagram
(CMD) where the pre-main sequence (PMS) joins the main sequence (MS). In the MS
luminosity function (LF) of the cluster, the TOn is identified as a peak
followed by a dip. We propose that by combining the CMD analysis with the
monitoring of the spatial distribution of MS stars it is possible to reliably
identify the TOn in extragalactic star forming regions. Compared to alternative
methods, this technique is complementary to the turn-off dating and avoids the
systematic biases affecting the PMS phase. We describe the method and its
uncertainties, and apply it to the star forming region NGC346, which has been
extensively imaged with the Hubble Space Telescope (HST). This study extends
the LF approach in crowded extragalactic regions and opens the way for future
studies with HST/WFC3, JWST and from the ground with adaptive optics.Comment: 6 pages, 4 figures. Accepted for publication in ApJ Letter
History and modes of star formation in the most active region of the Small Magellanic Cloud, NGC 346
We discuss the star formation history of the SMC region NGC 346 based on
Hubble Space Telescope images. The region contains both field stars and cluster
members. Using a classical synthetic CMD procedure applied to the field around
NGC 346 we find that there the star formation pace has been rising from a quite
low rate 13 Gyr ago to \approx 1.4 \times 10^{-8} Mo yr^{-1}pc^{-2} in the last
100 Myr. This value is significantly higher than in other star forming regions
of the SMC. For NGC 346 itself, we compare theoretical and observed
Color-Magnitude Diagrams (CMDs) of several stellar sub-clusters identified in
the region, and we derive their basic evolution parameters. We find that NGC
346 experienced different star formation regimes, including a dominant and
focused "high density mode", with the sub-clusters hosting both pre-main
sequence (PMS) and upper main sequence (UMS) stars, and a diffuse "low density
mode", as indicated by the presence of low-mass PMS sub-clusters.
Quantitatively, the star formation in the oldest sub-clusters started about 6
Myr ago with remarkable synchronization, it continued at high rate (up to 2
\times 10^{-5} Mo yr^{-1} pc^{-2}) for about 3 Myr and is now progressing at a
lower rate. Interestingly, sub-clusters mainly composed by low mass PMS stars
seem to experience now the first episode of star formation, following
multi-seeded spatial patterns instead of resulting from a coherent trigger. Two
speculative scenarios are put forth to explain the deficiency of UMS stars: the
first invokes under-threshold conditions of the parent gas; the second
speculates that the initial mass function (IMF) is a function of time, with the
youngest sub-clusters not having had sufficient time to form more massive
stars.Comment: 17 pages. Accepted for publication in A
STEP: Dissecting the SMC with the VLT Survey Telescope
We present the stellar catalog of g, r, i, H-alpha photometry from our survey STEP (SMC in Time: Evolution of a Prototype interacting late-type dwarf galaxy), a VST (the ESO VLT Survey Telescope) campaign of 53 deg2 dedicated to the study of the main body of the Small Magellanic Cloud and the Magellanic Bridge. Our photometry is able to resolve individual stars down to magnitudes well below the main-sequence turn-off of the oldest populations. We report some key results, including the detection of a prominent burst of star formation, which occurred in the SMC about 100-200 Myr ago, and the confirmation of a bimodal Red-Clump, evidence of a complex interaction history between the Clouds
American Culture as Seen Through Magazine Advertisements
This paper is designed as a resource for teachers who wish to teach American culture, and the influence of advertising on this culture, through the use of magazine advertisements. The lessons which can be derived from the materials are planned for advanced students of English who possess a good conversational ability in the language and who wish to learn more about American culture.
Part I presents for the reader a general overview of American advertising, describing how it developed, how it works, and how it mirrors and influences our culture and behavior. Part II describes the various appeals which advertising uses and contains advertisements which illustrate these appeals. For each category of advertisements there is an accompanying description which shows the reader what the trained eye can detect in the advertisements. Part III offers a list of ways in which the materials in Part II can be used in the classroom to generate discussions and lessons on culture. Also included is a look at how the language used in the advertisements can be examined for its cultural meaning.
Magazines were selected as a medium, rather than TV or radio, because of their easy availability and their wide use in America. Correspondingly, the advertisements which magazines contain reach a large portion of our population and have a significant impact on us. Ads were selected from magazines which appeal to young and middle-age readers, who make up the bulk of our population. Care was taken to choose magazines which appeal to both male and female readers, and ads which appeal to both middle-class and upper-class readers. It is hoped that the readers which the advertisements try to appeal to make up a fair representation of the American population
Star Formation History in two fields of the Small Magellanic Cloud Bar
The Bar is the most productive region of the Small Magellanic Cloud in terms
of star formation but also the least studied one. In this paper we investigate
the star formation history of two fields located in the SW and in the NE
portion of the Bar using two independent and well tested procedures applied to
the color-magnitude diagrams of their stellar populations resolved by means of
deep HST photometry. We find that the Bar experienced a negligible star
formation activity in the first few Gyr, followed by a dramatic enhancement
from 6 to 4 Gyr ago and a nearly constant activity since then. The two examined
fields differ both in the rate of star formation and in the ratio of recent
over past activity, but share the very low level of initial activity and its
sudden increase around 5 Gyr ago. The striking similarity between the timing of
the enhancement and the timing of the major episode in the Large Magellanic
Cloud is suggestive of a close encounter triggering star formation.Comment: 30 pages, 22 figures, accepted for publication in Ap
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