15 research outputs found
The morphology of the Magellanic Clouds revealed by stars of different age: results from the DENIS survey
The spatial distribution of sources populating different regions of the
colour-magnitude diagram (I-J, I) extracted from the DENIS catalogue towards
the Magellanic Clouds (DCMC -- Cioni et al. 2000) reveal significantly
different morphologies. Each region is associated to a different age group. The
Large Magellanic Cloud (LMC) shows an extended circular shape with a prominent,
off center bar, a nucleus and irregular spiral arms. The Small Magellanic Cloud
shows a perturbated structure with a prominent central concentration of stars.
Old and young populations are offset from one another.Comment: 4 pages and 7 figures, accepted for publication in A&A Journal
Letter
The Tip of the Red Giant Branch and Distance of the Magellanic Clouds: results from the DENIS survey
We present a precise determination of the apparent magnitude of the tip of
the red giant branch (TRGB) in the I (0.8 micron), J (1.25 micron), and K_S
(2.15 micron) bands from the luminosity function of a sample of data extracted
from the DENIS catalogue towards the Magellanic Clouds (Cioni et al. 2000).
From the J and Ks magnitudes we derive bolometric magnitudes m_bol. We
present a new algorithm for the determination of the TRGB magnitude, which we
describe in detail and test extensively using Monte-Carlo simulations. We note
that any method that searches for a peak in the first derivative (used by most
authors) or the second derivative (used by us) of the observed luminosity
function does not yield an unbiased estimate for the actual magnitude of the
TRGB discontinuity. We stress the importance of correcting for this bias, which
is not generally done. We combine the results of our algorithm with theoretical
predictions to derive the distance modulus of the Magellanic Clouds. We obtain
m-M = 18.55 (0.04 formal, 0.08 systematic) for the Large Magellanic Cloud
(LMC), and m-M = 18.99 (0.03 formal, 0.08 systematic) for the Small Magellanic
Cloud (SMC). These are among the most accurate determinations of these
quantities currently available, which is a direct consequence of the large size
of our sample and the insensitivity of near infrared observations to dust
extinction.Comment: 16 pages, 8 figures, revised version, accepted for publication in A&
Near-infrared spectra of Galactic stellar clusters detected on Spitzer/GLIMPSE images
We present near-infrared spectroscopic observations of massive stars in three
stellar clusters located in the direction of the inner Galaxy. One of them, the
Quartet, is a new discovery while the other two were previously reported as
candidate clusters identified on mid-infrared Spitzer images (GLIMPSE20 and
GLIMPSE13). Using medium-resolution (R=900-1320) H and K spectroscopy, we
firmly establish the nature of the brightest stars in these clusters, yielding
new identifications of an early WC and two Ofpe/WN9 stars in the Quartet and an
early WC star in GLIMPSE20. We combine this information with the available
photometric measurements from 2MASS, to estimate cluster masses, ages, and
distances. The presence of several massive stars places the Quartet and
GLIMPSE20 among the small sample of known Galactic stellar clusters with masses
of a few 10^3 Msun, and ages from 3 to 8 Myr. We estimate a distance of about
3.5 kpc for Glimpse 20, and 6.0 kpc for Quartet. The large number of giant
stars identified in GLIMPSE13 indicates that it is another massive (~ 6500
Msun) cluster, but older, with an age between 30 and 100 Myr, at a distance of
about 3 kpc.Comment: aastex macro, 21 pages, 15 figures. ApJ, accepte
The birth of modern astronomy
This richly illustrated book discusses the ways in which astronomy expanded after 1945 from a modest discipline to a robust and modern science. It begins with an introduction to the state of astronomy in 1945 before recounting how in the following years, initial observations were made in hitherto unexplored ranges of wavelengths, such as X-radiation, infrared radiation and radio waves. These led to the serendipitous discovery of more than a dozen new phenomena, including quasars and neutron stars, that each triggered a new area of research. The book goes on to discuss how after 1985, the further, systematic exploration of the earlier discoveries led to long-term planning and the construction of new, large telescopes on Earth and in Space. Key scientific highlights described in the text are the detection of exoplanets (1995), the unexpected discovery of the accelerated expansion of the Universe (1999), a generally accepted model for the large-scale properties of the Universe (2003) and the ΛCDM theory (2005) that explains how the galaxies and stars of the present Universe were formed from minute irregularities in the (almost) homogenous gas that filled the early Universe. All these major scientific achievements came at a price, namely the need to introduce two new phenomena that are as yet unexplained by physics: inflation and dark energy. Probably the deepest unsolved question has to be: Why did all of this start with a Big Bang