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&

Weeds for weed control: Asteraceae essential oils as natural herbicides.

The aim of this study was to test the botanical family of Asteraceae as a source of natural herbicides. Twenty Asteraceae species were collected during flowering time and evaluated in terms of the yield and quality of essential oils (germination inhibition and growth of weeds). Half the species showed a sufficient yield of essential oil (from about 0.1% to 1.43%) when testing these phytochemicals in vitro as germination inhibitors of two typical weeds, Amaranthus retroflexus and Setaria viridis. Despite the higher tolerance of S. viridis, the concentration of 100 lg L 1 of essential oils of the two Artemisia species and Xanthium strumarium could totally inhibit germination. In addition, at 10 lg L 1, the same essential oils showed full inhibition of A. retroflexus seeds. A comparison of their effectiveness at suboptimal doses led to a further selection of the most promising sources of essential oils. After their chemical characterisation, the essential oils were tested as post-emergence herbicides on seedlings of the above-cited weeds. After spraying the weeds at different concentrations (10, 100 and 1000 mg L 1) during two different phenological stages of weed seedlings (cotyledons and the third true leaf), the essential oils of Artemisia annua and X. strumarium showed the best performance. The essential oils of X. strumarium were then tested again on both weeds to monitor the dynamics of plant injury. A reduction in plant fresh weight (about 20%–30% after 10 days) and chlorophyll content (destroyed, after the same amount of time) was found, thus confirming the total and rapid effectiveness of these essential oils. In summary, A. annua and X. strumarium have elicited considerable agronomic interest and appear to be suitable as a source of essential oils to act as natural herbicides

Imaging of the Stellar Population of IC10 with Laser Guide Star Adaptive Optics and the Hubble Space Telescope

We present adaptive optics (AO) images of the central starburst region of the dwarf irregular galaxy IC10. The Keck 2 telescope laser guide star was used to achieve near diffraction-limited performance at H and K' (Strehls of 18% and 32%, respectively). The images are centered on the putative Wolf-Rayet (W-R) object [MAC92]24. We combine our AO images with F814W data from HST. By comparing the K' vs. [F814W]-K' color-magnitude diagram (CMD) with theoretical isochrones, we find that the stellar population is best represented by at least two bursts of star formation, one ~ 10 Myr ago and one much older (150-500 Myr). Young, blue stars are concentrated in the vicinity of [MAC92]24. This population represents an OB association with a half-light radius of about 3 pc. We resolve the W-R object [MAC92]24 into at least six blue stars. Four of these components have near-IR colors and luminosities that make them robust WN star candidates. By matching the location of C-stars in the CMD with those in the SMC we derive a distance modulus for IC10 of about 24.5 mag. and a foreground reddening of E(B-V) = 0.95. We find a more precise distance by locating the tip of the giant branch in the F814W, H, and K' luminosity functions. We find a weighted mean distance modulus of 24.48 +/- 0.08. The systematic error in this measurement, due to a possible difference in the properties of the RGB populations in IC10 and the SMC, is +/- 0.16 mag.Comment: 13 pages, 13 figures, ApJ in pres

AGB stars in the Magellanic Clouds. III. The rate of star formation across the SMC

This article compares the Ks magnitude distribution of Small Magellanic Cloud asymptotic giant branch stars obtained from the DENIS and 2MASS data with theoretical distributions. Theoretical Ks magnitude distributions have been constructed using up-to-date stellar evolution calculations for low and intermediate-mass stars, and in particular for thermally pulsing asymptotic giant branch stars. Separate fits of the magnitude distributions of carbon- and oxygen-rich stars allowed us to constrain the metallicity distribution across the galaxy and its star formation rate. The Small Magellanic Cloud stellar population is found to be on average 7-9 Gyr old but older stars are present at its periphery and younger stars are present in the direction of the companion galaxy the Large Magellanic Cloud. The metallicity distribution traces a ring-like structure that is more metal rich than the inner region of the galaxy. The C/M ratio discussed in Paper I is a tracer of the metallicity distribution only if the underlying stellar population is of intermediate-age.Comment: 8 pages, 7 figures, accepted by A&

Distances to Populous Clusters in the LMC via the K-Band Luminosity of the Red Clump

We present results from a study of the distances and distribution of a sample of intermediate-age clusters in the Large Magellanic Cloud. Using deep near-infrared photometry obtained with ISPI on the CTIO 4m, we have measured the apparent K-band magnitude of the core helium burning red clump stars in 17 LMC clusters. We combine cluster ages and metallicities with the work of Grocholski & Sarajedini to predict each cluster's absolute K-band red clump magnitude, and thereby calculate absolute cluster distances. An analysis of these data shows that the cluster distribution is in good agreement with the thick, inclined disk geometry of the LMC, as defined by its field stars. We also find that the old globular clusters follow the same distribution, suggesting that the LMC's disk formed at about the same time as the globular clusters, ~ 13 Gyr ago. Finally, we have used our cluster distances in conjunction with the disk geometry to calculate the distance to the LMC center, for which we find (m-M)o = 18.40 +/- 0.04_{ran} +/- 0.08_{sys}, or Do = 47.9 +/- 0.9 +/- 1.8 kpc.Comment: 31 pages including 5 figures and 7 tables. Accepted for publication in the August 2007 issue of A

AGB stars in the Magellanic Clouds II. The rate of star formation across the LMC

Original article can be found at: http://www.aanda.org/ Copyright The European Southern Observatory (ESO)This article compares the distribution of Ks magnitudes of Large Magellanic Cloud (LMC) asymptotic giant branch (AGB) stars obtained from the DENIS and 2MASS data with theoretical distributions. These have been constructed using up-to-date stellar evolution calculations for low and intermediate-mass stars, and in particular for thermally pulsing AGB stars. A fit of the magnitude distribution of both carbon- and oxygen-rich AGB stars allowed us to constrain the metallicity distribution across the LMC and its star formation rate (SFR). The LMC stellar population is found to be on average 5 − 6 Gyr old and is consistent with a mean metallicity corresponding to Z = 0.006. These values may however be affected by systematic errors in the underlying stellar models, and by the limited exploration of the possible SFR histories. Instead our method should be particularly useful for detecting variations in the mean metallicity and SFR across the LMC disk. There are well defined regions where both the metallicity and the mean-age of the underlying stellar population span the whole range of grid parameters. The C/M ratio discussed in paper I is a tracer of the metallicity distribution if the underlying stellar population is older than about a few Gyr. A similar study across the Small Magellanic Cloud is given in paper III of this series.Peer reviewe

Magellanic Cloud Structure from Near-IR Surveys I: The Viewing Angles of the LMC

We present a detailed study of the viewing angles of the LMC disk plane. We find that our viewing direction differs considerably from the commonly accepted values, which has important implications for the structure of the LMC. The discussion is based on an analysis of spatial variations in the apparent magnitude of features in the near-IR color-magnitude diagrams extracted from the DENIS and 2MASS surveys. Sinusoidal brightness variations with a peak-to-peak amplitude of approximately 0.25 mag are detected as function of position angle, for both AGB and RGB stars. This is naturally interpreted as the result of distance variations, due to one side of the LMC plane being closer to us than the opposite side. The best fitting geometric model of an inclined plane yields an inclination angle i = 34.7 +/- 6.2 degrees and line-of-nodes position angle Theta = 122.5 +/- 8.3 degrees. There is tentative evidence that the LMC disk plane may be warped. Traditional methods to estimate the position angle of the line of nodes have used either the major axis position angle Theta_maj of the spatial distribution of tracers on the sky, or the position angle Theta_max of the line of maximum gradient in the velocity field, given that for a circular disk Theta_maj = Theta_max = Theta. The present study does not rely on the assumption of circular symmetry, and is considerably more accurate than previous studies of its kind. We find that the actual position angle of the line of nodes differs considerably from both Theta_maj and Theta_max, for which measurements have fallen in the range 140-190 degrees. This indicates that the intrinsic shape of the LMC disk is not circular, but elliptical, as discussed further in Paper II. [Abridged]Comment: Astronomical Journal, in press. 44 pages, LaTeX, with 8 PostScript figures. Contains minor revisions with respect to previously posted version. Check out http://www.stsci.edu/~marel/lmc.html for a large scale (23x21 degree) stellar number-density image of the LMC constructed from RGB and AGB stars in the 2MASS and DENIS surveys. The paper is available with higher resolution figures from http://www.stsci.edu/~marel/abstracts/abs_R31.htm