اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدAnalysis of colour-magnitude diagrams of rich LMC clusters: NGC 1831
126
0
0.0
(
0
)
تأليف
L. O. Kerber
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
We present the analysis of a deep colour-magnitude diagram (CMD) of NGC 1831, a rich star cluster in the LMC. The data were obtained with HST/WFPC2 in the F555W (~V) and F814W (~I) filters, reaching m_555 ~ 25. We discuss and apply a method of correcting the CMD for sampling incompleteness and field star contamination. Efficient use of the CMD data was made by means of direct comparisons of the observed to model CMDs. The model CMDs are built by an algorithm that generates artificial stars from a single stellar population, characterized by an age, a metallicity, a distance, a reddening value, a present day mass function and a fraction of unresolved binaries. Photometric uncertainties are empirically determined from the data and incorporated into the models as well. Statistical techniques are presented and applied as an objective method to assess the compatibility between the model and data CMDs. By modelling the CMD of the central region in NGC 1831 we infer a metallicity Z = 0.012, 8.75 < log(tau) < 8.80, 18.54 < (m-M)_0 < 18.68 and 0.00 < E(B-V) < 0.03. For the position dependent PDMF slope (alpha = -dlog(Phi(M))/dlog(M)), we clearly observe the effect of mass segregation in the system: for projected distances R < 30 arcsec, alpha ~ 1.7, whereas 2.2 < alpha < 2.5 in the outer regions of NGC 1831.
قيم البحث
اقرأ أيضاً
We used the deep colour-magnitude diagrams (CMDs) of five rich LMC clusters (NGC1805, 1818, 1831, 1868, and Hodge14) observed with HST/WFPC2 to derive their present day mass function (PDMF) and its variation with position within the cluster. The PDMF
was parameterized as a power law in the available main-sequence mass range of each cluster, typically 0.9 <~ m/M_sun <~ 2.5; its slope was determined at different positions spanning from the very centre out to several core radii. The CMDs in the central regions of the clusters were carefully studied earlier, resulting in accurate age, metallicity, distance modulus, and reddening values. The slope alpha (where Salpeter is 2.35) was determined in annuli by following two distinct methods: 1) a power law fit to the PDMF obtained from the systemic luminosity function (LF); 2) a statistical comparison between observed and model CMDs. In all clusters, significant mass segregation is found from the positional dependence of the PDMF slope: alpha <~ 1.8 for R <= 1.0 R_core and alpha ~ Salpeter inside R=2~3 R_core (except for Hodge 14, where alpha ~ Salpeter for R ~ 4 R_core). The results are robust in the sense that they hold true for both methods used. The CMD method reveals that unresolved binaries flatten the PDMF obtained form the systemic LF, but this effect is smaller than the uncertainties in the alpha determination. For each cluster we estimated dynamical ages inside the core and for the entire system. In both cases we found a trend in the sense that older clusters have flatter PDMF, consistent with a dynamical mass segregation and stellar evaporation.
With the aim of increasing the sample of M31 clusters for which a colour magnitude diagram is available, we searched the HST archive for ACS images containing objects included in the Revised Bologna Catalogue of M31 globular clusters. Sixty-three suc
h objects were found. We used the ACS images to confirm or revise their classification and we obtained useful CMDs for 11 old globular clusters and 6 luminous young clusters. We obtained simultaneous estimates of the distance, reddening, and metallicity of old clusters by comparing their observed field-decontaminated CMDs with a grid of template clusters of the Milky Way. We estimated the age of the young clusters by fitting with theoretical isochrones. For the old clusters, we found metallicities in the range -0.4<=[Fe/H]<=-1.9, that generally agree with existing spectroscopic extimates. At least four of them display a clear blue HB, indicating ages >10 Gyr. All six candidate young clusters are found to have ages <1Gyr. With the present work the total number of M31 GCs with reliable optical CMD increases from 35 to 44 for the old clusters, and from 7 to 11 for the young ones. The old clusters show similar characteristics to those of the MW. We discuss the case of the cluster B407, with a metallicity [Fe/H] ~-0.6 and located at a large projected distance from the centre of M31 and from the galaxy major axis. Metal-rich globulars at large galactocentric distances are rare both in M31 and in the MW. B407, in addition, has a velocity in stark contrast with the rotation pattern shared by the bulk of M31 clusters of similar metallicity. This, along with other empirical evidence, supports the hypothesis that the cluster is physically associated with a substructure in the M31 halo that has been interpreted as the relic of a merging event.
We use ground-based and space-based eclipse measurements for the near-infrared ($JHK!s$) bands and Spitzer 3.6 $mu$m and 4.5 $mu$m bands to construct colour-colour and colour-magnitude diagrams for hot Jupiters. We compare the results with previous o
bservations of substellar objects and find that hot Jupiters, when corrected for their inflated radii, lie near the black body line and in the same region of the colour magnitude diagrams as brown dwarfs, including low gravity dwarfs that have been previously suggested as exoplanet analogs. We use theoretical emission spectra to investigate the effects of different metallicity, C/O ratios and temperatures on the IR colours. In general we find that while differences in C/O ratio and metallicity do correspond to different locations on these diagrams, the measurement errors are too large to use this method to put strong constraints on the composition of individual objects. However, as a class hot Jupiters cluster around the location expected for solar metallicity and C/O ratio.
We present a simple approach for obtaining robust values of astrophysical parameters from the observed colour-magnitude diagrams (CMDs) of star clusters. The basic inputs are the Hess diagram built with the photometric measurements of a star cluster
and a set of isochrones covering wide ranges of age and metallicity. In short, each isochrone is shifted in apparent distance modulus and colour excess until it crosses over the maximum possible Hess density. Repeating this step for all available isochrones leads to the construction of the solution map, in which the optimum values of age and metallicity - as well as foreground/background reddening and distance from the Sun - can be searched for. Controlled tests with simulated CMDs show that the approach is efficient in recovering the input values. We apply the approach to the open clusters M,67, NGC,6791, and NGC,2635, which are characterised by different ages, metallicities and distances from the Sun.
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 his
tories (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.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
