Carbon Star Survey in the Local Group. VI. The Dwarf Spheroidal Galaxy NGC 205

We present a CFH12K survey of the carbon stars in NGC 205 and its surrounding field. We find that the number of C stars in NGC 205 is low (~500) for its luminosity and that very few C stars are seen outside of the 10' isophote, suggesting tidal stripping by M31. Their MI = -4.54, a magnitude nearly identical to what has been found in other galaxies with numerous C stars. Stars with accurate (R-I) photometry, to I ≈ 22.5, are used to determine the outer profile of NGC 205. A King profile with rt = 1977 ± 300 fits the data for distances larger than 7'. This rt implies that NGC 205 reached a minimum distance to M31 of ~42 kpc. The spatial distribution of C stars in the surrounding field reveals that some C stars belong to to the disk of M31, seen in projection, and that a surplus of C stars seen west of NGC 205 could be part of tidal debris left along its orbit. Finally, we report evidence of a transient period of enhanced star formation that occurred 1–2 Gyr ago in the northwest half of NGC 205

A Local Group Polar Ring Galaxy: NGC 6822

Star counts, obtained from a 2 x 2 degree area centered on NGC 6822 have revealed an optical image of this galaxy composed of two components: in addition to the well-known HI disk with its young stellar component, there is a spheroidal stellar structure as extensive as its HI disk but with its major axis at roughly right angles to it which we traced to at least 36 arcmin. Radial velocities of over 100 intermediate-age carbon stars found within this structure display kinematics contrasting strongly with those of the HI disk. These C stars belong to the spheroid. Although devoid of gas, the spheroid rotation is consistent with the I-band Tully-Fisher relation. The orientation of the rotation axis which minimizes the stellar velocity dispersion coincides with the minor axis of the stellar population ellipsoid, lying very nearly in the plane of the HI disk. We conclude: that the HI disk is a polar ring and the spheroidal component an erstwhile disk, a fossil remainder of a past close encounter episode.Comment: ApJ Letter, accepted, in pres

The extent of NGC 6822 revealed by its C stars population

Using the CFH12K camera, we apply the four band photometric technique to identify 904 carbon stars in an area 28' x 42' centered on NGC 6822. A few C stars, outside of this area were also discovered with the Las Campanas Swope Telescope. The NGC 6822 C star population has an average I of 19.26 mag leading to an average absolute I magnitude of -4.70 mag, a value essentially identical to the mean magnitude obtained for the C stars in IC 1613. Contrary to stars highlighting the optical image of NGC 6822, C stars are seen at large radial distances and trace a huge slightly elliptical halo which do not coincide with the huge HI cloud surrounding NGC6822. The previously unknown stellar component of NGC 6822 has a exponential scale length of 3.0' +/- 0.1' and can be traced to five scale lengths. The C/M ratio of NGC 6822 is evaluated to br 1.0 +/- 0.2.Comment: accepted, to be published in A

Carbon Star Survey of Local Group Dwarf Galaxies. II. Pegasus, DDO 210 and Tucana

We present the latest results of our ongoing four filter photometric survey of C stars in Local Group dwarf irregular galaxies. Observations of the two low luminosity dwarf irregular galaxies, Pegasus and DDO 210, revealed respectively 40 and 3 C stars, assuming that the reddening of Pegasus is negligible. No C stars were identified in Tucana. Our observations permit the estimation of the CMD contamination by foreground M dwarf thus yielding reliable C/M ratios. Our R, I photometry of the C stars cannot be used to solve the extinction controversy toward Pegasus. The three C stars in DDO 210 are quite bright when compared to C star populations in dwarf galaxies. A larger fainter population in that galaxy seems however improbable. The statistics of C stars, currently on hand for dwarf galaxies, show a well-defined trend with the absolute magnitude of galaxies.Comment: 18 pages, 7 figures, 6 table

Hubble Space Telescope imaging of a peculiar stellar complex in NGC 6946

The stellar populations in a stellar complex in NGC 6946 are analyzed on images taken with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. The complex is peculiar by its very high density of stars and clusters and semicircular shape. Its physical dimensions are about the same as for the local Gould Belt, but the stellar density is 1 - 2 orders of magnitude higher. In addition to an extremely luminous, 15 Myr old cluster discussed in an earlier paper, accounting for about 17% of the integrated V-band light, we identify 18 stellar clusters within the complex with luminosities similar to the brightest open clusters in the Milky Way. The color-magnitude diagram of individual stars in the complex shows a paucity of red supergiants compared to model predictions in the 10-20 Myr age range for a uniform star formation rate. We thus find tentative evidence for a gap in the dispersed star formation history, with a concentration of star formation into a young globular cluster during this gap. Confirmation of this result must, however, await a better understanding of the late evolution of stars in the corresponding mass range (> 12 Msun). A reddening map based on individual reddenings for 373 early-type stars is presented, showing significant variations in the absorption across the complex. These may be responsible for some of the arc-like structures previously identified on ground-based images. We finally discuss various formation scenarios for the complex and the star clusters within it.Comment: 51 pages, including 19 figures and 4 tables. Accepted for publication in Ap

The CFHT Open Star Cluster Survey. IV. Two Rich, Young Open Star Clusters: NGC 2168 (M35) and NGC 2323 (M50)

We continue our study of rich Galactic clusters by presenting deep CCD observations of both NGC 2168 (M35) and NGC 2323 (M50). Both clusters are found to be rich (NGC 2168 contains at least 1000 stars brighter than V = 22 and NGC 2323 contains approximately 2100 stars brighter than our photometric limit of V = 23) and young (age of NGC 2168 = 180 Myrs, age of NGC 2323 = 130 Myrs). The color-magnitude diagrams for the clusters exhibit clear main sequences stretching over 14 magnitudes in the V, B-V plane. Comparing these long main sequences with those of earlier clusters in the survey, as well as with the Hyades, has allowed for accurate distances to be established for each cluster (dist. of NGC 2168 = 912 +/- 70/65 pc, dist. of NGC 2323 = 1000 +/- 81/75 pc). Analysis of the luminosity and mass functions suggest that despite their young ages, both clusters are somewhat dynamically relaxed exhibiting signs of mass-segregation. This is especially interesting in the case of NGC 2323, which has an age of only 1.3 times the dynamical relaxation time. The present photometry is also deep enough to detect all of the white dwarfs in both clusters. We discuss some interesting candidates which may be the remnants of quite massive (M > 5 Mo) progenitor stars. The white dwarf cooling age of NGC 2168 is found to be in good agreement with the main-sequence turn-off age. These objects are potentially very important for setting constraints on the white dwarf initial-final mass relationship and upper mass limit for white dwarf production.Comment: 34 pages, including 12 diagrams and 5 tables. Accepted for publication in AJ. Minor typos correcte

Un segno per il primo meridiano d’Italia a Roma

A sign for Italy's Prime Meridian Italy's Prime Meridian, defined in 1871, is situated in a brick tower at the top of Monte Mario - Rome's highest hill. The Italian Government chose the site to be the point of passage for the Prime Meridian on September 20 1870 the same time the italian army had arrived back in Rome. This meridian remained in use for official italian land maps until 1995 when a new cartography based on the WGS84 reference system was  adopted. For this reason Italy's Prime Meridian can be considered an historical geodetic monument. Studium Urbis began the first demarcation of the meridian in february 2007 with a plaque inside the Vatican Gardens. The Studium Urbis is currently continuing the ongoing project of the demarcation of the Italy's Prime Meridian in significant spots in Rome

Un segno per il primo meridiano d’Italia a Roma

A sign for Italy's Prime Meridian Italy's Prime Meridian, defined in 1871, is situated in a brick tower at the top of Monte Mario - Rome's highest hill. The Italian Government chose the site to be the point of passage for the Prime Meridian on September 20 1870 the same time the italian army had arrived back in Rome. This meridian remained in use for official italian land maps until 1995 when a new cartography based on the WGS84 reference system was  adopted. For this reason Italy's Prime Meridian can be considered an historical geodetic monument. Studium Urbis began the first demarcation of the meridian in february 2007 with a plaque inside the Vatican Gardens. The Studium Urbis is currently continuing the ongoing project of the demarcation of the Italy's Prime Meridian in significant spots in Rome