Serge Demers

The Dynamics of the Large Magellanic Cloud Periphery: Mass Limit and Polar Ring

Radial velocities of 759 carbon stars on the periphery of the LMC are used to determine the rotation curve from 3 to 12 kpc. After a peak of 42 km s-1 at 4 kpc, the velocities decline to 35 km s-1 at 6.5 kpc, suggesting that 90% of the LMCs matter lies inside this radius. The rising velocities seen at larger radii are explained by particle-particle N-body simulations as arising from tidal interactions between LMC and both the SMC and the Galaxy. For an inclination of 33° the mass contained within a 5 kpc radius is 6.2 ± 0.9 × 109 M☉. An upper limit on the LMC halo mass is determined.

A blue stellar link between the Magellanic Clouds

Three pairs of UK Schmidt plates of selected fields between the Magellanic Clouds have been processed using the APM facility in Cambridge. Color-magnitude diagrams obtained from the plate pairs revealed the presence of blue main-sequence stars between the wing of the SMC and the western halo of the LMC. The spatial distribution of these blue stars, B(J) - R less than 0.0, shows that many of them are in aggregates and newly identified clusters, stretching from an area surrounding and including the SMC wing tip all the way across to the LMC halo. Spectroscopic observations confirmed that the stars are indeed OB stars at a distance of about 50 kpc with radial velocities consistent with cloud membership. The number and distribution of horizontal branch stars identified in the same area also support the conclusion that the SMC wing is really the brightest section of the stellar link between the SMC and LMC. All of the individual blue stellar aggregates, including the region around the tip of the SMC wing, appear to have been formed about 10 to the 8th yr ago suggesting a common origin for all these features. 47 refs.

The calibration of the metallicity versus C/M relation

This paper presents a critical reassessment of the C/M ratio in the Local Group galaxies that have been surveyed by us in the last few years. We update distances, reddenings and metallicities with particular care to obtain coherent values for each galaxy. A new equation for the bolometric correction of M stars in terms of their (R − I) colours is obtained from Bessell et al. (1998) data. We present a critical discussion on the colour - spectral type relation for M stars which reveals a dramatic lack of reliable data for late M stars. Finally, we show that - when homogeneous data are used - the log(C/M0+) is well correlated to the (Fe/H) of the parent galaxy. Early objective prism surveys of the Milky Way revealed that C stars and late M stars were not spatially distributed in the same way. Blanco (1965) showed that M stars are found pref- erentially inside the Suns orbit while C stars favor the anti- center region. Westerlund (1965) also noticed that late M stars are much more numerous toward the Galactic center. More than ten years later, when deep late-type star surveys of the Magellanic Clouds were initiated, a pronounced difference be- tween galaxies was discovered. Blanco et al. (1978) were the first to point out the tremendous differences between the ratio of the number of C stars and of late M stars in the Galactic bulge, in the Large Magellanic Cloud (LMC) and in the Small Magellanic Cloud (SMC). They rightly suggested that the ra- tios were anti-correlated with the metal abundance. Iben & Renzini (1983) gave two reasons to explain the trend of the C/M ratio with the abundance of the parent pop- ulation. First, the effect of the carbon dredge-up on the stellar spectrum is more pronounced for stars of low metallicity: it is easier to transform an O-rich star into a C-rich star when there is little oxygen to start with. Second, the giant branch of metal- poor systems is steeper, therefore bluer than the metal-rich one, thus there are few if any M or late M stars in very metal-poor systems.

C stars as kinematic probes of the Milky Way disk from 9 to 15 kpc

Context. The availability, from 2MASS, of a large homogeneous sample of Galactic C stars and the recognition that their absolute magnitude can be accurately determined offer the possibility to use them as kinematical probes to investigate motions in the thin or thick disks. Aims. Determine the radial velocities for 70 C stars, a few degrees from the Galactic plane and distributed in longitudes from 60 ◦ to 220 ◦ . Methods. Spectra, with a resolution of 4300, were obtained with the DAO 1.8 m telescope during 6 beautiful nights in October 2006. Results. The rotation velocities of C stars with 60 ◦ <�< 150 ◦ suggest a flat rotation curve to 15 kpc. A number of stars have velocities that do not fit the thin disk rotation. Some of them, toward � = 200 ◦ are most probably members of the Canis Major overdensity. Conclusions. Effort should be made to extend the rotation curve to more that 20 kpc.

A Carbon Star Survey of the 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 40 and three C stars, respectively, assuming that the reddening of Pegasus is negligible. No C stars were identified in Tucana. Our observations permit the estimation of the color-magnitude diagram contamination by foreground M dwarfs thus yielding reliable C/M ratios. Our R and 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 with C star populations in other dwarf galaxies. A larger, fainter population in that galaxy seems improbable, however. The statistics of C stars currently on hand for dwarf galaxies show a well-defined trend with the absolute magnitude of dwarf galaxies.

Young stars between the Magellanic Clouds. III - Overall properties of associations: A shallow mass function

We present CCD B, V photometry of three more groups of blue stars located in the bridge of the Magellanic Clouds (MC). Two associations near the SMC Wing tip are members of large complexes of blue stars. The other one, located at equal angular distance between the SMC and the LMC, is isolated. ZAMS fittings yield distances to the Sun that are intermediate between the ones of the LMC and the SMC. We find that the stellar bridge has a large line-of-sight depth near the eastern tip of the SMC Wing.

2MASS Observations of Spectroscopically Identified Extragalactic C Stars

We matched spectroscopically identified C stars (from low-resolution objective prism surveys) in the Magellanic Clouds with Two Micron All Sky Survey (2MASS) sources. We confirm that C stars show a large spread in absolute magnitudes, even in the Ks band. We show that the I and Ks magnitude distributions of a population of C stars (in the LMC) have a similar narrow dispersion if the C stars are selected in a well-defined color range. Using magnitude and color criteria, we employ the 2MASS data to identify 26 C stars in the Fornax dwarf spheroidal galaxy. The mean Ks magnitude and the mean bolometric magnitude of C stars are found to be slightly brighter in the LMC and SMC when compared with those of the Fornax dwarf spheroidal galaxy. The difference could be explained by ages and/or abundance differences.

Magellanic Cloud Periphery Carbon Stars. IV. The SMC

The kinematics of 150 carbon stars observed at moderate dispersion on the periphery of the Small Magellanic Cloud are compared with the motions of neutral hydrogen and early-type stars in the intercloud region. The distribution of radial velocities implies a configuration of these stars as a sheet inclined at 73° ± 4° to the plane of the sky. The near side, to the south, is dominated by a stellar component; to the north, the far side contains fewer carbon stars and is dominated by the neutral gas. The upper velocity envelope of the stars is closely the same as that of the gas. This configuration is shown to be consistent with the known extension of the SMC along the line of sight and is attributed to a tidally induced disruption of the SMC that originated in a close encounter with the LMC some 0.3 to 0.4 Gyr ago. The dearth of gas on the near side of the sheet is attributed to ablation processes akin to those inferred in 1996 by Weiner & Williams to collisional excitation of the leading edges of Magellanic Stream clouds. Comparison with the 1989 kinematic data of Hardy, Suntzeff, & Azzopardi and Maurice, Martin, & Bouchet and the 1986 and 1988 data of Mathewson et al. leaves little doubt that forces other than gravity play a role in the dynamics of the H I.

The H I content of the local group dwarf (spheroidal or irregular?) galaxy phoenix

Neutral hydrogen was detected in the Local Group dwarf galaxy Phoenix. The 21 cm H I line emission, observed at a heliocentric radial velocity of 56 km/s, is clearly separated from a much larger scale component at ∼140 km/s which was previously asociated with the Magellanic Stream. The profile width ΔV 50 ⇒ 21 km/s, combined with the high inclination of the system, suggests that not much rotation is present in this system which would then be mainly supported by turbulent motions. Using a newly determined distance estimate, the integrated profile gives a total H I mass of ⇒1.0 × 10 5 M ⊙ for an M HI /L v ⇒ 0.07 M ⊙ /L ⊙ . Most of the known properties of Phoenix imply that it is intermediate between typical dwarf irregular and dwarf spheroidal galaxies

Carbon star survey in the Local Group ? VII. NGC 3109 a galaxy without a stellar halo

We present a CFH12K wide field survey of the carbon star population in and around NGC 3109. Carbon stars, the brightest members of the intermediate-age population, were found nearly exclusively in and near the disk of NGC 3109, ruling out the existence of an extensive intermediate-age halo like the one found in NGC 6822. Over 400 carbon stars identified havehMIi= 4:71, confirming the nearly universality of mean magnitude of C star populations in Local Group galaxies. Star counts over the field reveal that NGC 3109 is a truncated disk shaped galaxy without an extensive stellar halo. The minor axis star counts reach the foreground density between 4 0 and 5 0 , a distance that can be explained by an inclined disk rather than a spheroidal halo. We calculate a global C/M ratio of 1:75 0:20, a value expected for such a metal poor galaxy.