Preliminary Comments

1. Detection ofA young populationin the background of Open Clustersin the Third Galactic Quadrant of the Galaxy(implicancies)Grupo de Astrofísica de Cúmulos AbiertosFCAGLP – UNLP IALP – CONICET

2. Preliminary Comments • The detailed structure of the Galactic disk in the Third Galactic Quadrant (TGQ) is very poorly known and, unfortunately, little attention has been paid to this complex region over the years. • The situation however dramatically changed due to the detection of the Monoceros Ring (Mon. Ring,), considered the debris left behind by a satellite dwarf galaxy undergoing an in-plane accretion into the Milky Way. • Martin et al. (2004), Bellazini et al. (2004) report the discovery of the Canis Major over-density in the TGQ. • This putative galaxy is called Canis Major (CMa), centered at l = 240º, b = -8º, right in the middle of the TGQ at a distance between 7 and 8 kpc from the Sun. • The full understanding of this over-density raised a lively debate (Momany et al. 2004; Bellazzini et al. 2004; Dinescu et al. 2005).

3. Preliminary Comments • Martínez-Delgado et al (2005) obtained a deep BR Color-Magnitude Diagram which, according to these authors exhibits nice features, like a probable clump of He-burrning stars, typical of intermediate age population, and a blue plume (BP) of younger stars which, at the distance of CMa, should be around 1-2 Gyrs old. l = 240º, b = -8º l = 240º, b = +8º RC Comparison field BP Martínez-Delgado et al (2005)

4. Preliminary Comments 220 < l < 260 FWHM(b) =13.1 l = 240, b = -7.5 The CMa over density The CMa over density extension is about 1 kpc. Martin et al. 2004

5. Preliminary Comments • On the other hand… • Russeil (2003) using star forming complexes finds that both the Perseus and Norma-Cygnus arms are not visible at all in the TGQ, confirming previous results by May et al. (1997) who mapped the region with CO clouds, finding no grand design spiral features in this Galaxy Location. • May et al. (1997) show that bridges of material are present in a few anti-center directions and confirm previous suggestions about the shape and location of the Galactic warp.

6. Therefore, there are two relevant features: The BP (a stellar population 1-2 Gyr old associated to CMa) The CMa over density (irregular galaxy, 4-8 Gyr old) We propose another -model independent- explanation

7. Our investigation in the TGQ • We have carried out CCD UBVRI photometry of open clusters in the TGQ, in a zone (217º < l < 260º, −5º < b < +5º approximately) described in full detail in Moitinho (2001). • 30 open clusters were observed. Data are homogeneous with limiting magnitude of V=21. The analysis presented in Moitinho (2001) shows that photometry is accurate and consistent with other previously published works. • Nine open clusters, NGC 2302, NGC 2383, NGC 2384, NGC 2367, NGC 2362, NGC 2439, NGC 2533, NGC 2432 and Ruprecht 55, show a singular feature.

8. 219.28 -03.10 238.18 -05.55 246.41 -04.43 243.53 −0.93 All stars have σ < 0.1 It is remarkable that NGC 2453 does not show the BP feature! • The clusters population revealed by the upper, bluer main sequences, fitted the Schmidt-Kaler (1982) following a normal reddening law, that hold in this direction of the Galaxy (Moitinho 2001). • A fainter and more reddened young population indicated by filled squares in the NGC 2302 diagrams. We refer to this population as the Blue Plume (BP). The ZAMS fits to the BPs are indicated. • The Galactic disk field population.

9. Our investigation in the TGQ • The BP pattern behind our cluster sample covers over 30º in longitude, with distances going from 6 to 11 kpc from the sun. Three more cluster, not observed by us, with CCD and CMD that show also the BP pattern.

10. CMa over density BPs in clusters of our sample ▲ BPs in clusters not in our sample Our study clusters The TGQ: The Norma-Cygnus arm To facilitate the interpretation we also include the distribution of CO clouds from the recent study by May et al. (2005) depicted as open circles. Many of these clouds, by the way, harbour IRAS sources (Bronfman et al. 2005), suggesting that star formation is still on going at their location.

11. BPs in clusters of our sample ▲ BPs in clusters not in our sample Our study clusters The TGQ: The plane of the Galaxy warps

12. Simulated Field CM l = 240, b = -8 238.18 -05.55 NGC 2362 NGC 2362 Martinez-Delgado et al. 2005

13. First conclusions • Thanks to the use of U-B indices it is clear that BPs are young stars located at different distances from the Sun. • We found that the young population associated to CMa (the BP) is actually… Milky Way material !! Young stars and star clusters located inside the Perseus and … … the “Norma-Cygnus” Spiral arm! Carraro, Vázquez, Moitinho, Baume (2005, ApJ 630, L153)

14. Anyway, • This condenses previous findings in the TGQ… • Fitzgerald & Moffat (1974) firstly suggest the existence of a spiral arm 15 kpc from the Galactic Center at l = 241 • Vogt (1976) found an excess of OB stars towards Monoceros, Canis-Major, Vela-Puppis • Fitzgerald & Moffat (1980) found as well an excess of luminous stars at l=231 • Carney & Sietzer (1993) got the first optical CMD of the Galactic Warp below the galactic plane, l=245, b=-4, showing a BP • Kaltcheva & Hilditch (2000) found an excess of OB stars at 215<l<275 toward Canis-Major constellation

15. Results of a newer sample including 31 more clusters. More clusters showing BPs. Moffat et al. 1979 6 kpc Perseus arm Vazquez et al., 2005 (in preparation)

16. The gas and stars in the galactic plane warp Cluster+BPs CO clouds ▲ Norma-Cygnus population Perseus+Local arm population CO clouds Cluster+BPs There is a strong concentration of stars and gas in the direction of CMa overdensity Direction to CMa Moitinho et al. (2005 in preparation)

17. Longitude-Z appearance. Beyond 6 kpc and for l > 220º, the Norma-Cygnus arm abruptely falls down. While distant BPs and distant CO clouds warps, the nearest ones do not.

18. A simple solution: CMa is the result of looking tangently the Local Arm Extension in the Third Galactic Quadrant.