Detección de una población estelar jóven contra el fondo de

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					                    Detection of
         A young population
in the background of Open Clusters
in the Third Galactic Quadrant of the Galaxy

            Grupo de Astrofísica de Cúmulos Abiertos
              FCAGLP – UNLP IALP – CONICET
Preliminary Comments

q 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.

q 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.

q Martin et al. (2004), Bellazini et al. (2004) report the discovery of the Canis
  Major over-density in the TGQ.

q 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

q The full understanding of this over-density raised a lively debate (Momany et
  al. 2004; Bellazzini et al. 2004; Dinescu et al. 2005).
Preliminary Comments
q 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

      BP                                                          field

                     Martínez-Delgado et al (2005)
Preliminary Comments

    220 < l < 260      FWHM(b) =13.1

                                         l = 240, b = -7.5
                                           The CMa over

                                         The CMa over
                                       density extension
                                        is about 1 kpc.

                                       Martin et al. 2004
Preliminary Comments

On the other hand…

q 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

q 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.
Therefore, there are two relevant features:

     The BP
     (a stellar population 1-2 Gyr old
     associated to CMa)
                                         We propose another
                                         -model independent-
     The CMa over density
     (irregular galaxy, 4-8 Gyr old)
Our investigation in the TGQ

q 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).

q 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.

q 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.
     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!

q 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).

q 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.

q The Galactic disk field population.
  Our investigation in the TGQ

  q 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
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.
    CMa over density

                           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.

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

   ▲   BPs in clusters of our sample
       BPs in clusters not in our sample
       Our study clusters
     CM l = 240, b = -8        Simulated Field
                                  238.18 -05.55
                                       NGC 2362
                                  NGC 2362

Martinez-Delgado et al. 2005
First conclusions

q Thanks to the use of U-B indices it is clear that BPs
  are young stars located at different distances from
  the Sun.

q 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)
This condenses previous findings in the TGQ…

q Fitzgerald & Moffat (1974) firstly suggest the existence of a
  spiral arm 15 kpc from the Galactic Center at l = 241

q Vogt (1976) found an excess of OB stars towards
  Monoceros, Canis-Major, Vela-Puppis

q Fitzgerald & Moffat (1980) found as well an excess of
  luminous stars at l=231

q Carney & Sietzer (1993) got the first optical CMD of the
  Galactic Warp below the galactic plane, l=245, b=-4,
  showing a BP

q Kaltcheva & Hilditch (2000) found an excess of OB stars at
  215<l<275 toward Canis-Major constellation
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)
                                                           The gas and stars
                   Cluster+BPs      CO clouds
                                                           in the galactic
                                                           plane warp

▲ Norma-Cygnus population
  Perseus+Local arm population

                    Cluster+BPs     CO clouds

                 Direction to CMa
                                                            There is a strong
                                                            concentration of
                                                            stars and gas in
                                                            the direction of
                                                            CMa overdensity

                                     Moitinho et al. (2005 in preparation)
            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.
    A simple solution:

       CMa is the result of
        looking tangently
    the Local Arm Extension
in the Third Galactic Quadrant.

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