Asociación Argentina de Astronomı́a BAAA, Vol. 48, 2005 E.M. Arnal, A. Brunini, J.J. Clariá Olmedo, J.C. Forte, D.A. Gómez, D. Garcı́a Lambas, Z. López Garcı́a, Stella M. Malaroda, & G.E. Romero, eds. COMUNICACIÓN DE TRABAJO – CONTRIBUTED PAPER Detection of a young stellar population in the background of open clusters in the Third Galactic Quadrant (implicancies) Ruben A. Vázquez Facultad de Ciencias Astronómicas y Geofı́sicas de la UNLP, IALP-CONICET, Paseo del Bosque s/n, La Plata, Argentina Giovanni Carraro Departamento de Astronomı́a, Universidad de Chile, Casilla 36-D, Santiago, Chile Andre Moitinho CAAUL, Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal Gustavo Baume Facultad de Ciencias Astronómicas y Geofı́sicas de la UNLP, IALP-CONICET, Paseo del Bosque s/n, La Plata, Argentina Abstract. We report on the detection of a young stellar population (50 < age < 100 Myrs) in the background of young open clusters in the Third Galactic Quandrant that follows remarkably well the trace of the Norma-Cygnus spiral arm as defined by CO clouds. This population coincides with the one recently detected in 3 intermediate-age open clusters and suggested to be a 1-2 Gyr old population belonging to the Canis Major overdensity (Bellazzini et al. 2004). The distances and ages of the population we detected definitely rule out this possibility. Resumen. Reportamos la detección de población estelar jóven (50 < edad < 100 Myrs) por detrás de cúmulos abiertos jóvenes en el Tercer Cuadrante Galactico que sigue apretadamente la traza del brazo espiral de Norma-Cygnus definido por nubes de CO. Esta población coincide con otra recientemente detectada en 3 cúmulos de edad intermedia y que se sugirió era una población de 1-2 Gyr perteneciente a la sobredensidad de Canis Major (Bellazzini et al. 2004). La distancia y edades de la población que detectamos deja fuera esta posibilidad. 1. Introduction The Canis Major (CMa) overdensity detection by Martin et al. (2004) produced great interest for the Third Galactic Quadrant (TGQ) of the Milky Way. The 1 2 Vázquez et al. lively debate in the last years (Momany et al. 2004, Bellazzini et al. 2004) on this overdensity clearly demands a better picture of the Galaxy structure in this region. Surprisingly, the shape and extension of the Perseus and CygnusNorma arms in the TGQ are far from being clear and settled. Russeil (2003) 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). In fact, May et al. mapped the region with CO clouds finding no grand design spiral features in this Galaxy location but confirming the shape and location of the Galactic warp. No attempts have been done so far to probe the structure of the TGQ with star clusters. Clearly, modern surveys of young stars clusters and CO clouds are needed to better trace the spiral pattern in this interesting region of the Galaxy. 2. Data and analysis Our group conducted a systematic UBVRI photometric survey of more than 30 open clusters in the TGQ, in a zone (270◦ < l < 180◦ , −8◦ < b < +5◦ , approximately) described in full detail in Moitinho (2001) (see also Giorgi et al. (2005) and Carraro et al. (2005) and references therein)). Young open clusters are widely recognized to be ideal spiral arm tracers (Feinstein 1994) because of their ages which make them close to the spiral arm where they formed, and because of the precise determinations of reddening and distances. Our photometry is homogeneous with typical errors < 0.1 mag in all the color combinations down to the limiting magnitude of V = 21; the photometry is accurate and consistent with other previously published works (Moitinho 2001). We distinguish in nine open clusters three populations as shown in the template of Fig. 1, where the color-color and color-magnitude diagrams of NGC 2302, NGC 2349 and NGC 2362 are included. They are: • The cluster populations (the upper, bluer main sequences, fitted by the Schmidt-Kaler (1982) intrinsic relation -dotted lines) following a normal reddening law -long–dashed line- that hold in this direction of the Galaxy (Moitinho 2001). • A fainter, more reddened and distant young population indicated by filled squares in the NGC 2302 diagrams (fitted by the Schmidt-Kaler (1982) intrinsic relation -solid lines). We refer to this population as the Blue Plume (BP). • The Galactic disk field population. For comparison purposes, we also show the photometric diagrams of the open cluster NGC 2453 (l = 243.53◦ , b = −0.93◦ ) where the BP is completely absent. It is remarkable not only that the BPs with the same age and shape are visible in all the nine clusters irrespective of its position but also that the BP feature we find appears also in the fields of NGC 2477 (Momany et al. 2001), Tombaugh 1 (Carraro & Patat 1995) and Berkeley 33 (Carraro et al. 2005), 3 other negative latitude clusters. We recall that this feature has been currently suggested to be an intermediate-age (1-2 Gyr) population associated with the CMa overdensity. Young population in the Third Galactic Quadrant 3 Figure 1. Template showing 4 open clusters in the TGQ. In all cases, the Color-color and Color-magnitude diagramas show the intrinsic line (dooted line) fitted to the cluster population and the Blue Plume (solid line). We want to remark that the BP pattern shown in our nine clusters covers over 40◦ -see Fig 2, lower left panel, in advance- in galactic longitude, with distances going from 6 to 11 kpc from the sun (Carraro et al. 2005b). For the sake of completeness we now include also six more clusters: NGC 2477 (l = 253.58◦ , b = −5.84◦ ), Bellazzini et al. 2004), NGC 2168 (l = 186.59◦ , b = +2.19◦ ) and NGC 2323 (l = 221.68◦ , b = −01.31◦ ) (Kalirai et al. 2003, Fig.1 and 2) and three more from Moffat et al (1979), the S groups -distant early type stars. We show in Fig. 2 their distribution in the X-Z (left upper panel) and the X-Y planes (lower left panel). To facilitate further interpretations we also indicate in the figure the distribution of CO clouds from the recent study by May et al. (2005) depicted as open squares (right panels, Fig. 2). Many of these clouds, by the way, harbour IRAS sources (Bronfman et al. 2005), suggesting that star formation is still going on at their location. Figure 2 is the distribution of clusters, BPs and CO clouds in rectangular coordinates centered in the sun, whose position is always at 0-0. According to our analysis we distinguish spatially two groups, filled triangles indicate BPs 4 Vázquez et al. Figure 2. Upper panel: X-Z distribution of open clusters and BP patterns -left- and CO clouds -right. Lower panel: the same in X-Y. Left panels show the Norma-Cygnus components with filled triangles and filled circles for those ones in Perseus and/or Local-arm. The dashed straight lines in the panels indicate the direction toward the CMa overdensity is assumed to lie and clusters located at the Norma-Cygnus arm while filled circles show objects belonging to the Perseus-arm and/or the Local arm. As seen in the upper panels of Fig. 2, distant clusters and BP objects fall all down b = 0◦ exactly as CO clouds do being this a clear effect of the Galactic warp. Lower right panel shows that CO clouds trace very well the expected position of the Norma-Cygnus spiral arm -Russeil 2003- and that the BP and open clusters closely follow distance, longitude and latitude of the outer spiral arm. We found two clusters which deviate from that, NGC 2533 and Ruprecht 55 which lie both above the Galactic Young population in the Third Galactic Quadrant 5 plane, closer to the sun, and follow the expected extension of the Perseus arm in the TGQ (May et al. 1997, 2005). Comparing the locations of the BP and open clusters with the ones of CO clouds it is clear that the PB is a good spiral arm indicator. Therefore, the BP seems to be young, distant population. Notice that several BPs coincide with the direction toward CMa. The dashed lines in both upper panels correspond to the direction toward which the CMa overdensity is detected. Certainly, along this direction not only clusters but also field stars are detected thanks to absorption windows. It is then possible that CMa overdensity is the result of the superimposition of layers of intermediate and old stars. 3. Conclusions Though our new results confirm a recent work by Carraro et al. (2005b) we want to emphasize the fact that a significant star concentration is visible in the color-magnitude diagram of NGC 2362 at V ≈ 14 − 19, remarkably similar to that seen in NGC 2477 (Bellazzini et al. 2004). According to the current interpretation, this population lies between the cluster and the BP, so in the inter-arm zone, since it is the same population ascribed to CMa overdensity, which lies at 8.5 kpc from the Sun covering around 12◦ in galactic longitude. But our observations include the U filter and the combination of the B − V and U − B indices allow a precise estimation of the reddening undergone for stars in the BP. Consequently, we can give a better distance estimation of the BP objects. Therefore, the BP seen in the background of NGC 2477 and NGC 2362 is the same star population but located beyond the usual distance attributed to the CMa overdensity. Another interesting argument against the CMa overdensity comes from the CMD of NGC 2168 (Kalirai et al. 2003, Fig.1 and 2): there is the BP and the same MS as in NGC 2477 and the F-XMM field shown in. Now, the two clusters lie 35◦ apart in longitude and 3.5◦ in latitude, and while NGC 2477 is clearly in the CMa region, NGC 2168 is without any doubt out of reach of the overdensity main body. The BP in NGC 2168 belongs to the Perseus arm, the one in NGC 2477 to the outer arm. Since intermediate-age and old stars are expected to populate the inter-arm region, our findings support the idea that the BP in the CMa direction is simply young population associated to the Norma-Cygnus arm, since it turns out to be younger and distant than suggested before and situated in an area encompassing a significant sector of the TGQ, where the Norma-Cygnus spiral arm is expected to lie. Due to space limitations we will remark briefly that what Fig. 2 is additionally showing is that the galactic plane goes below b = 0◦ for distances larger than 3 kpc from the sun. Indeed, both arms, Norma-Cygnus and Perseus enter the TGQ not far from b = 0◦ but at l = 220◦ while Perseus holds the relative location respect the mean galactic equator, Norma-Cygnus falls, reaching a depth of about 1 kpc. This effect is usually known as the warp and its trace is outlined by very far open clusters, BP objects and CO clouds. It is worth mentioning to support our conclusions that May et al (1997) show at l = 240◦ a clear bridge of material extending for more than 6 kpc, exactly toward the CMa overdensity direction. 6 Vázquez et al. References Bronfman L., May J., Alvarez H., 2005 AJ submitted Bellazzini M., Ibata R.A., Monaco L., Martin N., Irwin M.J., Lewis G.F., 2004, MNRAS 354, 1278 Carraro G., Patat F., 1995 MNRAS 276, 563 Carraro G., Geisler D., Baume G., Vázquez R.A, Moitinho A., 2005, MNRAS, in press Carraro G., Vázquez R., Moitinho A., Baume G., 2005b, ApJ 630 L153 Feinstein A., 1994, Rev. Mex. Astron. Astroph. 29, 141 Giorgi E.E., Baume G., Solivella G., Vázquez R.A., 2005, A&A 432, 491 Kalirai J.S., Fahlman G.G., Richer, H.B., Ventura, P., 2003, AJ 126, 1402 Martin N., Ibata R.A., Bellazzini M., Irwin M.J., Lewis G.F., Dehnen W., 2004, MNRAS 348, 12 May J., Alvarez H., Bronfman L., 1997, A&A 327, 325 May J., Alvarez H., Bronfman L., 1997, A&A submitted Moffat A., Jackson P.D., Fitzgerald M.P., 1979 A&AS 38, 197 Moitinho A., 2001, A&A 370, 436 Moitinho A., Alves J., Huélamo, N. Lada, C. J. Momany Y. et al, 2001 A&A 379, 436 Momany Y., Zaggia S.R., Bonifacio P., Piotto G., De Angeli F., Bedin L., Carraro G., 2004, A&A 421, L29 Russeil D., 2003, A&A 397, 133 Schmidt-Kaler, Th. 1982, Landolt-Bornstein, Numerical data and Funct ional Relationships in Science and Technology, New Series, Group VI, Vol. 2(b), K. Schaifers and H.H. Voigt Eds., Springer Verlag, Berlin, p.14