Detection of a young stellar population in the background of open

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
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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
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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
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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
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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.
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Vázquez et al.
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