NGC 4337: an overlooked old cluster in the inner disc of the Milky Way

Galactic open clusters do not survive long in the high-density regions of the inner Galactic disc. Inside the solar ring only 11 open clusters are known with ages older than 1 Gyr. We show here, basing on deep, high-quality photometry, that NGC 4337, contrary to earlier findings, is indeed an old open cluster. The cluster is located very close to the conspicuous star cluster Trumpler 20, as well misclassified in the past, and that has received so much attention in recent years. NGC 4337 shows a significant clump of He-burning stars which was not detected previously. Its beautiful colour-magnitude diagram is strikingly similar to the one of the classical old open clusters IC 4651, NGC 752, and NGC 3680, and this suggests similar age and composition. A spectroscopic study is much needed to confirm our findings. This, in turn, would also allow us to better define the inner disc radial abundance gradient and its temporal evolution. To this aim, a list of clump star candidates is provided.Instituto de Astrofísica de La Plat

The Vela-Puppis open clusters Pismis 8 and Pismis 13

CCD UBVI imaging photometry was carried out in the fields of the open clusters Pismis 8 and Pismis 13, located in the Vela-Puppis region in our Galaxy. MK spectral types have also been determined for a number of stars located in the fields of these two clusters which were used to secure membership among the brightest stars. Since our photometry goes to a fainter limit than previous studies we could provide better reddening, distance and age determinations. Both clusters are located close to the edge of the local arm in the third quadrant. Pismis 8 is a cluster about 5-7 My old located at 2000 pc from the Sun, while Pismis 13 was found at 2750 pc with a probable age of about 100 My. The estimate of the slopes of the mass functions in both cases yielded x ≈ 1.7 and x ≈ 2.1 for Pismis 8 and 13 respectively.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

Early-type stars in the open cluster NGC 4852

The southern open cluster NGC 4852 was re-observed in the UBVRI passbands to refine its intrinsic fundamental parameters. An extended spectral coverage was also undertaken aimed at obtaining a clear picture of the kind of stars inhabiting the cluster upper main sequence. We detected an unusual number of B-type stars including a new weak-emission Be star that raises to three the number of confirmed emission objects known in the cluster area. Our analysis, however, is conclusive in that none of them belongs to the cluster. With the new data set, we improved the fundamental parameters of NGC 4852: it is little bit farther than formerly assumed; the reddening, which increases smoothly with the distance, is lower than early estimates; and more importantly, it is really a young object of nearly 40-60Myr, contrary to the age of 200 Myr stated before. By means of spectroscopic parallaxes we secured the star distances, finding a good agreement between spectroscopic and zero-age main-sequence fitting distances, within the errors. The spatial distribution of the stars allows us to infer that NGC 4852 is the final, closer to the Sun, structure of a star-forming region and that a distant group of early-B-type stars, located immediately beyond the cluster, seems to be part of an extended OB association. The three emission stars detected in the region are related to this farther group. Finally, two extremely faint and blue stars were also found in the cluster zone. They have no connection with NGC 4852 but seem to be white-dwarf candidates.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

NGC 2580 and NGC 2588 : Two open clusters in the Third Galactic Quadrant

We present CCD broad band photometric observations in the fields of the Third Galactic Quadrant open clusters NGC 2580 and NGC 2588 (V(I)C and UBV(RI)C respectively). From the analysis of our data we found that NGC 2580 is located at a distance of about 4 kpc and its age is close to 160 Myr. As for NGC 2588, it is placed at about 5 kpc from the Sun and is 450 Myr old. This means that NGC 2588 belongs to the extension of the Perseus arm, whereas NGC 2580 is closer to the local arm structure. The luminosity functions (LFs) have been constructed for both clusters down to V ∼ 20 together with their initial mass functions (IMFs) for stars with masses above M ∼ 1-1.5 M⊙. The IMF slopes for the most massive bins yielded values of x ≈ 1.3 for NGC 2580 and x ≈ 2 for NGC 2588. In the case of this latter cluster we found evidence of a core-corona structure produced probably by dynamical effect. In the main sequences of both clusters we detected gaps, which we suggest could be real features.Facultad de Ciencias Astronómicas y Geofísica

The Vela-Puppis open clusters Pismis 8 and Pismis 13

CCD UBVI imaging photometry was carried out in the fields of the open clusters Pismis 8 and Pismis 13, located in the Vela-Puppis region in our Galaxy. MK spectral types have also been determined for a number of stars located in the fields of these two clusters which were used to secure membership among the brightest stars. Since our photometry goes to a fainter limit than previous studies we could provide better reddening, distance and age determinations. Both clusters are located close to the edge of the local arm in the third quadrant. Pismis 8 is a cluster about 5-7 My old located at 2000 pc from the Sun, while Pismis 13 was found at 2750 pc with a probable age of about 100 My. The estimate of the slopes of the mass functions in both cases yielded x ≈ 1.7 and x ≈ 2.1 for Pismis 8 and 13 respectively.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

The thickening of the thin disk in the third galactic quadrant

In the third Galactic quadrant (180° ≤ l ≤ 270°) of the Milky Way, the Galactic thin disk exhibits a significant Warp-shown both by gas and young stars-bending down a few kiloparsecs below the formal Galactic plane (b = 0°). This warp shows its maximum at l ∼ 240°, in the direction of the Canis Major constellation. In a series of papers, we have traced the detailed structure of this region using open star clusters, putting particular emphasis on the spiral structure of the outer disk. We noted a conspicuous accumulation of young star clusters within 2-3 kpc from the Sun and close to b = 0°, which we interpreted as the continuation of the Local (Orion) arm toward the outer disk. While most clusters (and young stars in their background) closely follow the warp of the disk, our decade-old survey of the spiral structure of this region led us to identify three clusters, Haffner 18 (1 and 2) and Haffner 19, which remain very close to b = 0° and lie at distances (4.5, ∼8.0, and 6.4 kpc) where most of the material is already significantly warped. Here, we report on a search for clusters that share the same properties as Haffner 18 and 19, and investigate the possible reasons for such an unexpected occurrence. We present UBVRI photometry of five young clusters, namely NGC 2345, NGC 2374, Trumpler 9, Haffner 20, and Haffner 21, which also lie close to the formal Galactic plane. With the exception of Haffner 20, in the background of these clusters we detected young stars that appear close to b = 0° and are located at distances up to ∼8 kpc from the Sun, thus deviating significantly from the warp. These populations define a structure that distributes over almost the entire third Galactic quadrant. We discuss this structure in the context of a possible thin disk flaring, similar to the Galactic thick disk.Instituto de Astrofísica de La PlataFacultad de Ciencias Astronómicas y Geofísica

The thickening of the thin disk in the third galactic quadrant

In the third Galactic quadrant (180° ≤ l ≤ 270°) of the Milky Way, the Galactic thin disk exhibits a significant Warp-shown both by gas and young stars-bending down a few kiloparsecs below the formal Galactic plane (b = 0°). This warp shows its maximum at l ∼ 240°, in the direction of the Canis Major constellation. In a series of papers, we have traced the detailed structure of this region using open star clusters, putting particular emphasis on the spiral structure of the outer disk. We noted a conspicuous accumulation of young star clusters within 2-3 kpc from the Sun and close to b = 0°, which we interpreted as the continuation of the Local (Orion) arm toward the outer disk. While most clusters (and young stars in their background) closely follow the warp of the disk, our decade-old survey of the spiral structure of this region led us to identify three clusters, Haffner 18 (1 and 2) and Haffner 19, which remain very close to b = 0° and lie at distances (4.5, ∼8.0, and 6.4 kpc) where most of the material is already significantly warped. Here, we report on a search for clusters that share the same properties as Haffner 18 and 19, and investigate the possible reasons for such an unexpected occurrence. We present UBVRI photometry of five young clusters, namely NGC 2345, NGC 2374, Trumpler 9, Haffner 20, and Haffner 21, which also lie close to the formal Galactic plane. With the exception of Haffner 20, in the background of these clusters we detected young stars that appear close to b = 0° and are located at distances up to ∼8 kpc from the Sun, thus deviating significantly from the warp. These populations define a structure that distributes over almost the entire third Galactic quadrant. We discuss this structure in the context of a possible thin disk flaring, similar to the Galactic thick disk.Instituto de Astrofísica de La PlataFacultad de Ciencias Astronómicas y Geofísica

NGC 2571: An intermediate-age open cluster with a White Dwarf candidate

CCD UBVI imaging photometry was carried out in the field of the open cluster NGC 2571. From the analysis of our data we state the cluster is at a distance of 1380 ± 130 pc and its age is 504±10 ×106 yr. The cluster mass function has a slope larger than a typical Salpeter's law. There are two notorious features in NGC 2571: the cluster contains a high proportion of stars located below the reference line that are serious candidates to be metallic line stars (probably Am-Fm), and shows also a sharp gap along its main sequence that cannot be explained by a random process nor by a biased rejection of cluster members. A striking blue object was detected in the cluster field that could be a white dwarf candidate.Facultad de Ciencias Astronómicas y Geofísica

NGC 2571: An intermediate-age open cluster with a White Dwarf candidate

CCD UBVI imaging photometry was carried out in the field of the open cluster NGC 2571. From the analysis of our data we state the cluster is at a distance of 1380 ± 130 pc and its age is 504±10 ×106 yr. The cluster mass function has a slope larger than a typical Salpeter's law. There are two notorious features in NGC 2571: the cluster contains a high proportion of stars located below the reference line that are serious candidates to be metallic line stars (probably Am-Fm), and shows also a sharp gap along its main sequence that cannot be explained by a random process nor by a biased rejection of cluster members. A striking blue object was detected in the cluster field that could be a white dwarf candidate.Facultad de Ciencias Astronómicas y Geofísica

NGC 4337: an overlooked old cluster in the inner disc of the Milky Way

Galactic open clusters do not survive long in the high-density regions of the inner Galactic disc. Inside the solar ring only 11 open clusters are known with ages older than 1 Gyr. We show here, basing on deep, high-quality photometry, that NGC 4337, contrary to earlier findings, is indeed an old open cluster. The cluster is located very close to the conspicuous star cluster Trumpler 20, as well misclassified in the past, and that has received so much attention in recent years. NGC 4337 shows a significant clump of He-burning stars which was not detected previously. Its beautiful colour-magnitude diagram is strikingly similar to the one of the classical old open clusters IC 4651, NGC 752, and NGC 3680, and this suggests similar age and composition. A spectroscopic study is much needed to confirm our findings. This, in turn, would also allow us to better define the inner disc radial abundance gradient and its temporal evolution. To this aim, a list of clump star candidates is provided.Instituto de Astrofísica de La Plat