600 research outputs found
Constraining the star formation rate in the Solar neighbourhood with star clusters
This paper investigates the star formation rate (SFR) in the Solar
neighbourhood. First, we build the local age distribution function (ADF) with
an updated sample of 442 star clusters located at less than 1\,kpc from the
Sun. Next, we define the SFR, compute the individual mass evolution of a
population of artificial clusters covering the broad range of parameters
observed in actual clusters, and assume 100\,\ms\ as the low-mass limit for
effective cluster observation. This leads to a simulated ADF, which is compared
to the low-noise Solar neighbourhood ADF. The best match corresponds to a
non-constant SFR presenting two conspicuous excesses for ages \,Myr and
between 220-600\,Myr (the local starburst). The average formation rate is
\bar{SFR}\approx(2500\pm500)\,\mmy, corresponding to the average surface
formation rate \bar{\ssfr}\approx(790\pm160)\,\mmk. These values are
consistent with the formation rate inferred from embedded clusters (ECs), but
much lower (\la16%) than that implied by field stars. Both the local
starburst and the recent star formation period require
to be described. The simulations show that
of the clusters created in the Solar neighbourhood do not survive
the first 10\,Myr, which is consistent with the rate of EC dissolution.Comment: Accepted by MNRA
Astrophysical parameters of 14 open clusters projected close to the Galactic plane
Astrophysical parameters (\textit{age, reddening, distance, core and cluster
radii}) of 14 open clusters (OCs) projected close to the Galactic plane are
derived with 2MASS photometry. The OCs are Be 63, Be 84, Cz 6, Cz 7, Cz 12, Ru
141, Ru 144, Ru 172, FSR 101, FSR 1430, FSR 1471, FSR 162, FSR 178 and FSR 198.
The OCs Be 63, Be 84, Ru 141, Ru 144, and Ru 172 are studied in more detail
than in previous works, while the others have astrophysical parameters derived
for the first time. The open clusters of the sample are located at
kpc from the Sun and at Galactocentric distances
kpc, with age in the range 10 Myr to 1.5 Gyr and reddening in the
range mag. The core and cluster radii are in the range
pc and pc, respectively. Cz 6 and FSR 198 are the youngest OCs of
this sample, with a population of pre-main sequence (PMS) stars, while FSR 178
is the oldest cluster.Comment: 11 pages, 14 figures - accepted by A&
From proper motions to star cluster dynamics: measuring velocity dispersion in deconvolved distribution functions
We investigate the effect that the usually large errors associated with
ground-based proper motion (PM) components have on the determination of a star
cluster's velocity dispersion (\sv). Rather than histograms, we work with PM
distribution functions (PMDFs), taking the uncertainties formally
into account. In this context, a cluster's intrinsic PMDF is broadened by the
error distribution function (eDF) that, given the average error amplitude, has
a width usually comparable to the cluster PMDF. Thus, we apply a
Richardson-Lucy (RL) deconvolution to the PMDFs of a set of relatively nearby
and populous open clusters (OCs), using the eDFs as point spread functions
(PSFs). The OCs are NGC\,1039 (M\,34), NGC\,2477, NGC\,2516, NGC\,2682 (M\,67),
and NGC\,7762. The deconvolved PMDFs are approximately Gaussian in shape, with
dispersions lower than the observed ones by a factor of 4-10. NGC\,1039 and
NGC\,2516, the nearest OCs of the sample, have deconvolved \sv\ compatible with
those of bound OCs of mass \,\ms. NGC\,2477 and NGC\,2682 have
deconvolved PMDFs with a secondary bump, shifted towards higher average
velocities, which may be an artefact of the RL deconvolution when applied to
asymmetric profiles. Alternatively, it may originate from cluster merger,
large-scale mass segregation or, least probably, binaries.Comment: Accepted by MNRA
Probing the age and structure of the nearby very young open clusters NGC 2244 and NGC 2239
The very young open cluster (OC) NGC 2244 in the Rosette Nebula was studied
with field-star-decontaminated 2MASS photometry, which shows the main-sequence
(MS) stars and an abundant pre-MS (PMS) population. Fundamental and structural
parameters were derived with colour-magnitude diagrams (CMDs), stellar radial
density profiles (RDPs) and mass functions (MFs). Most previous studies centred
NGC 2244 close to the bright K0V star 12 Monocerotis, which is not a cluster
member. Instead, the near-IR RDP indicates a pronounced core near the O5 star
HD 46150. We derive an age within 1--6 Myr, an absorption \aV=1.7\pm0.2, a
distance from the Sun \ds=1.6\pm0.2 kpc ( kpc outside the Solar
circle), an MF slope and a total (MS+PMS) stellar mass of
\sim625 \ms. Its RDP is characterised by the core and cluster radii
\rc\approx5.6\arcmin ( pc) and \rl\approx10\arcmin
( pc), respectively. Departure from dynamical equilibrium is
suggested by the abnormally large core radius and the marked central stellar
excess. We also investigate the elusive neighbouring OC NGC 2239, which is
low-mass (m_{MS+PMS}\approx301 \ms), young ( Myr) rather absorbed
(\aV=3.4\pm0.2), and located in the background of NGC 2244 at \ds=3.9\pm0.4
kpc. Its RDP follows a King-like function of \rc\approx0.5\arcmin\approx0.5
pc and \rl\approx5.0\arcmin\approx5.6 pc. The MF slope, ,
is essentially Salpeter's IMF. NGC 2244 is probably doomed to dissolution in a
few yr. Wide-field extractions and field-star decontamination increase
the stellar statistics and enhance both CMDs and RDPs, which is essential for
faint and bright star clusters.Comment: Accepted by MNRA
Towards a census of the Galactic anticentre star clusters: colour-magnitude diagram and structural analyses of a sample of 50 objects
In this work we investigate the nature of 50 overdensities from the catalogue
of Froebrich, Scholz, and Raftery (FSR) projected towards the Galactic
anticentre, in the sector 160{\deg} \leq \ell \leq 200{\deg}. The sample
contains candidates with |b| \leq 20{\deg} classified by FSR as probable open
cluster (OC) and labelled with quality flags 2 and 3. Our main purpose is to
determine the nature of these OC candidates and the fraction of these objects
that are unknown OCs, as well as to derive astrophysical parameters (age,
reddening, distance, core and cluster radii) for the clusters and to
investigate the relationship among parameters. The analysis is based on 2MASS
J, (J-H), and (J-Ks) colour-magnitude diagrams (CMDs), and stellar radial
density profiles (RDPs) built with decontamination tools. The tools are a field
star decontamination algorithm, used to uncover the cluster's intrinsic CMD
morphology, and colour-magnitude filters to isolate stars with a high
probability of being cluster members. Out of the 50 objects, 16 (32%) are star
clusters. We show that 9 (18%) overdensities are new OCs (FSR 735, FSR 807, FSR
812, FSR 826, FSR 852, FSR 904, FSR 941, FSR 953, and FSR 955) and 7 (14%) are
previously studied or catalogued OCs (KKC1, FSR 795, Cz 22, FSR 828, FSR 856,
Cz 24, and NGC 2234). These are OCs with ages in the range 5 Myr to 1 Gyr, at
distances from the Sun 1.28 \precnapprox d_Sun(kpc) \precnapprox 5.78 and
Galactocentric distances 8.5 R_GC(kpc) \precnapprox 12.9. We also derive
parameters for the previously analysed OCs Cz 22 and NGC 2234. Five (10%)
candidates are classified as uncertain cases, and the remaining objects are
probable field fluctuations.Comment: 14 pages, 15 figure
Characterisation of 15 overlooked Ruprecht clusters with ages within 400Myr and 3Gyr
We derive fundamental, structural, and photometric parameters of 15
overlooked Ruprecht (hereafter Ru) star clusters by means of 2MASS photometry
and field-star decontamination. Ru\,1, 10, 23, 26, 27, 34, 35, 37, 41, 54, 60,
63, 66, and 152 are located in the third Galactic quadrant, while Ru\,174 is in
the first. With the constraints imposed by the field-decontaminated
colour-magnitude diagrams (CMDs) and stellar radial density profiles (RDPs), we
derive ages in the range 400\,Myr --- 1\,Gyr, except for the older Ru\,37, with
\,Gyr. Distances from the Sun are within \rm1.5\la\ds(kpc)\la8.0. The
RDPs are well-defined and can be described by a King-like profile for most of
the radial range, except for Ru\,23, 27, 41, 63, and 174, which present a
conspicuous stellar density excess in the central region. The clusters dwell
between (or close to) the Perseus and Sagittarius-Carina arms. We derive
evidence in favour of cluster size increasing with distance to the Galactic
plane (\zgc), which is consistent with a low frequency of tidal stress
associated with high-|\zgc| regions. The clusters are rather faint even in
the near-infrared, with apparent integrated \jj\ magnitudes within 6.4\la
m_J\la9.8, while their absolute magnitudes are -6.6\la M_J\la-2.6.
Extrapolation of the relation between and , derived for globular
clusters, suggests that they are low-luminosity optical clusters, with -5\la
M_V\la-1.Comment: The paper contains 11 figures and 3 tables. Accepted by MNRAS
Discovery of three optical open clusters in the Galaxy
We report the discovery of three optical open clusters in the Milky Way. Two
clusters are in Scutum (Cluster1 at l=18.44 degrees and b=-0.42 degrees, and
Cluster2 at l=19.60 degrees and b=-1.02 degrees), thus projected not far from
the Galactic center direction, and the other is in Canis Major (Cluster3 at
l=235.61 degrees and b=-4.10 degrees), near the anti-center direction. Cluster3
is less populous than Clusters 1 and 2, but presents evidence of being a
physical system. The objects were found optically by inspecting maps obtained
from the Guide Star Catalogue and images from the Digitized Sky Survey. No
previous identification of cluster has been reported in each area so far. The
analysis was carried out with 2MASS photometry in J and H. For Cluster1 we
derive an age of 25 Myr, a reddening E(B-V)=2.18 and a distance from the Sun
1.64kpc; for Cluster2, age of 500Myr, E(B-V)=0.91 and distance 2.19kpc; finally
for Cluster3, age 32-100Myr, E(B-V)=0.94 and distance of 3.93kpc. Luminosity
and mass functions are derived for Clusters1 and 2 which, in turn, allowed us
to estimate their observed masses as 147 and 89 solar masses, respectively.
Estimated total masses, by extrapolating the mass functions to 0.08 solar mass,
amount to 382 and 614 solar masses, for the two clusters. Cluster3 has an
observed mass of 55 solar masses. The present results indicate that further
searches in the optical might still reveal new open clusters, and more so in
infrared bands.Comment: accepted to Astronomy and Astrophysics, 9 figure
Uniform detection of the pre-main sequence population in the 5 embedded clusters related to the H\,II region NGC\,2174 (Sh2-252)
We investigate 5 embedded clusters (ECs) and the extended stellar group
itself of the prominent H\,II region NGC\,2174 (Sh2-252), which presents scarce
and heterogeneous information, coming from the optical and infrared.
Considering the discrepant values of distance and age, the clusters and the
H\,II region appear to be physically unrelated. The analysis is based on
field-star decontaminated 2MASS photometry, which allows sampling the pre-main
sequence (PMS). We find that Sh2-252A, C, E, NGC\,2175s, and Teu\,136 are small
ECs (radius within \,pc) characterised by a similar age
(\,Myr), reddening (\aV\sim1), distance from the Sun
(\ds\sim1.4\,kpc), and low mass (60-200\,\ms). This age is consistent with
the H\,II region, the presence of O and B stars still in the MS, and the
dominance (\ga95% in number) of PMS stars in colour-magnitude diagrams
(CMDs). NGC\,2175 is not a star cluster, but an extended stellar group that
encompasses the ECs Sh2-252\,A and C. It contains of the member stars
(essentially PMS) in the area, with the remaining belonging to the 2 ECs. CMDs
of the overall star-forming region and the ECs provide \ds=1.4\pm0.4\,kpc for
the NGC\,2174 complex, consistent with the value estimated for the
physically-related association Gem\,OB1. Our uniform approach shows that
NGC\,2174 and its related ECs (except, perhaps, for Teu\,136) are part of a
single star-forming complex. CMD similarities among the ECs and the overall
region suggest a coeval (to within \,Myr) star-forming event extending
for several Myr. At least 4 ECs originated in the event, together with the
off-cluster star formation that probably gave rise to the scattered stars of
NGC\,2175.Comment: Accepted by MNRA
A possible sequential star formation in the giant molecular cloud G174+2.5
We investigate the nature of 14 embedded clusters (ECs) related to a group of
four H II regions Sh2-235, Sh2-233, Sh2-232, and Sh2-231 in the giant molecular
cloud G174 + 2.5. Projected towards the Galactic anticentre, these objects are
a possible example of the collect and collapse scenario. We derive
astrophysical parameters (age, reddening, distance, core and cluster radii) for
the ECs and investigate the relationship among their parameters. Parameters are
derived with field decontaminated 2MASS colour-magnitude diagrams (CMDs) and
stellar radial density profiles (RDPs). The CMDs of these young clusters are
characterised by a poorly-populated main sequence and a significant number of
pre-main sequence stars, affected by differential reddening. The ECs are KKC
11, FSR 784, Sh2-235 E2, Sh2-235 Cluster, Sh2-233SE Cluster, BDSB 73, Sh2-235B
Cluster, BDSB 72, BDSB 71, Sh2-232 IR, PCS 2, and the newly found clusters CBB
1 and CBB 2. We were able to derive fundamental parameters for all ECs in the
sample. Structural parameters are derived for FSR 784, Sh2-235 Cluster and
Sh2-235E2.Comment: 14 pages and 15 figures; MNRAS 201
Open cluster survival within the solar circle: Teutsch145 and Teutsch146
Teutsch145 and Teutsch146 are shown to be open clusters (OCs) orbiting well
inside the Solar circle, a region where several dynamical processes combine to
disrupt most OCs on a time-scale of a few 10^8yrs. BVI photometry from the
GALILEO telescope is used to investigate the nature and derive the fundamental
and structural parameters of the optically faint and poorly-known OCs
Teutsch145 and 146. These parameters are computed by means of field-star
decontaminated colour-magnitude diagrams (CMDs) and stellar radial density
profiles (RDPs). Cluster mass estimates are made based on the intrinsic mass
functions (MFs). We derive the ages 200+100-50Myr and 400+/-100Myr, and the
distances from the Sun 2.7+/-0.3kpc and 3.8+/-0.2kpc, respectively for
Teutsch145 and 146. Their integrated apparent and absolute magnitudes are m_V ~
12.4, m_V ~ 13.3, M_V ~- 5.6 and M_V ~- 5.3. The MFs (detected for stars with
m>1Msun) have slopes similar to Salpeter's IMF. Extrapolated to the H-burning
limit, the MFs would produce total stellar masses of ~1400Msun, typical of
relatively massive OCs. Both OCs are located deep into the inner Galaxy and
close to the Crux-Scutum arm. Since cluster-disruption processes are important,
their primordial masses must have been higher than the present-day values. The
conspicuous stellar density excess observed in the innermost bin of both RDPs
might reflect the dynamical effects induced by a few 10^8yrs of external tidal
stress.Comment: 8 pagas with 9 figs. Accepted by MNRA
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