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The mismatch between the mass function of the Milky Way's embedded clusters (ECs) and that of open clusters (OCs) raises the question of whether each OC originates from a single EC. In this work, we explore a scenario in which OCs form as a result of post-gas expulsion coalescence of ECs within the same parental molecular cloud. We model this process using N-body simulations of ECs undergoing expansion due to gas expulsion. Our initial conditions are based on the observed spatial, kinematic, and mass distributions of ECs in three representative massive star-forming regions (MSFRs). Initially, ECs are isolated. After further expansion, interactions between ECs begin, mutually influencing their evolution. We examine this process as a function of gas expulsion timescales, spatial separations between ECs, and their relative velocities. Our results demonstrate that, within a reasonable range of these parameters, the coalescence of ECs is robust and largely insensitive to initial conditions. The mass of ECs plays a critical role in the coalescence process. More massive ECs form stable gravitational cores, which greatly facilitate coalescence and help the resulting cluster resist expansion and Galactic tidal forces. Additionally, the number of ECs also enhances coalescence. The current mass distribution of clumps in the Milky Way suggests that directly forming massive ECs is challenging. However, the coalescence of multiple low-mass ECs can account for the observed parameter space of OCs in the Milky Way., Comment: 8 pages, 6 figures, Accepted for publication

We categorized clumps, embedded clusters, and open clusters and conducted a comparative analysis of their physical properties. Overall, the radii of open clusters are significantly larger than those of embedded clusters and clumps. The radii of embedded clusters are larger than those of clumps, which may be due to the expansion of embedded clusters. The open clusters have significantly higher masses than embedded clusters, by about one order of magnitude. Given the current mass distribution of clumps in the Milky Way, the evolutionary sequence from a single clump evolving into an embedded cluster and subsequently into an open cluster cannot account for the observed open clusters with old ages and high masses, which is also supported by N-body simulations of individual embedded clusters. To explain the mass and radius distributions of the observed open clusters, initial embedded clusters with masses higher than 3000 M$_{\odot}$ are necessary. However, the upper limit of the embedded cluster sample is less than 1000 M$_{\odot}$, and only a few ATLASGAL clumps have a mass higher than 3000 M$_{\odot}$. Thus, the currently observed clumps cannot be the "direct" precursors of the currently observed open clusters. If the Milky Way has a burst-like and time-dependent star formation history, the currently observed open clusters with old ages and high masses may come from massive clumps in the past. There is also a very real possibility that these open clusters originate from post-gas expulsion coalescence of multiple embedded clusters. We compared the separation of open clusters and the typical size of molecular clouds, and find that most molecular clouds may only form one open cluster, which supports the scenario of post-gas expulsion coalescence. Further study is necessary to distinguish between the different scenarios., Comment: Accepted for publication in A&A, 8 pages, 6 figures. arXiv admin note: text overlap with arXiv:2409.20271

We decomposed the molecular gas in the spiral galaxy NGC 628 (M74) into multi-scale hub-filament structures using the CO (2-1) line by the dendrogram algorithm. All leaf structures as potential hubs were classified into three categories, i.e. leaf-HFs-A, leaf-HFs-B and leaf-HFs-C. leaf-HFs-A exhibit the best hub-filament morphology, which also have the highest density contrast, the largest mass and the lowest virial ratio. We employed the FILFINDER algorithm to identify and characterize filaments within 185 leaf-HFs-A structures, and fitted the velocity gradients around the intensity peaks. Measurements of velocity gradients provide evidence for gas inflow within these structures. The numbers of the associated 21 $\mu$m and H$_{\alpha}$ structures and the peak intensities of 7.7 $\mu$m, 21 $\mu$m and H$_{\alpha}$ emissions decrease from leaf-HFs-A to leaf-HFs-C. The spatial separations between the intensity peaks of CO and 21 $\mu$m structures of leaf-HFs-A are larger than those of leaf-HFs-C. These evidence indicate that leaf-HFs-A are more evolved than leaf-HFs-C. There may be an evolutionary sequence from leaf-HFs-C to leaf-HFs-A. Currently, leaf-HFs-C lack a distinct gravitational collapse process that would result in a significant density contrast. The density contrast can effectively measure the extent of the gravitational collapse and the depth of the gravitational potential of the structure which, in turn, shapes the hub-filament morphology. Combined with the kinematic analysis presented in previous studies, a picture emerges that molecular gas in spiral galaxies is organized into network structures through the gravitational coupling of multi-scale hub-filament structures. Molecular clouds, acting as knots within these networks, serve as hubs, which are local gravitational centers and the main sites of star formation., Comment: 11 pages, 14 figures. Accepted for publication

1226 ATLASGAL clumps with HII regions were matched with radio sources in the CORNISH-North/South surveys, and 392 of them have corresponding radio sources. We determined the stellar luminosity according to the Lyman continuum flux. When the bolometric luminosity of HII-clumps is less than $\approx$ 10$^{3.7}$ L$_{\odot}$, corresponding to a clump mass $\approx$ 10$^{2.55}$ M$_{\odot}$, the stellar luminosities derived from the Lyman continuum flux overestimate the actual stellar luminosities, because the accretion onto the protostars contributes significantly to the radio emission. After subtracting the accretion luminosity, we obtained reasonable estimates of the stellar luminosity. Using the 0.5 Myr isochrone, we calculated the stellar masses according to the stellar luminosities, and found that they roughly follow the $m_{\rm max}-M_{\rm ecl}$ relation of embedded clusters, consistent with the ionizing sources representing the most massive stars in the embedded clusters of HII-clumps. We also studied the contribution of the possible flaring activity to the observed stellar luminosity and found that they can be neglected. We further studied the change of SFE with the clump mass. According to the derived mass of the most massive star in each HII-clump, using the theoretical $m_{\rm max}-M_{\rm ecl}$ relation, we calculated the mass of the corresponding embedded cluster and then the SFE of the clump. The SFE decreases with increasing clump mass, with a median value of $\approx$0.3. We also independently derived the SFE for each HII-clump based on the model developed in our previous work. The SFEs of HII-clumps derived from the observation and the model are in good agreement. Concerning the star formation histories of the ATLASGAL clumps, low-mass clumps may reach the peak of star formation earlier than high-mass clumps, consistent with the shorter free-fall time of low-mass clumps., Comment: 9 pages, 14 figures, Accepted for publication

Using the optimal sampling model, we synthesized the embedded clusters of ATLASGAL clumps with HII regions (HII-clumps). The 0.1 Myr isochrone was used to estimate the bolometric luminosity of each star in an embedded cluster, we also added the accretion luminosity of each star in the embeded cluster. The total bolometric luminosity of synthetic embedded clusters can well fit the observed bolometric luminosity of HII-clumps. More realistically, we considered the age spread in the young star and protostar populations in embedded clusters of HII-clumps by modeling both constant and time-varying star formation histories (SFHs). According to the age distribution of the stellar population, we distributed the appropriate isochrones to each star, and sorted out the fraction of stellar objects that are still protostars (Class 0 and Class I phases) to properly add their accretion luminosities. Compared to a constant SFH, burst-like and time-dependent SFHs can better fit the observational data. We found that as long as 20\% of the stars within the embedded cluster are still accreting, the contribution of accretion luminosity will be significant to the total bolometric luminosity of low-mass HII-clumps with mass log$_{10}$(M$_{\rm cl}$/M$_{\odot}$) $<$ 2.8. Variations in the accretion rate, the SFE and the initial mass function (IMF) and more physical processes like the external heating from HII regions and the flaring from pre-main sequence (PMS) stars and protostars need to be investigated to further explain the excess luminosity of low-mass HII-clumps., Comment: 7 pages, 10 figures, Accepted for publication

We used the dendrogram algorithm to decompose the surface density distributions of stars into hierarchical structures. These structures were tied to the multiscale structures of star clusters. A similar power-law for the mass-size relation of star clusters measured at different scales suggests a self-similar structure of star clusters. We used the minimum spanning tree method to measure the separations between clusters and gas clumps in each massive star-forming region. The separations between clusters, between clumps, and between clusters and clumps were comparable, which indicates that the evolution from clump to embedded cluster proceeds in isolation and locally, and does not affect the surrounding objects significantly. By comparing the mass functions of the ATLASGAL clumps and the identified embedded clusters, we confirm that a constant star formation efficiency of $\approx$ 0.33 can be a typical value for the ATLASGAL clumps., Comment: 6 pages, 8 figures, Accepted for publication in A&A

Star formation estimates based on the counting of YSOs is commonly applied to nearby star-forming regions in the Galaxy. With this method, the SFRs are measured using the counts of YSOs in a particular protostellar Class, a typical protostellar mass, and the lifetime associated with this Class. However, the assumptions underlying the validity of the method such as that of a constant star formation history (SFH) and whether the method is valid for all protostellar Classes has never been fully tested. In this work, we use Monte Carlo models to test the validity of the method. We build synthetic clusters in which stars form at times that are randomly drawn from a specified SFH. The latter is either constant or time-dependent with a burst like behavior. The masses of the protostars are randomly drawn from an IMF which can be either similar to that of the Milky Way field or be variable . For each star in every cluster, the lifetimes associated with the different protostellar classes are also randomly drawn from Gaussian distribution functions centered around their most likely value as suggested by the observations. We find that only the SFR derived using the Class 0 population can reproduce the true SFR at all epochs, and this is true irrespective of the shape of the SFH. For a constant SFH, the SFR derived using the more evolved populations of protostars (Classes I, F, II, and III) reproduce the real SFR only at later epochs which correspond to epochs at which their numbers have reached a steady state. For a time-dependent burst-like SFH, all SFR estimates based on the number counts of the evolved populations fail to reproduce the true SFR. We also show how the offsets between Class I and Class II based SFRs and the true SFR plotted as a function of the number ratios of Class I and Class II versus Class III YSOs can be used in order to constrain the SFH of observed molecular clouds., Comment: Accepted for publication in Astronomy & Astrophysics. Missing references included. Matches the published version

We have recently presented observational evidence which suggests that the origin of the second generation (G2) stars in globular clusters (GCs) is due to the binary-mediated collision of primordial (G1) low-mass main-sequence (MS) stars. This mechanism avoids both the mass budget problem and the need of external gas for dilution. Here, we report on another piece of evidence supporting this scenario: (1) the fraction of MS binaries is proportional to the fraction of G1 stars in GCs and, at the same time, (2) the smaller the fraction of G1 stars is, the more deficient binaries of higher mass ratio (q$>0.7$) are. They are, on average, harder than their smaller mass-ratio counterparts due to higher binding energy at a given primary mass. Then (2) implies that (1) is due to the merging\slash collisions of hard binaries rather than to their disruption. These new results complemented by the present-day data on binaries lead to the following conclusions: (i) the mass-ratio distribution of binaries, particularly short-period ones, with low-mass primaries, $M_{\rm P} < 1.5$ M$_{\sun}$, is strongly peaked close to q$=1.0$, whereas (ii) dynamical processes at high stellar density tend to destroy softer binaries and make hard (nearly) twin binaries to become even harder and favor their mergers and collisions. G2 stars formed this way gain mass that virtually doubles the primary one, $2M_{\rm P}$, at which the number of G1 stars is $\sim5$ times smaller than at $M_{\rm P}$ according to the slope of a Milky Way-like IMF at $M_{\rm MS} < 1.0$ M$_{\sun}$., Comment: 9 pages, 1 figure, 1 table; Accepted for publication in Monthly Notices of the Royal Astronomical Society

We present observational evidence of the correlation between the high-mass slope of the stellar initial mass function (IMF) in young star clusters and their stellar surface density, $\sigma_{*}$. When the high-mass end of the IMF is described by a power law of the form $dN/d{\rm log}{M_{*}}\propto M_{*}^{-\Gamma}$, the value of $\Gamma$ is seen to weakly decrease with increasing $\sigma_{*}$, following a $\Gamma=1.31~\sigma_{*}^{-0.095}$ relation. We also present a model that can explain these observations. The model is based on the idea that the coalescence of protostellar cores in a protocluster forming clump is more efficient in high density environments where cores are more closely packed. The efficiency of the coalescence process is calculated as a function of the parental clump properties and in particular the relation between its mass and radius as well as its core formation efficiency. The main result of this model is that the increased efficiency of the coalescence process leads to shallower slopes of the IMF in agreement with the observations of young clusters, and the observations are best reproduced with compact protocluster forming clumps. These results have significant implications for the shape of the IMF in different Galactic and extragalactic environments and have very important consequences for galactic evolution., Comment: Accepted to ApJ

According to the $\Lambda$CDM cosmology, present-day galaxies with stellar masses $M_\star>10^{11} {\rm M}_\odot$ should contain a sizable fraction of dark matter within their stellar body. Models indicate that in massive early-type galaxies (ETGs) dark matter should account for $\sim60\%$ of the dynamical mass within five effective radii ($5 R_{\rm e}$). Most massive ETGs have been shaped through a two-phase process: the rapid growth of a compact core was followed by the accretion of an extended envelope through mergers. The exceedingly rare galaxies that have avoided the second phase, the so-called relic galaxies, are thought to be the frozen remains of the massive ETG population at $z\gtrsim2$. The best relic galaxy candidate discovered to date is NGC 1277, in the Perseus cluster. We used deep integral field GCMS data to revisit NGC 1277 out to an unprecedented radius of 6 kpc (corresponding to $5 R_{\rm e}$). By using Jeans anisotropic modelling we find a negligible dark matter fraction within $5 R_{\rm e}$ ($f_{\rm DM}(5 R_{\rm e})<0.05$; two-sigma confidence level), which is in tension with the expectation. Since the lack of an extended envelope would reduce dynamical friction and prevent the accretion of an envelope, we propose that NGC 1277 lost its dark matter very early or that it was dark matter deficient ab initio. We discuss our discovery in the framework of recent proposals suggesting that some relic galaxies may result from dark matter stripping as they fell in and interacted within galaxy clusters. Alternatively, NGC 1277 might have been born in a high-velocity collision of gas-rich proto-galactic fragments, where dark matter left behind a disc of dissipative baryons. We speculate that the relative velocities of $\approx2000 {\rm km/s}$ required for the latter process to happen were possible in the progenitors of the present-day rich galaxy clusters., Comment: Accepted for publication in A&A

We analyze the structure of 15 protocluster forming regions in the Milky Way using their $1.3$ mm continuum emission maps from the ALMA-IMF large program. The analysis of the clouds structure is performed using the delta-variance spectrum technique. The calculated spectra display a self-similar regime on small scales as well as the presence of a prominent bump on larger scales and whose physical size, $L_{\rm hub}$, falls in the range of $\approx 7000$ au to $60000$ au. These scales correspond to the sizes of the most compact clumps within the protocluster forming clouds. A significant correlation is found between $L_{\rm hub}$ and the surface density of the free-free emission estimated from the integrated flux of the H41$\alpha$ recombination line $\left(\Sigma_{\rm H41\alpha}^{\rm free-free}\right)$ as well as a significant anti-correlation between $L_{\rm hub}$ and the ratio of the 1.3 mm to 3 mm continuum emission fluxes $\left(S_{\rm 1.3 mm}^{\rm cloud}/S_{\rm 3 mm}^{\rm cloud}\right)$. Smaller values of $\left(S_{\rm 1.3 mm}^{\rm cloud}/S_{\rm 3 mm}^{\rm cloud}\right)$ and larger values of $\Sigma_{\rm H41\alpha}^{\rm free-free}$ correspond to more advanced evolutionary stages of the protocluster forming clumps. Hence, our results suggest that the sizes of the densest regions in the clouds are directly linked to their evolutionary stage and to their star formation activity with more evolved clouds having larger protocluster forming clumps. This is an indication that gravity plays a vital role in regulating the size and mass growth and star formation activity of these clumps with ongoing gas accretion, Comment: Accepted in MNRAS. Matches the published version

We study brane-world models and demonstrate that such models do not admit self-similar solutions through the matter collineation approach. By introducing the hypothesis of variable brane tension, $\lambda,$ we outline the new effective field equation (EFE) in the most simple case (symmetric embedding) under the assumption that the fundamental constants in 5D are constants. In this case, we find the exact form that each physical quantity may take in order that the EFE become invariant under scale transformations. By taking into account such assumptions, we find that in 4D, the gravitational constant $\kappa^{2}\thicksim\lambda$ while the cosmological constant $\Lambda\thicksim\lambda^{2}$ are always decreasing. These results are quite general and valid for any homogeneous self-similar metric. Nevertheless, the study of the EFE under scale symmetries suggests that $\rho\thicksim\lambda$ (as a functional relationship). This allows to get a growing $\kappa^{2}$ but, in this case, the fundamental constants in 5D must vary as well. We outline a toy model allowing such a possibility., Comment: Accepted to Canadian Journal of Physics

We calculate the integrated galactic initial stellar mass function (IGIMF) in the presence of IMF variations in clusters. IMF Variations for a population of clusters are taken into account in the form of Gaussian distributions of the IMF parameters. For the tapered power law function used here, these are the slopes at the high and low mass ends, $\Gamma$ and $\gamma$, and the characteristic mass $M_{ch}$. Variations are modeled by varying the width of the Gaussian distributions. The reference values are the standard deviations of the parameters observed for young clusters in the present-day Milky Way $\sigma_{\Gamma}=0.6$, $\sigma_{\gamma}=0.25$, and $\sigma_{M_{ch}}=0.27$ M$_{\odot}$. Increasing the dispersions of $\gamma$ and $\Gamma$ moderately flattens the IGIMF at the low and high mass ends. Increasing $\sigma_{M_{ch}}$ shifts the peak of the IGIMF to lower masses, rendering the IGIMF more bottom heavy. This can explain the bottom heavy stellar mass function of Early-type galaxies as they are the result of the merger of disk galaxies where the physical conditions of the star forming gas vary significantly both in time and space. The effect of IMF variations is compared to that due to other effects such as variations in the shape of the initial cluster mass function, metallicity, and galactic SFR. We find that the effect of IMF variations is a dominant factor that always affects the characteristic mass of the IGIMF. We compare our results to a sample of ultra-faint dwarf satellite galaxies (UFDs). Their present-day stellar mass function is an analog to their IGIMF at the time their stellar populations have formed. We show that the slope of the IGIMF of the UFDs can only be reproduced when IMF variations of the same order as those measured in the present-day Milky Way are included. (Abridged), Comment: Accepted to A&A. Additional Figures. Figure 7 and 8 are updated, but conclusions are unchanged

Globular clusters display an anticorrelation between the fraction of the first generation of stars ($N({\rm G1})/N({\rm tot})$) and the slope of the present-day mass function of the clusters ($\alpha_{pd}$), which is particularly significant for massive clusters. In the framework of the binary-mediated collision scenario for the formation of the second-generation stars in globular clusters, we test the effect of a varying stellar initial mass function (IMF) of the G1 stars on the $(N({\rm G1})/N({\rm tot}))-\alpha_{pd}$ anticorrelation. We use a simple collision model that has only two input parameters, the shape of the IMF of G1 stars and the fraction of G1 stars that coalesce to form second-generation stars. We show that a variable efficiency of the collision process is necessary in order to explain the $(N({\rm G1})/N({\rm tot}))-\alpha_{pd}$ anticorrelation; however, the scatter in the anticorrelation can only be explained by variations in the IMF, and in particular by variations in the slope in the mass interval $\approx$ (0.1-0.5) M$_{\odot}$. Our results indicate that in order to explain the scatter in the $(N({\rm G1})/N({\rm tot}))-\alpha_{pd}$ relation, it is necessary to invoke variations in the slope in this mass range between $\approx -0.9$ and $\approx -1.9$. Interpreted in terms of a Kroupa-like broken power law, this translates into variations in the mean mass of between $\approx 0.2$ and $0.55$ M$_{\odot}$. This level of variation is consistent with what is observed for young stellar clusters in the Milky Way and may reflect variations in the physical conditions of the globular cluster progenitor clouds at the time the G1 population formed or may indicate the occurrence of collisions between protostellar embryos before stars settle on the main sequence., Comment: Accepted to A&A. Matches the published version. Typo for the sign of alpha1 and alpha2 corrected in Figures 1-4

Two generations of stars, G1 and G2, typically populate Galactic globular clusters (GCs). The origin of G2 stars is unclear. We uncover two empirical dependencies between GC characteristics, which can be explained by the formation of G2 Main-Sequence (MS) stars due to collision\merging of their primordial counterparts (G1). A similar genesis of both G2 stars and peculiar objects like LMXBs and millisecond pulsars is also implied. Indeed, we find a significant (at a confidence level > 99,9%) anti-correlation between the fraction of G1 red giants ($N_{G1}/N_{tot}$) and stellar encounter rates among 51 GCs. Moreover, a Milky Way-like initial mass function (IMF) requires at least ~50% of MS stars located in the mass range [0.1-0.5] $M\odot$. Unlike cluster mass loss, stellar collisions\merging retain these G1 stars by converting them into more massive G2 ones, with mainly M_ms > 0.5 $M\odot$. This process coupled with a decreasing relative mass loss with increasing GC masses implies a smaller ($N_{G1}/N_{tot}$) in more massive GCs with a shallower present day MF. From data for 35 GCs, we find that such an anti-correlation is significant at 98.3% confidence level (Spearman's correlation) for 12 most massive GCs ($M_{GC} > 10^{5.3}$) and it is at a confidence level of 89% for the 12 least massive GCs ($M_{GC} < 10^{5.1} M_\odot$). Other fractions of G1 and G2 stars observed at the bottom of the MS as compared with the red giant branch in a few GCs are consistent with the scenario proposed., Comment: 10 pages, 3 figures, 1 table; accepted for publication in MNRAS (slightly abridged abstract)

The Cepheus Flare region consists of a group of dark cloud complexes that are currently active in star formation. The aim of this work is to estimate the motion of four clouds, L1147/1158, L1172/1174, L1228 and L1251 located at relatively high Galactic latitude (b $\gt$ 14$^{\circ}$) in the Cepheus Flare region. We study the relationship between the motion of the cloud with respect to the magnetic field and the clump orientations with respect to both the magnetic field and the motion. We estimated the motion of the molecular clouds using the proper motion and the distance estimates of the young stellar objects (YSOs) associated with them using the Gaia EDR3 data. By assuming that the YSOs are associated with the clouds and share the same velocity, the projected direction of motion of the clouds are estimated. We estimated a distance of 371$\pm$22 pc for L1228 and 340$\pm$ 7 pc for L1251 implying that all four complexes are located at almost the same distance. Assuming that both the clouds and YSOs are kinematically coupled, we estimated the projected direction of motion of the clouds using the proper motions of the YSOs. All the clouds in motion are making an offset of $\sim$ 30$^{\circ}$ with respect to the ambient magnetic fields except in L1172/1174 where the offset is $\sim$ 45$^{\circ}$. In L1147/1158, the starless clumps are found to be oriented predominantly parallel to the magnetic fields while prestellar clumps show random distribution. In L1172/1174, L1228 and L1251,the clumps are oriented randomly with respect to magnetic field. With respect to the motion of the clouds, there is a marginal trend that the starless clumps are oriented more parallel in L1147/1158 and L1172/1174. In L1228, the clumps major axis are oriented more randomly. In L1251, we find a bimodal trend in case of starless clumps., Comment: 13 pages, 24 figures, accepted for publication in A&A, Abstract has been shortened up due to word limit

We study the $f(R,T)$ cosmological models under the self-similarity hypothesis. We determine the exact form that each physical and geometrical quantity may take in order that the Field Equations (FE) admit exact self-similar solutions through the matter collineation approach. We study two models: the case$\ f(R,T)=f_{1}(R)+f_{2}(T)$ and the case $f(R,T)=f_{1}(R)+f_{2} (R)f_{3}(T)$. In each case, we state general theorems which determine completely the form of the unknown functions $f_{i}$ such that the field equations admit self-similar solutions. We also state some corollaries as limiting cases. These results are quite general and valid for any homogeneous self-similar metric$.$ In this way, we are able to generate new cosmological scenarios. As examples, we study two cases by finding exact solutions to these particular models., Comment: accepted to IJGMMP. Matches the published version. arXiv admin note: text overlap with arXiv:1407.2013 by other authors

The spatial distribution of the HI gas in galaxies holds important clues on the physical processes that shape the structure and dynamics of the interstellar medium (ISM). In this work, we quantify the structure of the HI gas in a sample of 33 nearby galaxies taken from the THINGS Survey using the delta-variance spectrum. The THINGS galaxies display a large diversity in their spectra, however, there are a number of recurrent features. In many galaxies, we observe a bump in the spectrum on scales of a few to several hundred pc. We find the characteristic scales associated with the bump to be correlated with galactic SFR for values of the SFR > 0.5 M$_{sol}$ yr$^{-1}$ and also with the median size of the HI shells detected in those galaxies. On larger scales, we observe the existence of two self-similar regimes. The first one, on intermediate scales is shallow and the power law that describes this regime has an exponent in the range [0.1-1] with a mean value of 0.55 which is compatible with the density field being generated by supersonic turbulence in the cold phase of the HI gas. The second power law is steeper, with a range of exponents between [0.5-1.5] and a mean value of 1.5. These values are associated with subsonic turbulence which is characteristic of the warm phase of the HI gas. The spatial scale at which the transition between the two regimes occurs is found to be $\approx 0.5 R_{25}$ which is similar to the size of the molecular disk in the THINGS galaxies. Overall, our results suggest that on scales < $0.5 R_{25}$, the structure of the ISM is affected by the effects of supernova explosions. On larger scales (> 0.5 $R_{25}$), stellar feedback has no significant impact, and the structure of the ISM is determined by large scale processes that govern the dynamics of the gas in the warm neutral medium such as the flaring of the HI disk and the effects of ram pressure stripping., Comment: Accepted to A&A. Matches the published version

We present an analysis of the optical properties of three Ultra Luminous X-ray (ULX) sources identified in NGC 925. We use Integral field unit data from the George Mitchel spectrograph in the context of the Metal-THINGS survey. The optical properties for ULX-1 and ULX-3 are presented, while the spaxel associated with ULX-2 had a low S/N, which prevented its analysis. We also report the kinematics and dimensions of the optical nebula associated with each ULX using ancillary data from the PUMA Fabry-Perot spectrograph. A BPT analysis demonstrates that most spaxels in NGC 925 are dominated by star-forming regions, including those associated with ULX-1 and ULX-3. Using the resolved gas-phase metallicities, a negative metallicity gradient is found, consistent with previous results for spiral galaxies, while the ionization parameter tends to increase radially throughout the galaxy. Interestingly, ULX-1 shows a very low gas metallicity for its galactocentric distance, identified by two independent methods, while exhibiting a typical ionization. We find that such low gas metallicity is best explained in the context of the high-mass X-ray binary population, where the low-metallicity environment favours active Roche lobe overflows that can drive much higher accretion rates. An alternative scenario invoking accretion of a low-mass galaxy is not supported by the data in this region. Finally, ULX-3 shows both a high metallicity and ionization parameter, which is consistent with the progenitor being a highly-accreting neutron star within an evolved stellar population region., Comment: Accepted by ApJ

The structure of molecular clouds (MCs) holds important clues on the physical processes that lead to their formation and subsequent evolution. While it is well established that turbulence imprints a self-similar structure to the clouds, other processes, such as gravity and stellar feedback, can break their scale-free nature. The break of self-similarity can manifest itself in the existence of characteristic scales that stand out from the underlying structure generated by turbulent motions. We investigate the structure of the Cygnus-X North and the Polaris MCs which represent two extremes in terms of their star formation activity. We characterize the structure of the clouds using the delta-variance ($\Delta$-variance) spectrum. In Polaris, the structure of the cloud is self-similar over more than one order of magnitude in spatial scales. In contrast, the $\Delta$-variance spectrum of Cygnus-X exhibits an excess and a plateau on physical scales of ~0.5-1.2 pc. In order to explain the observations for Cygnus-X, we use synthetic maps in which we overlay populations of discrete structures on top of a fractal Brownian motion (fBm) image. The properties of these structures such as their major axis sizes, aspect ratios, and column density contrasts are randomly drawn from parameterized distribution functions. We show that it is possible to reproduce a $\Delta$-variance spectrum that resembles the one of the Cygnus-X cloud. We also use a "reverse engineering" approach in which we extract the compact structures in the Cygnus-X cloud and re-inject them on an fBm map. The calculated $\Delta$-variance using this approach deviates from the observations and is an indication that the range of characteristic scales observed in Cygnus-X is not only due to the existence of compact sources, but is a signature of the whole population of structures, including more extended and elongated structures, Comment: Accepted to A&A. Version after language and other minor corrections

We aim to reveal the physical properties and chemical composition of the cores in the California molecular cloud (CMC), so as to better understand the initial conditions of star formation. We made a high-resolution column density map (18.2") with Herschel data, and extracted a complete sample of the cores in the CMC with the \textsl{fellwalker} algorithm. We performed new single-pointing observations of molecular lines near 90 GHz with the IRAM 30m telescope along the main filament of the CMC. In addition, we also performed a numerical modeling of chemical evolution for the cores under the physical conditions. We extracted 300 cores, of which 33 are protostellar and 267 are starless cores. About 51\% (137 of 267) of the starless cores are prestellar cores. Three cores have the potential to evolve into high-mass stars. The prestellar core mass function (CMF) can be well fit by a log-normal form. The high-mass end of the prestellar CMF shows a power-law form with an index $\alpha=-0.9\pm 0.1$ that is shallower than that of the Galactic field stellar mass function. Combining the mass transformation efficiency ($\varepsilon$) from the prestellar core to the star of $15\pm 1\%$ and the core formation efficiency (CFE) of 5.5\%, we suggest an overall star formation efficiency of about 1\% in the CMC. In the single-pointing observations with the IRAM 30m telescope, we find that 6 cores show blue-skewed profile, while 4 cores show red-skewed profile. [$\rm {HCO}^{+}$]/[HNC] and [$\rm {HCO}^{+}$]/$\rm [N_{2}H^{+}]$ in protostellar cores are higher than those in prestellar cores; this can be used as chemical clocks. The best-fit chemical age of the cores with line observations is $\sim 5\times 10^4$~years., Comment: Accepted by Astronomy & Astrophysics (A&A)

Recent numerical simulations of magnetized accretion discs show that the radial-azimuthal component of the stress tensor due to the magnetorotational instability (MRI) is well represented by a power-law function of the gas pressure rather than a linear relation which has been used in most of the accretion disc studies. The exponent of this power-law function which depends on the net flux of the imposed magnetic field is reported in the range between zero and unity. However, the physical consequences of this power-law stress-pressure relation within the framework of the standard disc model have not been explored so far. In this study, the structure of an accretion disc with a power-law stress-pressure relation is studied using analytical solutions in the steady-state and time-dependent cases. The derived solutions are applicable to different accreting systems, and as an illustrative example, we explore structure of protoplanetary discs using these solutions. We show that the slopes of the radial surface density and temperature distributions become steeper with decreasing the stress exponent. However, if the disc opacity is dominated by icy grains and value of the stress exponent is less than about $0.5$, the surface density and temperature profiles become so steep that make them unreliable. We also obtain analytical solutions for the protoplanetary discs which are irradiated by the host star. Using these solutions, we find that the effect of the irradiation becomes more significant with decreasing the stress exponent., Comment: Accepted for publication in MNRAS

We examine the spatial distribution and mass segregation of dense molecular cloud cores in a number of nearby star forming regions that span about four orders of magnitude in star formation activity. We use an approach based on the calculation of the minimum spanning tree, and for each region, we calculate the structure parameter Q and the mass segregation ratio measured for various numbers of the most massive cores. Our results indicate that the distribution of dense cores in young star forming regions is very substructured and that it is likely that this substructure will be imprinted onto the nascent clusters that will emerge out of these clouds. With the exception of Taurus in which there is nearly no mass segregation, we observe mild-to-significant levels of mass segregation for the ensemble of the 6, 10, and 14 most massive cores in Aquila, CrA, and W43, respectively. Our results suggest that the clouds' star formation activity are linked to their structure, as traced by their population of dense cores. We also find that the fraction of massive cores that are the most mass segregated in each region correlates with the surface density of star formation in the clouds. The Taurus region with low star-forming activity is associated with a highly hierarchical spatial distribution of the cores (low Q value) and the cores show no sign of being mass segregated. On the other extreme, the mini-starburst region W43-MM1 has a higher Q that is suggestive of a more centrally condensed structure and it possesses a higher fraction of massive cores that are segregated by mass. While some limited evolutionary effects might be present, we attribute the correlation between the star formation activity of the clouds and their structure to a dependence on the physical conditions that have been imprinted on them by the large scale environment at the time they started to assemble, Comment: Accepted to A&A, 11 pages, 5 figures. Text and discussion significantly expanded. Conclusions unchanged

We investigate how a single generation galactic mass function (SGMF) depends on the existence of variations in the initial stellar mass functions (IMF) of stellar clusters. We show that cluster-to-cluster variations of the IMF lead to a multicomponent SGMF where each component in a given mass range can be described by a distinct power-law function. We also show that a dispersion of $\approx 0.3$ M$_{\odot}$ in the characteristic mass of the IMF, as observed for young Galactic clusters, leads to a low mass slope of the SGMF that matches the observed Galactic stellar mass function even when the IMFs in the low mass end of individual clusters are much steeper., Comment: Accepted to A&A. Matches the published version

We quantify the structure of a very large number of Galactic open clusters and look for evidence of mass segregation for the most massive stars in the clusters. We characterise the structure and mass segregation ratios of 1276 clusters in the Milky Way Stellar Cluster (MWSC) catalogue containing each at least 40 stars and that are located at a distance of up to $\approx 2$ kpc from the Sun. We use an approach based on the calculation of the minimum spanning tree of the clusters, and for each one of them, we calculate the structure parameter \Q\ and the mass segregation ratio $\Lambda_{\rm MSR}$. Our findings indicate that most clusters possess a \Q\ parameter that falls in the range 0.7-0.8 and are thus neither strongly concentrated nor do they show significant substructure. Only 27\% can be considered centrally concentrated with \Q\ values $> 0.8$. Of the 1276 clusters, only 14\% show indication of significant mass segregation ($\Lambda_{\rm MSR} > 1.5$). Furthermore, no correlation is found between the structure of the clusters or the degree of mass segregation with their position in the Galaxy. A comparison of the measured \Q\ values for the young open clusters in the MWSC to N-body numerical simulations that follow the evolution of the \Q\ parameter over the first 10 Myrs of the clusters life suggests that the young clusters found in the MWSC catalogue initially possessed local mean volume densities of $\rho_{*} \approx 10-100$ M$_{\odot}$ pc$^{-3}$., Comment: Accepted to MNRAS

We present a model for the origin of the extended law of star formation in which the surface density of star formation ($\Sigma_{\rm SFR}$) depends not only on the local surface density of the gas ($\Sigma_{g}$), but also on the stellar surface density ($\Sigma_{*}$), the velocity dispersion of the stars, and on the scaling laws of turbulence in the gas. We compare our model with the spiral, face-on galaxy NGC 628 and show that the dependence of the star formation rate on the entire set of physical quantities for both gas and stars can help explain both the observed general trends in the $\Sigma_{g}-\Sigma_{\rm SFR}$ and $\Sigma_{*}-\Sigma_{\rm SFR}$ relations, but also, and equally important, the scatter in these relations at any value of $\Sigma_{g}$ and $\Sigma_{*}$. Our results point out to the crucial role played by existing stars along with the gaseous component in setting the conditions for large scale gravitational instabilities and star formation in galactic disks., Comment: aceepted to MNRAS

Self-gravitating accretion discs in a gravitoturbulent state, including radiation and gas pressures, are studied using a set of new analytical solutions. While the Toomre parameter of the disc remains close to its critical value for the onset of gravitational instability, the dimensionless stress parameter is uniquely determined from the thermal energy reservoir of the disc and its cooling rate. Our solutions are applicable to the accretion discs with dynamically important radiation pressure like in the quasars discs. We show that physical quantities of a gravitoturbulent disc in the presence of radiation are significantly modified compared to solutions with only gas pressure. We show that the dimensionless stress parameter is an increasing function of the radial distance so that its steepness strongly depends on the accretion rate. In a disc without radiation its slope is 4.5, however, we show that in the presence of radiation, it varies between 2 and 4.5 depending on the accretion rate and the central mass. As for the surface density, we find a shallower profile with an exponent -2 in a disc with sub-Eddington accretion rate compared to a similar disc, but without radiation, where its surface density slope is -3 independent of the accretion rate. We then investigate gravitational stability of the disc when the stress parameter reaches to its critical value. In order to self-consistently determine the fragmentation boundary, however, it is shown that the critical value of the stress parameter is a power-law function of the ratio of gas pressure and the total pressure and its exponent is around 1.7. We also estimate the maximum mass of the central black hole using our analytical solutions., Comment: Accepted for publication in MNRAS

We investigate whether the stellar initial mass function (IMF) is universal, or whether it varies significantly among young stellar clusters in the Milky Way. We propose a method to uncover the range of variation of the parameters that describe the shape of the IMF for the population of young Galactic clusters. These parameters are the slopes in the low and high stellar mass regimes, $\gamma$ and $\Gamma$, respectively, and the characteristic mass, $M_{ch}$. The method relies exclusively on the high mass content of the clusters, but is able to yield information on the distributions of parameters that describe the IMF over the entire stellar mass range. This is achieved by comparing the fractions of single and lonely massive O stars in a recent catalog of the Milky Way clusters with a library of simulated clusters built with various distribution functions of the IMF parameters. The synthetic clusters are corrected for the effects of the binary population, stellar evolution, sample incompleteness, and ejected O stars. Our findings indicate that broad distributions of the IMF parameters are required in order to reproduce the fractions of single and lonely O stars in Galactic clusters. They also do not lend support to the existence of a cluster mass-maximum stellar mass relation. We propose a probabilistic formulation of the IMF whereby the parameters of the IMF are described by Gaussian distribution functions centered around $\gamma=0.91$, $\Gamma=1.37$, and $M_{ch}=0.41$ M$_{\odot}$, and with dispersions of $\sigma_{\gamma}=0.25$, $\sigma_{\Gamma}=0.60$, and $\sigma_{M_{ch}}=0.27$ M$_{\odot}$ around these values., Comment: Accepted to MNRAS, 17 pages, 13 figures. Larger observational sample. Conclusions strengthened

[Abridged] We carried out optical polarimetry of five dense cores, (IRAM 04191, L1521F, L328, L673-7, and L1014) which are found to harbour VeLLO. This study was conducted mainly to understand the role played by the magnetic field in the formation of very low and substellar mass range objects using optical polarisation. The angular offsets between the envelope magnetic field direction (inferred from optical polarisation measurements) and the outflow position angles from the VeLLOs in IRAM 04191, L1521F, L328, L673-7, and L1014 are found to be 84$^\circ$, 53$^\circ$, 24$^\circ$, 08$^\circ$, and 15$^\circ$, respectively. The mean value of the offsets for all the five clouds is $\sim37^\circ$. If we exclude IRAM 04191, the mean value reduces to become $\sim25^\circ$. In IRAM 04191, the offset between the projected envelope and the inner magnetic field (inferred from the submillimetre data from SCUPOL) is found to be $\sim68^\circ$. The inner magnetic field, however, is found to be nearly aligned with the projected position angles of the minor axis, the rotation axis of the cloud, and the outflow from the IRAM 04191-IRS. We discuss a possible explanation for the nearly perpendicular orientation between the envelope and core scale magnetic fields in IRAM04191. The angular offset between the envelope magnetic field direction and the minor axis of IRAM 04191, L1521F, L673-7, and L1014 are 82$^\circ$, 60$^\circ$, 47$^\circ$, and 55$^\circ$, respectively. The mean value of the offsets between the envelope magnetic field and the minor axis position angles for the four cores is found to be $\sim60^\circ$. The results obtained from our study on the limited sample of five cores with VeLLOs show that the outflows in three of them tend to nearly align with the envelope magnetic field., Comment: 18 pages, 18 figures, 5 tables, due to length limitation, the abstract appearing here is abridged

The universality of the stellar initial mass function (IMF) is tested using Bayesian statistics with a sample of eight young Galactic stellar clusters (IC 348, ONC, NGC 2024, NGC 6611, NGC 2264, $\rho$ Ophiuchi, Chameleon I, and Taurus). We infer the posterior probability distribution function (pPDF) of the IMF parameters when the likelihood function is described by a tapered power law function, a lognormal distribution at low masses coupled to a power law at higher masses, and a multi-component power law function. The inter-cluster comparison of the pPDFs of the IMF parameters for each likelihood function shows that these distributions do not overlap within the $1\sigma$ uncertainty level. Furthermore, the most probable values of the IMF parameters for most of the clusters deviate substantially from their values for the Galactic field stellar IMF. We also quantify the effects of taking into account the completeness correction as well as the uncertainties on the measured masses. The inclusion of the former affects the inferred pPDFs of the slope of the IMF at the low mass end while considering the latter affects the pPDFs of the slope of the IMF in the intermediate- to high mass regime. As variations are observed in all of the IMF parameters at once and for each of the considered likelihood functions, even for completeness corrected samples, we argue that the observed variations are real and significant, at least for the sample of eight clusters considered in this work. The results presented here clearly show that the IMF is not universal., Comment: accepted to MNRAS

We show that the termination of the star formation process by winds from massive stars in protocluster forming clumps imposes dual constraints on the star formation efficiencies (SFEs) and stellar age spreads ($\Delta \tau_{*}$) in stellar clusters. We have considered two main classes of clump models. One class of models in one in which the core formation efficiency (CFE) per unit time and as a consequence the star formation rate (SFR) is constant in time and another class of models in which the CFE per unit time, and as a consequence the SFR, increases with time. Models with an increasing mode of star formation yield shorter age spreads (a few 0.1 Myrs) and typically higher SFEs than models in which star formation is uniform in time. We find that the former models reproduce remarkably well the SFE$-\Delta \tau_{*}$ values of starburst clusters such as NGC 3603 YC and Westerlund 1, while the latter describe better the star formation process in lower density environments such as in the Orion Nebula Cluster. We also show that the SFE and $\Delta \tau_{*}$ of massive clusters are expected to be higher in low metallicity environments. This could be tested with future large extragalactic surveys of stellar clusters. We advocate that placing a stellar cluster on the SFE-$\Delta \tau_{*}$ diagram is a powerful method to distinguish between different stellar clusters formation scenarios such as between generic gravitational instability of a gas cloud/clump or as the result of cloud-cloud collisions. It is also a very useful tool for testing star formation theories and numerical models versus the observations., Comment: Accepted to MNRAS

We show how the mass function of dense cores (CMF) which results from the gravoturbulent fragmentation of a molecular cloud evolves in time under the effect of gas accretion. Accretion onto the cores leads to the formation of larger numbers of massive cores and to a flattening of the CMF. This effect should be visible in the CMF of star forming regions that are massive enough to contain high mass cores and when comparing the CMF of cores in and off dense filaments which have different environmental gas densities., Comment: 2 page, 1 figure. To appear in the proceedings of The Labyrinth of Star Formation, eds. D. Stamatellos, S. Goodwin, and D. Ward-Thompson. Springer

When a gravitating object moves across a given mass distribution, it creates an overdense wake behind it. Here, we performed an analytical study of the structure of the flow far from object when the flow is isentropic and the object moves subsonically within it. We show that the dynamical friction force is the main drag force on the object and by using a perturbation theory, we obtain the density, velocity and pressure of the perturbed flow far from the mass. We derive the expression of the dynamical friction force in an isentropic flow and show its dependence on the Mach number of the flow and on the adiabatic index. We find that the dynamical friction force becomes lower as the adiabatic index increases. We show analytically that the wakes are less dense in our isentropic case in comparison to the isothermal ones., Comment: Accepted for publication in Astrophysics and Space Science

We analyse the role played by shear in regulating star formation in the Galaxy on the scale of individual molecular clouds. The clouds are selected from the 13^CO J=1-0 line of the Galactic Ring Survey. For each cloud, we estimate the shear parameter which describes the ability of density perturbations to grow within the cloud. We find that for almost all molecular clouds considered, there is no evidence that shear is playing a significant role in opposing the effects of self-gravity. We also find that the shear parameter of the clouds does not depend on their position in the Galaxy. Furthermore, we find no correlations between the shear parameter of the clouds with several indicators of their star formation activity. No significant correlation is found between the shear parameter and the star formation efficiency of the clouds which is measured using the ratio of the massive young stellar objects luminosities, measured in the Red MSX survey, to the cloud mass. There are also no significant correlations between the shear parameter and the fraction of their mass that is found in denser clumps which is a proxy for their clump formation efficiency, nor with their level of fragmentation expressed in the number of clumps per unit mass. Our results strongly suggest that shear is playing only a minor role in affecting the rates and efficiencies at which molecular clouds convert their gas into dense cores and thereafter into stars., Comment: Accepted to ApJ. 30 pages, 11 figures. Content substantially enlarged and includes quantitative correlations between the SFE of molecular clouds and their shear parameters. Some references corrected

We check how the change in surface conditions between the Sun and red giant branch stars changes the characteristic surface convection length scale to be used in models. We investigate the question in the case of the mixing length theory and of the phenomenology of full spectrum of turbulence. For the observational part, we rely on independent measurements of effective temperatures and interferometric radii of nearby red giants. We find that the local red giant branch cannot be explained taking into account the solar calibrated convective length scale., Comment: In the proceedings of the French Astronomical Society meeting SF2A 2011

The aim of the paper is to estimate distances to dense molecular cores that harbour Very Low Luminosity Objects (VeLLO) detected by Spitzer Space Telescope and to confirm their VeLLO nature. The cloud distances are estimated using near-IR photometric method. We use a technique that provides spectral classification of stars lying towards the fields containing the clouds into main sequence and giants. In this technique, the observed (J-H) and (H-Ks) colours are dereddened simultaneously using trial values of A_V and a normal interstellar extinction law. The best fit of the dereddened colours to the intrinsic colours giving a minimum value of chi^{2} then yields the corresponding spectral type and A_V for the star. The main sequence stars, thus classified, are then utilized in an A_V versus distance plot to bracket the cloud distances. The typical error in the estimation of distances to the clouds are found to be ~18%.We estimated distances to seven cloud cores, IRAM04191, L1521F, BHR111, L328, L673-7, L1014, and L1148 using the above method. These clouds contain VeLLO candidates. The estimated distances to the cores are found to be 127\pm25 pc (IRAM04191), 136\pm36 pc (L1521F), 355\pm65 pc (BHR111), 217\pm30 pc (L328), 240\pm45 pc (L673-7), 258\pm50 pc (L1014), and 301\pm55 pc (L1148). We re-evaluated the internal luminosities of the VeLLOs discovered in these seven clouds using the distances estimated from this work. Except for L1014-IRS (L_{int}=0.15 L_{\odot}), all other VeLLO candidates are found to be consistent with the definition of a VeLLO (L_{int}<=0.1L_{\odot}). In addition to the cores that harbour VeLLO candidates, we also obtained distances to the clouds L323, L675, L676, CB 188, L1122, L1152, L1155, L1157 and L1158 which are located in the directions of the above seven cores. Towards L1521F and L1148 we found evidence of the presence of multiple dust layers., Comment: 23 figures, accepted for publication in A&A

We describe results from semi-analytical modelling of star formation in protocluster clumps of different metallicities. In this model, gravitationally bound cores form uniformly in the clump following a prescribed core formation efficiency per unit time. After a contraction timescale which is equal to a few times their free-fall times, the cores collapse into stars and populate the IMF. Feedback from the newly formed OB stars is taken into account in the form of stellar winds. When the ratio of the effective energy of the winds to the gravitational energy of the system reaches unity, gas is removed from the clump and core and star formation are quenched. The power of the radiation driven winds has a strong dependence on metallicity and it increases with increasing metallicity. Thus, winds from stars in the high metallicity models lead to a rapid evacuation of the gas from the protocluster clump and to a reduced star formation efficiency, as compared to their low metallicity counterparts. We derive the metallicity dependent star formation efficiency per unit time in this model as a function of the gas surface density Sigma_g. This is combined with the molecular gas fraction in order to derive the dependence of the surface density of star formation Sigma_SFR on Sigma_g. This feedback regulated model of star formation reproduces very well the observed star formation laws in galaxies extending from low gas surface densities up to the starburst regime. Furthermore, the results show a dependence of Sigma_SFR on metallicity over the entire range of gas surface densities, and can also explain part of the scatter in the observations., Comment: In the proceedings of the French Astronomical Society meeting SF2A 2011. 8 pages, 5 figures

This paper summarises results from semi-analytical modelling of star formation in protocluster clumps of different metallicities. In this model, gravitationally bound cores form uniformly in the clump following a prescribed core formation efficiency per unit time. After a contraction timescale which is equal to a few times their free-fall times, the cores collapse into stars and populate the IMF. Feedback from the newly formed OB stars is taken into account in the form of stellar winds. When the ratio of the effective wind energy of the winds to the gravitational energy of the system reaches unity, gas is removed from the clump and core and star formation are quenched. The power of the radiation driven winds has a strong dependence on metallicity and increases with increasing metallicity. Thus, winds from stars in the high metallicity models lead to a rapid evacuation of the gas from the protocluster clump and to a reduced star formation efficiency, SFE_{exp}, as compared to their low metallicity counterparts. By combining SFE_{exp} with the timescales on which gas expulsion occurs, we derive the metallicity dependent star formation rate per unit time in this model as a function of the gas surface density Sigma_{g}. This is combined with the molecular gas fraction in order to derive the dependence of the surface density of star formation Sigma_{SFR} on Sigma_{g}. This feedback regulated model of star formation reproduces very well the observed star formation laws extending from low gas surface densities up to the starburst regime. Furthermore, the results show a dependence of Sigma_{SFR} on metallicity over the entire range of gas surface densities, and can also explain part of the scatter in the observations., Comment: 21 pages, 13 figures, proceedings of "Stellar Clusters and Associations- A RIA workshop on GAIA", 23-27 May 2011, Granada, Spain

We derive a metallicity dependent relation between the surface density of the star formation rate (Sigma_{SFR}) and the gas surface density (Sigma_{g}) in a feedback regulated model of star formation in galactic disks. In this model, star formation occurs in gravitationally bound protocluster clumps embedded in larger giant molecular clouds with the protocluster clump mass function following a power law function with a slope of -2. Metallicity dependent feedback is generated by the winds of OB stars (M > 5 Msol) that form in the clumps. The quenching of star formation in clumps of decreasing metallicity occurs at later epochs due to weaker wind luminosities, thus resulting in higher final star formation efficiencies (SFE_{exp}). By combining SFE_{exp} with the timescales on which gas expulsion occurs, we derive the metallicity dependent star formation rate per unit time in this model as a function of Sigma_{g}. This is combined with the molecular gas fraction in order to derive the global dependence of Sigma_{SFR} on Sigma_{g}. The model reproduces very well the observed star formation laws extending from low gas surface densities up to the starburst regime. Furthermore, our results show a dependence of $\Sigma_{SFR}$ on metallicity over the entire range of gas surface densities in contrast to other models, and can also explain part of the scatter in the observations. We provide a tabulated form of the star formation laws that can be easily incorporated into numerical simulations or semi-analytical models of galaxy formation and evolution., Comment: accepted to ApJL. Slopes of the KS relations in different gas surface density regimes are now quoted in the text. Figures and conclusions unchanged

We explore how the star formation efficiency in a protocluster clump is regulated by metallicity dependent stellar winds from the newly formed massive OB stars (Mstar >5 Msol). The model describes the co-evolution of the mass function of gravitationally bound cores and of the IMF in a protocluster clump. Dense cores are generated uniformly in time at different locations in the clump, and contract over lifetimes that are a few times their free fall times. The cores collapse to form stars that power strong stellar winds whose cumulative kinetic energy evacuates the gas from the clump and quenches further core and star formation. This sets the final star formation efficiency, SFEf. Models are run with various metallicities in the range Z/Zsol=[0.1,2]. We find that the SFEf decreases strongly with increasing metallicity.The SFEf-metallicity relation is well described by a decaying exponential whose exact parameters depend weakly on the value of the core formation efficiency. We find that there is almost no dependence of the SFEf-metallicity relation on the clump mass. This is due to the fact that an increase (decrease) in the clump mass leads to an increase (decrease) in the feedback from OB stars which is opposed by an increase (decrease) in the gravitational potential of the clump. The clump mass-cluster mass relations we find for all of the different metallicity cases imply a negligible difference between the exponent of the mass function of the protocluster clumps and that of the young clusters mass function. By normalizing the SFEs to their value for the solar metallicity case, we compare our results to SFE-metallicity relations derived on galactic scales and find a good agreement. As a by-product of this study, we also provide ready-to-use prescriptions for the power of stellar winds of main sequence OB stars in the mass range [5,80] Msol in the metallicity range we have considered, Comment: accepted to MNRAS. More discussion added. Figures and conclusions unchanged

Aims: We aim to develop a method to determine distances to molecular clouds using JHK near-infrared photometry. Methods: The method is based on a technique that aids spectral classification of stars lying towards the fields containing the clouds into main sequence and giants. In this technique, the observed (J-H) and (H-K_s) colours are dereddened simultaneously using trial values of A V and a normal interstellar extinction law. The best fit of the dereddened colours to the intrinsic colours giving a minimum value of Chi^2 then yields the corresponding spectral type and A_V for the star. The main sequence stars, thus classified, are then utilized in an A V versus distance plot to bracket the cloud distances. Results: We applied the method to four clouds, L1517, Chamaeleon I, Lupus 3 and NGC 7023 and estimated their distances as 167+-30, 151+-28, 157+-29 and 408+-76 pc respectively, which are in good agreement with the previous distance estimations available in the literature, Comment: Published in A&A, 2010, 509, 44. Accepted 23 september 2009

We study the rotational properties of magnetized and self-gravitating molecular cloud cores formed in 2 very high resolution 3D molecular cloud simulations.The simulations have been performed using the code RAMSES at an effective resolution of 4096^3.One simulation represents a mildly magnetically-supercritical cloud and the other a strongly magnetically-supercritical cloud.A noticeable difference between the 2 simulations is the core formation efficiency (CFE) of the high density cores.In the strongly supercritical simulations the CFE is ~33 % per free-fall time of the cloud tff,cl, whereas in the mildly supercritical simulations this value goes down to ~6%/tff,cl. A comparison of the intrinsic specific angular momentum j3D distributions of the cores with the distribitions of j2D derived using synthetic 2D velocity maps of the cores,shows that the synthetic observations tend to overestimate the true value of j by a factor of ~10.The origin of this discrepancy lies in the fact that contrary to the intrinsic determination which sums up the individual gas parcels contributions to j, the determination of j using the observational procedure which is based on a measurement on the global velocity gradient under the hypothesis of uniform rotation smoothes out the complex fluctuations present in the 3D velocity field. Our results provide a natural explanation for the discrepancy by a factor ~10 observed between the intrinsic 3D distributions of j and the corresponding distributions derived in real observations.We suggest that measurements of j which are based on the measurement of the observed global velocity gradients may need to be reduced by a factor of ~10 in order to derive a more accurate estimate of j in the cores., Comment: accepted to ApJ

(abridged) We develop a model which describes the coevolution of the mass function of dense cores and of the IMF in a protocluster clump. In the model, cores injected in the clump evolve under the effect of gas accretion. Accretion onto the cores follows a time-dependent accretion rate that describes accretion in a turbulent medium. Once the accretion timescales of cores exceed their contraction timescales, they are turned into stars. We include the effect of feedback by the newly formed massive stars through their stellar winds. A fraction of the wind's energy is assumed to counter gravity and disperse the gas from the protocluster and as a consequence, quench further star formation. The latter effect sets the final IMF of the cluster. We apply our model to a clump that is expected to resemble the progenitor clump of the Orion Nebula Cluster (ONC). Our model is able to reproduce both the shape and normalization of the ONC's IMF and the mass function of dense cores in Orion. The complex features of the ONC's IMF,i.e., a shallow slope in the mass range ~0.3-2.5 Msol,a steeper slope in the mass range ~2.5-12 Msol, and a nearly flat tail at the high mass end are reproduced. The model predicts a 'rapid' star formation process with an age spread for the stars of 2.3 10^5 yr which is consistent with the fact that 80% of the ONC's stars have ages of <=0.3 Myr. The model predicts a primordial mass segregation with the most massive stars being born in the region between 2-4 times the core radius of the cluster. In parallel, the model also reproduces, simultaneously, the mass function of dense cores in Orion. We study the effects of varying the model parameters on the resulting IMF and show that the IMF of stellar clusters is expected to show significant variations, provided variations in the clumps and cores properties exist., Comment: accepted to MNRAS. 22 pages, 17 figures. 2 new sections added to better present the model parameters and compare to previous work. Some figures are now better presented. Main conclusions unchanged

Supernova explosions inject a considerable amount of energy into the interstellar medium (ISM) in regions with high to moderate star formation rates. In order to assess whether the driving of turbulence by supernovae is also important in the outer Galactic disk, where the star formation rates are lower, we study the spatial distribution of molecular cloud (MC) inclinations with respect to the Galactic plane. The latter contains important information on the nature of the mechanism of energy injection into the ISM. We analyze the spatial correlations between the position angles (PAs) of a selected sample of MCs (the largest clouds in the catalogue of the outer Galaxy published by Heyer et al. 2001). Our results show that when the PAs of the clouds are all mapped to values into the [0,90]degrees interval, there is a significant degree of spatial correlation between the $PA$s on spatial scales in the range of 100-800 pc. These scales are of the order of the sizes of individual SN shells in low density environments such as those prevailing in the outer Galaxy and where the metallicity of the ambient gas is of the order of the solar value or smaller. These findings suggest that individual SN explosions, occurring in the outer regions of the Galaxy and in likewise spiral galaxies, albeit at lower rates, continue to play an important role in shaping the structure and dynamics of the ISM in those regions. The SN explosions we postulate here are likely associated with the existence of young stellar clusters in the far outer regions of the Galaxy and the UV emission and low levels of star formation observed with the GALEX satellite in the outer regions of local galaxies., Comment: Accepted to MNRAS main journal. Additional discussion and and one figure with error estimates is added. 7 pages, 7 figures. Main conclusions unchanged

In this paper, we review some of the properties of dense molecular cloud cores. The results presented here rely on three-dimensional numerical simulations of isothermal, magnetized, turbulent, and self-gravitating molecular clouds (MCs) in which dense core form as a consequence of the gravo-turbulent fragmentation of the clouds. In particular we discuss issues related to the mass spectrum of the cores, their lifetimes and their virial balance., Comment: Paper of invited talk at the 2008 french astronomical society meeting (SF2A-2008). 4 pages; 4 figures. A few typos corrected in the text and references

We study thermal instability in a magnetized and partially ionized plasma with charged dust particles. Our linear analysis shows that the growth rate of the unstable modes in the presence of dust particles strongly depends on the ratio of the cooling rate and the modified dust-cyclotron frequency. If the cooling rate is less than the modified dust-cyclotron frequency, then growth rate of the condensation modes does not modify due to the existence of the charged dust particles. But when the cooling rate is greater than (or comparable to) the modified dust-cyclotron frequency, the growth rate of unstable modes increases because of the dust particles. Also, wavenumber of the perturbations corresponding to the maximum growth rate shifts to the smaller values (larger wavelengths) as the cooling rate becomes larger than the modified dust-cyclotron frequency. We show that growth rate of the condensation modes increases with the electrical charge of the dust particles., Comment: accepted by MNRAS