arXiv subjects

Nestor Sanchez

Publications and source records attributed to Nestor Sanchez.

32 records · Page 2Linked to original sources

arXiv2010

The fractal dimension of star-forming regions at different spatial scales in M33

We study the distribution of stars, HII regions, molecular gas, and individual giant molecular clouds in M33 over a wide range of spatial scales. The clustering strength of these components is systematically estimated through the fractal dimension. We find scale-free behavior at small spatial scales and a transition to a larger correlation dimension (consistent with a nearly uniform distribution) at larger scales. The transition region lies in the range 500-1000 pc. This transition defines a characteristic size that separates the regime of small-scale turbulent motion from that of large-scale galactic dynamics. At small spatial scales, bright young stars and molecular gas are distributed with nearly the same three-dimensional fractal dimension (Df <= 1.9), whereas fainter stars and HII regions exhibit higher values (Df = 2.2-2.5). Our results indicate that the interstellar medium in M33 is on average more fragmented and irregular than in the Milky Way.

astro-ph.CO↗

arXiv2009

Cluster radius and sampling radius in the determination of cluster membership probabilities

We analyze the dependence of the membership probabilities obtained from kinematical variables on the radius of the field of view around open clusters (the sampling radius, Rs). From simulated data, we show that the best discrimination between cluster members and non-members is obtained when the sampling radius is very close to the cluster radius. At higher Rs values more field stars tend to be erroneously assigned as cluster members. From real data of two open clusters (NGC 2323 and NGC 2311) we obtain that the number of identified cluster members always increases with increasing Rs. However, there is a threshold Rs value above which the identified cluster members are severely contaminated by field stars and the effectiveness of membership determination is relatively small. This optimal sampling radius is \sim 14 arcmin for NGC 2323 and \sim 13 arcmin for NGC 2311. We discuss the reasons for such behavior and the relationship between cluster radius and optimal sampling radius. We suggest that, independently of the method used to estimate membership probabilities, several tests using different sampling radius should be performed in order to evaluate the existence of possible biases.

astro-ph.GA↗

arXiv2009

The spatial distribution of stars in open clusters

In this work we study the internal spatial structure of 16 open clusters in the Milky Way spanning a wide range of ages. For this, we use the minimum spanning tree method (the Q parameter, which enables one to classify the star distribution as either radially or fractally clustered), King profile fitting, and the correlation dimension (Dc) for those clusters with fractal patterns. On average, clusters with fractal-like structure are younger than those exhibiting radial star density profiles. There is a significant correlation between Q and the cluster age measured in crossing time units. For fractal clusters there is a significant correlation between the fractal dimension and age. These results support the idea that stars in new-born clusters likely follow the fractal patterns of their parent molecular clouds, and eventually evolve toward more centrally concentrated structures. However, there can exist stellar clusters as old as 100 Myr that have not totally destroyed their fractal structure. Finally, we have found the intriguing result that the lowest fractal dimensions obtained for the open clusters seem to be considerably smaller than the average value measured in galactic molecular cloud complexes.

astro-ph.GA↗

arXiv2009

The spatial distribution of stars in open clusters

The analysis of the distribution of stars in open clusters may yield important information on the star formation process and early dynamical evolution of stellar clusters. Here we address this issue by systematically characterizing the internal spatial structure of 16 open clusters in the Milky Way spanning a wide range of ages. Cluster stars have been selected from a membership probability analysis based on a non-parametric method that uses both positions and proper motions and does not make any a priori assumption on the underlying distributions. The internal structure is then characterized by means of the minimum spanning tree method (Q parameter), King profile fitting, and the correlation dimension (Dc) for those clusters with fractal patterns. On average, clusters with fractal-like structure are younger than those exhibiting radial star density profiles and an apparent trend between Q and age is observed in agreement with previous ideas about the dynamical evolution of the internal spatial structure of stellar clusters. However, some new results are obtained from a more detailed analysis: (a) a clear correlation between Q and the concentration parameter of the King model for those cluster with radial density profiles, (b) the presence of spatial substructure in clusters as old as 100 Myr, and (c) a significant correlation between fractal dimension and age for those clusters with internal substructure. Moreover, the lowest fractal dimensions seem to be considerably smaller than the average value measured in galactic molecular cloud complexes.

astro-ph.SR↗

arXiv2008

On the Fractal Distribution of HII Regions in Disk Galaxies

In this work we quantify the degree to which star-forming events are clumped. We apply a precise and accurate technique to calculate the correlation dimension Dc of the distribution of HII regions in a sample of disk galaxies. Our reliable results are distributed in the range 1.5<Dc<2.0. We get significant variations in the fractal dimension among galaxies, contrary to a universal picture sometimes claimed in literature. The faintest galaxies tend to distribute their HII regions in more clustered (less uniform) patterns. Moreover, the fractal dimension for the brightest HII regions within the same galaxy seems to be smaller than for the faintest ones suggesting some kind of evolutionary effect.

astro-ph↗

arXiv2008

The Fractal Distribution of HII Regions in Disk Galaxies

It is known that the gas has a fractal structure in a wide range of spatial scales with a fractal dimension that seems to be a constant around Df = 2.7. It is expected that stars forming from this fractal medium exhibit similar fractal patterns. Here we address this issue by quantifying the degree to which star-forming events are clumped. We develop, test, and apply a precise and accurate technique to calculate the correlation dimension Dc of the distribution of HII regions in a sample of disk galaxies. We find that the determination of Dc is limited by the number of HII regions, since if there are < 100 regions available then a bias tending to underestimate the dimension is produced. The reliable results are distributed in the range 1.5 < Dc < 2.0 with an average value Dc = 1.81. This corresponds to a three-dimensional dimension of Df = 2.73, very similar to the value measured in the interstellar clouds. However, we get significant variations in the fractal dimension among galaxies, contrary to a universal picture sometimes claimed in literature. The fractal dimension exhibits a weak but significant correlation with the absolute magnitude and, to a lesser extent, with the galactic radius. The faintest galaxies tend to distribute their HII regions in more clustered (less uniform) patterns. The fractal dimension for the brightest HII regions within the same galaxy seems to be smaller than for the faintest ones suggesting some kind of evolutionary efffect, but the obtained correlation remains unchanged if only the brightest regions are taken into account.

astro-ph↗

arXiv2008

Determining the Fractal Dimension of the Interstellar Medium

The Interstellar Medium seems to have an underlying fractal structure, which can be characterized through its fractal dimension (Df). However, several factors may affect the determination of Df, such as distortions due to projection, low image resolution, opacity of the cloud, and low signal-to-noise ratio. Here we use both simulated clouds and real molecular cloud maps to study these effects in order to estimate Df in a reliable way. Our results indicate in a self-consistent way that the fractal dimension of the Interstellar Medium is in the range 2.6 < Df < 2.8, which is significantly higher than the value Df = 2.3 usually assumed in the literature.

astro-ph↗

arXiv2008

Stability of thermal structures with an internal heating source

We study the thermal equilibrium and stability of isobaric, spherical structures having a radiation source located at its center. The thermal conduction coefficient, external heating and cooling rates are represented as power laws of the temperature. The internal heating decreases with distance from the source r approximately as exp(-tau)/(r**2), being tau the optical depth. We find that the influence of the radiation source is important only in the central region, but its effect is enough to make the system thermally unstable above a certain threshold central temperature. This threshold temperature decreases as the internal heating efficiency increases, but, otherwise, it does not depend on the structure size. Our results suggest that a solar-like star migrating into a diffuse interstellar region may destabilize the surrounding medium.

astro-ph↗

arXiv2006

Fractal dimension of interstellar clouds: opacity and noise effects

There exists observational evidence that the interstellar medium has a fractal structure in a wide range of spatial scales. The measurement of the fractal dimension (Df) of interstellar clouds is a simple way to characterize this fractal structure, but several factors, both intrinsic to the clouds and to the observations, may contribute to affect the values obtained. In this work we study the effects that opacity and noise have on the determination of Df. We focus on two different fractal dimension estimators: the perimeter-area based dimension (Dper) and the mass-size dimension (Dm). We first use simulated fractal clouds to show that opacity does not affect the estimation of Dper. However, Dm tends to increase as opacity increases and this estimator fails when applied to optically thick regions. In addition, very noisy maps can seriously affect the estimation of both Dper and Dm, decreasing the final estimation of Df. We apply these methods to emission maps of Ophiuchus, Perseus and Orion molecular clouds in different molecular lines and we obtain that the fractal dimension is always in the range 2.6 < Df < 2.8 for these regions. These results support the idea of a relatively high (> 2.3) average fractal dimension for the interstellar medium, as traced by different chemical species.

astro-ph↗

arXiv2006

Development of accurate solutions for a classical oscillator

We present a method to obtain arbitrarily accurate solutions for conservative classical oscillators. The method that we propose here works both for small and large nonlinearities and provides simple analytical approximations. A comparison with the standard Lindstedt-Poincaré method is presented, from which the advantages of our method are clear.

math-ph↗

arXiv2005

On the properties of fractal cloud complexes

We study the physical properties derived from interstellar cloud complexes having a fractal structure. We first generate fractal clouds with a given fractal dimension and associate each clump with a maximum in the resulting density field. Then, we discuss the effect that different criteria for clump selection has on the derived global properties. We calculate the masses, sizes and average densities of the clumps as a function of the fractal dimension (D_f) and the fraction of the total mass in the form of clumps (epsilon). In general, clump mass does not fulfill a simple power law with size of the type M_cl ~ (R_cl)**(gamma), instead the power changes, from gamma ~ 3 at small sizes to gamma<3 at larger sizes. The number of clumps per logarithmic mass interval can be fitted to a power law N_cl ~ (M_cl)**(-alpha_M) in the range of relatively large masses, and the corresponding size distribution is N_cl ~ (R_cl)**(-alpha_R) at large sizes. When all the mass is forming clumps (epsilon=1) we obtain that as D_f increases from 2 to 3 alpha_M increases from ~0.3 to ~0.6 and alpha_R increases from ~1.0 to ~2.1. Comparison with observations suggests that D_f ~ 2.6 is roughly consistent with the average properties of the ISM. On the other hand, as the fraction of mass in clumps decreases (epsilon<1) alpha_M increases and alpha_R decreases. When only ~10% of the complex mass is in the form of dense clumps we obtain alpha_M ~ 1.2 for D_f=2.6 (not very different from the Salpeter value 1.35), suggesting this a likely link between the stellar initial mass function and the internal structure of molecular cloud complexes.

astro-ph↗

arXiv2005

The Fractal Dimension of Projected Clouds

The interstellar medium seems to have an underlying fractal structure which can be characterized through its fractal dimension. However, interstellar clouds are observed as projected two-dimensional images, and the projection of a tri-dimensional fractal distorts its measured properties. Here we use simulated fractal clouds to study the relationship between the tri-dimensional fractal dimension (D_f) of modeled clouds and the dimension resulting from their projected images. We analyze different fractal dimension estimators: the correlation and mass dimensions of the clouds, and the perimeter-based dimension of their boundaries (D_per). We find the functional forms relating D_f with the projected fractal dimensions, as well as the dependence on the image resolution, which allow to estimatethe "real" D_f value of a cloud from its projection. The application of these results to Orion A indicates in a self-consistent way that 2.5 < D_f < 2.7 for this molecular cloud, a value higher than the result D_per+1 = 2.3 some times assumed in literature for interstellar clouds.

astro-ph↗

arXiv2000

Neutrino decay and the thermochemical equilibrium of the interstellar medium

We calculate the thermochemical equilibrium of the diffuse interstellar medium, including ionization by a photon flux F_{nu} from neutrino decay. The main heating mechanism considered is photoelectrons from grains and PAHs. For the studied range of F_{nu} values, there always exists two regions of stability (a warm and a cold phase) that can coexist in equilibrium if the thermal interstellar pressure is between a maximum value P_{max} and a minimum value P_{min}. High F_{nu} values (~10^4-10^5 cm^{-2} s^{-1}) can be consistent with observed interstellar pressures only if more efficient sources are heating the gas. It is shown that a neutrino flux increase (due, for example, to an increase in the supernova explosion rate) may stimulate the condensation of cold gas by decreasing P_{max} below the interstellar pressure value.

astro-ph↗

arXiv1998

Hierarchical Model for the Evolution of Cloud Complexes

The structure of cloud complexes appears to be well described by a "tree structure" representation when the image is partitioned into "clouds". In this representation, the parent-child relationships are assigned according to containment. Based on this picture, a hierarchical model for the evolution of Cloud Complexes, including star formation, is constructed, that follows the mass evolution of each sub-structure by computing its mass exchange (evaporation or condensation) with its parent and children, which depends on the radiation density at the interphase. For the set of parameters used as a reference model, the system produces IMFs with a maximum at too high mass (~2 M_sun) and the characteristic times for evolution seem too long. We show that these properties can be improved by adjusting model parameters. However, the emphasis here is to illustrate some general properties of this nonlinear model for the star formation process. Notwithstanding the simplifications involved, the model reveals an essential feature that will likely remain if additional physical processes are included. That is: the detailed behavior of the system is very sensitive to variations on the initial and external conditions, suggesting that a "universal" IMF is very unlikely. When an ensemble of IMFs corresponding to a variety of initial or external conditions is examined, the slope of the IMF at high masses shows variations comparable to the range derived from observational data. (Abridged)

astro-ph↗