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Joao Alves

Publications and source records attributed to Joao Alves.

At least 37 records · Page 2Linked to original sources
arXiv2011

2MASS wide field extinction maps: IV. The Orion, Mon R2, Rosette, and Canis Major star forming regions

We present a near-infrared extinction map of a large region (approximately 2200 deg^2) covering the Orion, the Monoceros R2, the Rosette, and the Canis Major molecular clouds. We used robust and optimal methods to map the dust column density in the near-infrared (NICER and NICEST) towards ~19 million stars of the Two Micron All Sky Survey (2MASS) point source catalog. Over the relevant regions of the field, we reached a 1-sigma error of 0.03 mag in the K-band extinction with a resolution of 3 arcmin. We measured the cloud distances by comparing the observed density of foreground stars with the prediction of galactic models, thus obtaining d_{Orion A} = (371 +/- 10) pc, d_{Orion B} = (398 +/- 12) pc, $d_{Mon R2} = (905 +/- 37) pc, $d_{Rosette} = (1330 +/- 48) pc, and $d_{CMa} = (1150 +/- 64) pc, values that compare very well with independent estimates.

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arXiv2010

Larson's third law and the universality of molecular cloud structure

Larson (1981) first noted a scaling relation between masses and sizes in molecular clouds that implies that these objects have approximately constant column densities. This original claim, based upon millimeter observations of carbon monoxide lines, has been challenged by many theorists, arguing that the apparent constant column density observed is merely the result of the limited dynamic range of observations, and that in reality clouds have column density variations over two orders of magnitudes. In this letter we investigate a set of nearby molecular clouds with near-infrared excess methods, which guarantee very large dynamic ranges and robust column density measurements, to test the validity of Larson's third law. We verify that different clouds have almost identical average column densities above a given extinction threshold; this holds regardless of the extinction threshold, but the actual average surface mass density is a function of the specific threshold used. We show that a second version of Larson's third law, involving the mass-radius relation for single clouds and cores, does not hold in our sample, indicating that individual clouds are not objects that can be described by constant column density. Our results instead indicate that molecular clouds are characterized by a universal structure. Finally we point out that this universal structure can be linked to the log-normal nature of cloud column density distributions.

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arXiv2008

2MASS wide field extinction maps: II. The Ophiuchus and the Lupus cloud complexe

We present an extinction map of a ~1,700 deg sq region that encloses the Ophiuchus, the Lupus, and the Pipe dark complexes using 42 million stars from the Two Micron All Sky Survey (2MASS) point source catalog. The use of a robust and optimal near-infrared method to map dust column density (Nicer, described in Lombardi & Alves 2001) allow us to detect extinction as low as A_K = 0.05 mag with a 2-sigma significance, and still to have a resolution of 3 arcmin on our map. We also present a novel, statistically sound method to characterize the small-scale inhomogeneities in molecular clouds. Finally, we investigate the cloud structure function, and show that significant deviations from the results predicted by turbulent models are observed.

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arXiv2008

The Inevitable Future of the Starless Core Barnard 68

Dense, small molecular cloud cores have been identified as the direct progenitors of stars. One of the best studied examples is Barnard 68 which is considered a prototype stable, spherical gas core, confined by a diffuse high-pressure environment. Observations of its radial density structure however indicate that Barnard 68 should be gravitationally unstable and collapsing which appears to be inconsistent with its inferred long lifetime and stability. We argue that Barnard 68 is currently experiencing a fatal collision with another small core which will lead to gravitational collapse. Despite the fact that this system is still in an early phase of interaction, our numerical simulations imply that the future gravitational collapse is already detectable in the outer surface density structure of the globule which mimicks the profile of a gravitationally unstable Bonnor-Ebert sphere. Within the next 200,000 years Barnard 68 will condense into a low-mass solar-type star(s), formed in isolation, and surrounded by diffuse, hot interstellar gas. As witnessed in situ for Barnard 68, core mergers might in general play an important role in triggering star formation and shaping the molecular core mass distribution and by that also the stellar initial mass function.

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arXiv2008

Near-infrared reddening of extra-galactic GMCs in a face-on geometry

[Abridged] We describe the near-infrared reddening signature of giant molecular clouds (GMCs) in external galaxies. In particular, we examine the E(J-H) and E(H-K) color-excesses, and the effective extinction law observed in discrete GMC regions. We also study the effect of the relative scale height of the GMC distribution to the color-excesses, and to the observed mass function of GMCs. We perform Monte Carlo radiative transfer simulations with 3D models of stellar radiation and clumpy dust distributions, resembling a face-on geometry. The scattered light is included in the models, and near-infrared color maps are calculated from the simulated data. The effective near-infrared reddening law, i.e. the ratio E(J-H)/E(H-K), has a value close to unity in GMC regions. The ratio depends on the relative scale height of GMCs, xi, and for xi values 0.1...0.75 we find the typical ratios of 0.6...1.1. The effective extinction law turns out to be very flat in GMC regions. We find the ratios of apparent extinctions of A(H)/A(K)=1.35...1.55 and A(J)/A(H)=1.15. The effect of the scattered flux on the effective reddening law, as well as on the effective extinction law, is significant. Regarding the GMC mass function, we find no correlation between the input and observed slopes of the mass functions. Rather, the observed slope reflects the parameter $ξ$ and the dynamical range of the mass function. We estimate that only a fraction of 10...20 % of the total mass of GMCs is recovered, if the observed color-excess values are transformed to masses using the Galactic reddening law. In the case of individual clouds the fraction can vary between ~0...50 %.

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arXiv2007

Determination of the mass function of extra-galactic GMCs via NIR color maps. Testing the method in a disk-like geometry

[Abridged] The giant molecular clouds (GMCs) of external galaxies can be mapped with sub-arcsecond resolution using multiband observations in the near-infrared. However, the interpretation of the observed features, and their transformation into physical quantities, is greatly hampered by the effects arising from the unknown geometry and the scattering of light by dust particles. We examine the relation between the observed NIR reddening and the column density of the dust clouds. In this paper we particularly assess the feasibility of deriving the mass function of GMCs from NIR color-excess data. We perform Monte Carlo radiative transfer simulations with 3D models of stellar radiation and clumpy dust distributions. We include the scattered light in the models and calculate NIR color maps from the simulated data. We extract clumps from the color maps and compare the observed mass function to the true mass function. For the physical configuration chosen in this study, essentially a face-on geometry, the observed mass function is a non-trivial function of the true mass function. The dynamical range of the observed mass function is confined to ~10^3.5... 10^5.5 M_\odot regardless of the dynamical range of the true mass function. The color maps are more sensitive in detecting the high-mass end of the mass function, and on average the masses of clouds are underestimated by a factor of ~10. The simulations show that the cloud mass function derived from JHK color excess data using simple foreground screening geometry cannot be regarded as a one-to-one tracer of the underlying mass function.

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arXiv2006

2MASS wide field extinction maps - I. The Pipe nebula

We present a 8 deg x 6 deg, high resolution extinction map of the Pipe nebula using 4.5 million stars from the Two Micron All Sky Survey (2MASS) point source catalog. The use of NICER, a robust and optimal technique to map the dust column density, allows us to detect a Av = 0.5 mag extinction at a 3-sigma level with a 1 arcmin resolution. We find for the Pipe nebula a normal reddening law, E(J-H) = (1.85 +/- 0.15) E(H-K). We measure the cloud distance using Hipparchos and Tycho parallaxes, and obtain ~130 pc. This, together with the total estimated mass, 10^4 Msun, makes the Pipe the closest massive cloud complex to Earth. We compare the NICER extinction map to the NANTEN 12CO observations and derive with unprecedented accuracy the relationship between the near-infrared extinction and the 12CO column density and hence (indirectly) the 12CO X-factor, that we estimate to be 2.91 10^20 cm^-2 K^-1 km^-1 s in the range Av <- [0.9, 5.4] mag. We identify ~1500 OH/IR stars located within the Galactic bulge in the direction of the Pipe field. This represents a significant increase of the known numbers of such stars in the Galaxy. Our analysis confirms the power and simplicity of the color excess technique to study molecular clouds. The comparison with the NANTEN 12CO data corroborates the insensitivity of CO observations to low column densities (up to approximately 2 mag in Av), and shows also an irreducible uncertainty in the dust-CO correlation of about 1 mag of visual extinction.

astro-ph↗
arXiv2006

X-ray and IR Point Source Identification and Characteristics In the Embedded, Massive Star-Forming Region RCW 38

We report on results of a 96.7 ks Chandra observation of one of the youngest, most embedded and massive young stellar clusters studied in X-rays - RCW 38. We detect 460 sources in the field of which 360 are confirmed to be associated with the RCW~38 cluster. The cluster members range in luminosity from ~10^30 ergs s^-1 to 10^33.5 ergs s^-1. Over ten percent of the cluster members with over 100 counts exhibit flares while about 15% of cluster members with over 30 counts are variable. Of the sources identified as cluster members, 160 have near-infrared (NIR) counterparts either in the 2MASS database or detected via VLT observations. Of these about 20% appear to have optically thick disks. An additional 353 members are identified through NIR observations of which at least 50% possess optically thick disks. We fit over 100 X-ray sources as absorbed Raymond-Smith type plasmas and find the column to the cluster members varies from 10^21.5 to 10^23 per sq cm. We compare the gas to dust absorption signatures in these stars and find Nh= A_v x 2 x 10^21 cm^-2. We find that the cluster contains 31 candidate OB stars and is centered about 10" (0.1 pc) west of the primary source of the ionization, the O5 star IRS 2. The cluster has a peak central density of about 400 X-ray sources pc^-2. We estimate that the total cluster membership exceeds 2000 stars.

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arXiv2005

Constraints on Inner Disk Evolution Timescales: A Disk Census of the eta Chamaeleontis Young Cluster

We present new L' (3.8-micron) observations of stars in the nearby (~97 pc) young (~6 Myr) compact cluster around eta Chamaeleontis, obtained with the European Southern Observatory's Very Large Telescope in Paranal, Chile. Our data, combined with J,H, Ks photometry from the 2-Micron All Sky Survey, reveal that only two of the 12 members surveyed harbor L'-band excesses consistent with optically thick inner disks; both are also likely accretors. Intriguingly, two other stars with possible evidence for on-going accretion, albeit at very low rates, do not show significant infrared excess: this may imply substantial grain growth and/or partial clearing of the inner disk region, as expected in planet formation scenarios. Our findings suggest that eta Cha stars are in an epoch when disks are rapidly evolving, perhaps due to processes related to planet building, and provide further constraints on inner disk lifetimes.

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arXiv2003

Envelope Structure of Starless Core L694-2 Derived from a Near-Infrared Extinction Map

We present a near-infrared extinction study of the dark globule L694-2, a starless core that shows strong evidence for inward motions in molecular line profiles. The J,H, and K band data were taken using the European Southern Observatory New Technology Telescope. The best fit simple spherical power law model has index p=2.6 +/- 0.2, over the 0.036--0.1 pc range in radius sampled in extinction. This power law slope is steeper than the value of p=2 for a singular isothermal sphere, the initial condition of the inside-out model for protostellar collapse. Including an additional extinction component along the line of sight further steepens the inferred profile. Fitting a Bonnor-Ebert sphere results in a super-critical value of the dimensionless radius xi_max=25 +/- 3. The unstable configuration of material may be related to the observed inward motions. The Bonnor-Ebert model matches the shape of the observed profile, but significantly underestimates the amount of extinction (by a factor of ~4). This discrepancy in normalization has also been found for the nearby protostellar core B335 (Harvey et al. 2001). A cylindrical density model with scale height H=0.0164+/- 0.002 pc viewed at a small inclination to the cylinder axis provides an equally good radial profile as a power law model, and reproduces the asymmetry of the core remarkably well. In addition, this model provides a basis for understanding the discrepancy in the normalization of the Bonnor-Ebert model, namely that L694-2 has prolate structure, with the full extent (mass) of the core being missed by assuming symmetry between the profiles in the plane of the sky and along the line-of-sight. If the core is sufficiently magnetized then fragmentation may be avoided, and later evolution might produce a protostar similar to B335.

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arXiv2003

Uncovering the Beast: Discovery of Embedded Massive Stellar Clusters in W49A

We present subarcsecond J, H, and Ks images (FWHM ~ 0.5") of an unbiased 5'x 5' (16pc x 16pc) survey of the densest region of the W49 giant molecular cloud. The observations reveal 4 massive stellar clusters (with stars as massive as \~120 Msun), the larger (Cluster 1) about 3 pc East of the well known Welch ring of ultra-compact Hii regions. Cluster 1 is a) extincted by at least Av > 20 mag of foreground (unrelated and local) extinction, b) has more than 30 mag of internal inhomogeneous visual extinction implying that it is still deeply buried in its parental molecular cloud, and c) is powering a 6 pc diameter giant Hii region seen both at the NIR and radio continuum. We also identify the exciting sources of several UCHii regions. The census of massive stars in W49A agrees or is slightly overabundant when compared with the number of Lyman continuum photons derived from radio observations. We argue that although the formation of the Welch ring could have been triggered by Cluster 1, the entire W49A starburst region seems to have been multi-seeded instead of resulting from a coherent trigger.

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arXiv2002

HST, VLT, and NTT imaging search for wide companions to bona-fide and candidate brown dwarfs in the Cha I dark cloud

We present results from a deep imaging search for companions around the young bona-fide and candidate brown dwarfs Cha Ha 1 to 12 in the Cha I dark cloud, performed with HST WFPC2 (R, I, Ha), VLT FORS1 (VRI), and NTT SofI (JHK). We find 16 faint companion candidates around five primaries with separations between 1.5" and 7" and magnitudes in R & I from 19 to 25 mag, i.e. up to 8 mag fainter than the primaries. While most of these companion candidates are probably unrelated background objects, there is one promising candidate, namely 1.5" SW off the M6-dwarf Cha Ha 5. This candidate is 3.8 to 4.7 mag fainter than the primary and its colors are consistent with an early- to mid-L spectral type. Assuming the same distance (140 pc) and absorption (0.47 mag in I) as towards the primary, the companion candidate has log (L(bol)/L(odot) = -3.0 +- 0.3. At the age of the primary (1 to 5 Myrs), the faint object would have a mass of 3 to 15 Jupiter masses according to Burrows et al. (1997) and Chabrier & Baraffe (2000) models. The probability for this companion candidate to be an unrelated fore- or background object is smaller than 0.7%, its colors are marginally consistent with a strongly reddened background K giant. One other companion candidate has infrared colors consistent with an early T-dwarf. In addition, we present indications for Cha Ha 2 being a close (0.2") binary with both components very close to the sub-stellar limit. Our detection limits are such that we should have detected all companions above 1 Jup with separations above 2" (320 AU) and all above 5 Jup at 0.35" (50 AU).

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arXiv2002

Imaging and spectroscopy of DENIS-P J104814-395606

We obtained deep H- and K-band images of DENIS-P J104814-395606 using SofI and the speckle camera SHARP-I at the ESO-3.5m-NTT as well as QUIRC at the Mauna Kea 2.2m telescope between December 2000 and June 2001. The target was recently discovered as nearby M9-dwarf among DENIS sources (Delfosse et al. 2001). We detect parallactic motion on our images and determine the distance to be 4.6 pm 0.3 pc, more precise than previously known. From the available colors, the distance, and the spectral type, we conclude from theoretical models that the star has a mass of ~0.075 to 0.09 M_sun and an age of ~1 to 2 Gyrs. We also obtained H- and K-band spectra of this star with ISAAC at the VLT. A faint companion candidate is detected 6 arc sec NNW of the star, which is 6.4 pm 0.5 mag fainter in H. However, according to another image taken several month later, the companion candidate is not co-moving with the M9-dwarf. Instead, it is a non-moving background object. Limits for undetected companion candidates are such that we can exclude any stellar companions outside of ~0.25 arc sec (1 AU), any brown dwarf companions (above the deuterium burning mass limit) outside of ~2 arc sec (9 AU), and also any companion down to ~40 M_jup with \ge 0.15 arc sec (0.7 AU) separation, all calculated for an age of 2 Gyrs. Our observations show that direct imaging of sub-stellar companions near the deuterium burning mass limit in orbit around nearby ultra-cool dwarfs is possible, even with separations that are smaller than the semi-major axis of the outermost planet in our solar system, namely a few tens of AU.

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arXiv2002

N2H+ and C18O Depletion in a Cold Dark Cloud

We present sensitive, high angular resolution molecular-line observations of C18O and N2H+ toward the dark globule B68. We directly compare these data with the near-infrared extinction measurements of Alves, Lada, & Lada (2001) to derive the first evidence for the depletion of N2H+, and by inference N2, in a pre-stellar dark cloud. We also find widespread C18O depletion throughout the centrally condensed core of the B68 cloud. Specifically, we find the N2H+ emission to peak in a shell partially surrounding the peak of dust extinction. Moreover, the N2H+ peaks inside the much larger C18O depletion hole and has a smaller depletion zone, confirming theoretical predictions. These data are analyzed through a direct coupling of time dependent chemical models to a radiation transfer code. This analysis highlights the importance of photodissociation at cloud edges and suggests that the CO abundance declines by two orders of magnitude from edge to center. In contrast N2H+ declines in abundance, at minimum, by at least a factor of two. Indeed it is entirely possible that both N2H+ and N2 are completely absent from the central regions of the B68 core. The depletion of N2H+, and its parent molecule N2, opens the possibility that the centers of dense cores, prior to the formation of a star, may evade detection by conventional methods of probing cores using molecular emission. Under these conditions H2D+ may be the sole viable molecular probe of the innermost regions of star forming cores.

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arXiv2001

Discovery of new embedded Herbig-Haro objects in the rho Ophiuchi dark cloud

We report here the discovery of a 30"-chain of embedded Herbig-Haro (HH) objects in the rho Ophiuchi dark cloud. These HH objects were first detected during a deep K_S-band observation (completeness magnitude for point source~19) made with NTT/SOFI. We confirm their nature with follow-up observations made with H_2 v=1-0 S(1) narrow-band filter. We argue that they belong to two different jets emanating from two Class I protostars: the main component of the recently resolved subarcsecond radio binary YLW15 (also called IRS43), and IRS54. We propose also to identify the [S II] knot HH224NW1 (Gomez et al 1998) as emanating from a counterjet of YLW15. The alignment between these HH objects and the thermal jet candidate found in YLW15 by Girart et al. (2000) implies that this jet is not precessing at least on timescale ~(2-4)x1E4 yr.

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arXiv2001

Mapping the interstellar dust with near-infrared observations: An optimized multi-band technique

We generalize the technique of Lada et al. (1994) to map dust column density through a molecular cloud (NICE) to an optimized multi-band technique (NICER) that can be applied to any multi-band survey of molecular clouds. We present a first application to a ~625 deg^2 subset of the Two Micron All Sky Survey (2MASS) data and show that when compared to NICE, the optimized NICER technique (i) achieves the same extinction peak values, (ii) improves the noise variance of the map by a factor of 2 and (iii) is able to reach 3 σdust extinction measurements as low as A_V = 0.5 magnitudes, better than or equivalent to classical optical star count techniques and below the threshold column density for the formation of CO, the brightest H2 tracer in radio-spectroscopy techniques. The application of the NICER techniques to near-infrared data obtained with a 8 meter-class telescope with a state-of-the-art NIR camera, such as the VLT-ISAAC combination, will be able to achieve dynamic ranges from below 10^21 protons cm^-2 to over 10^23 protons cm^-2 (A_V in the range [0.3, 60]) and spatial resolutions < 10'', making use of a single and straightforward column density tracer, extinction by interstellar dust.

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arXiv2001

Evidence for Circumstellar Disks around Young Brown Dwarfs in the Trapezium Cluster

We report the results of deep infrared observations of brown dwarf candidates in the Trapezium cluster in Orion. Analysis of the JHK color-color diagram indicates that a large fraction (approx 65% +/- 15%) of the observed sources exhibit infrared excess emission. This suggests the extreme youth of these objects and in turn, provides strong independent confirmation of the existence of a large population of substellar objects in the cluster. Moreover, this suggests that the majority of these substellar objects are presently surrounded by circumstellar disks similar to the situation for the stellar population of the cluster. This evidence for a high initial disk frequency (> 50%) around cluster members of all masses combined with the smooth continuity of the cluster's initial mass function across the hydrogen burning limit suggests that a single physical mechanism is likely responsible for producing the entire cluster mass spectrum down to near the deuterium burning limit. The results may also indicate that even substellar objects are capable of forming with systems of planetary companions.

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