arXiv subjects

Cesar Briceno

Publications and source records attributed to Cesar Briceno.

47 records · Page 3Linked to original sources

arXiv2006

The Low-mass Populations in OB Associations

Low-mass stars 0.1 ~< M ~< 1 Msun) in OB associations are key to addressing some of the most fundamental problems in star formation. The low-mass stellar populations of OB associations provide a snapshot of the fossil star-formation record of giant molecular cloud complexes. Large scale surveys have identified hundreds of members of nearby OB associations, and revealed that low-mass stars exist wherever high-mass stars have recently formed. The spatial distribution of low-mass members of OB associations demonstrate the existence of significant substructure ("subgroups"). This "discretized" sequence of stellar groups is consistent with an origin in short-lived parent molecular clouds within a Giant Molecular Cloud Complex. The low-mass population in each subgroup within an OB association exhibits little evidence for significant age spreads on time scales of ~10 Myr or greater, in agreement with a scenario of rapid star formation and cloud dissipation. The Initial Mass Function (IMF) of the stellar populations in OB associations in the mass range 0.1 ~< M ~< 1 Msun is largely consistent with the field IMF, and most low-mass pre-main sequence stars in the solar vicinity are in OB associations. These findings agree with early suggestions that the majority of stars in the Galaxy were born in OB associations. The most recent work further suggests that a significant fraction of the stellar population may have their origin in the more spread out regions of OB associations, instead of all being born in dense clusters. (Abridged version).

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arXiv2004

Disk evolution in the Ori OB1 association

We analyze multi-band photometry of a subsample of low mass stars in the associations Ori OB1a and 1b discovered during the CIDA Orion Variability Survey, which have ages of 7 - 10 Myr and 3 - 5 Myr, respectively. We obtained UBVRcIc photometry at Mt. Hopkins for 6 Classical T Tauri stars (CTTS) and 26 Weak T Tauri stars (WTTS) in Ori OB1a, and for 21 CTTS and 2 WTTS in Ori OB1b. We also obtained L band photometry for 14 CTTS at Mt. Hopkins, and 10um and 18um photometry with OSCIR at Gemini for 6 CTTS; of these, all 6 were detected at 10um while only one was detected at 18um. We estimate mass accretion rates from the excess luminosity at U, and find that they are consistent with determinations for a number of other associations, with or without high mass star formation. The observed decrease of mass accretion rate with age is qualitatively consistent with predictions of viscous evolution of accretion disks. We find an overall decrease of disk emission from Taurus to Ori OB1b to Ori OB1a. This decrease implies that significant grain growth and settling towards the midplane has taken place in the inner disks of Ori OB1. We compare the SED of the star detected at both 10um and 18um with disk models for similar stellar and accretion parameters. We find that the low <= 18 um fluxes of this Ori OB1b star cannot be due to the smaller disk radius expected from viscous evolution in the presence of the FUV radiation fields from the OB stars in the association. Instead, we find that the disk of this star is essentially a flat disk, with little if any flaring, indicating a a significant degree of dust settling towards the midplane, as expected from dust evolution in protoplanetary disks.

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arXiv2004

The CIDA Variability Survey of Orion OB1. I: the low-mass population of Ori OB 1a and 1b

We present results of a large scale, multi-epoch optical survey of the Ori OB1 association, carried out with the QuEST camera at the Venezuela National Astronomical Observatory. We identify for the first time the widely spread low-mass, young population in the Orion OB1a and OB1b sub-associations. Candidate members were picked up by their variability in the V-band and position in color-magnitude diagrams. We obtained spectra to confirm membership. In a region spanning ~ 68 deg^2 we found 197 new young stars; of these, 56 are located in the Ori OB1a subassociation and 142 in Ori OB1b. Comparison with the spatial extent of molecular gas and extinction maps indicates that the subassociation Ori 1b is concentrated within a ring-like structure of radius ~2 deg (~15 pc at 440 pc), centered roughly on the star epsilon Ori in the Orion belt. The ring is apparent in 13CO and corresponds to a region with an extinction Av>=1. The stars exhibiting strong Ha emission, an indicator of active accretion, are found along this ring, while the center is populated with weak Ha emitting stars. In contrast, Ori OB1a is located in a region devoid of gas and dust. We identify a grouping of stars within a ~3 deg^2 area located in 1a, roughly clustered around the B2 star 25 Ori. The Herbig Ae/Be star V346 Ori is also associated with this grouping, which could be an older analog of sigma Ori. Using using several sets of evolutionary tracks we find an age of 7 - 10 Myr for Ori 1a and of ~4 - 6 Myr for Ori OB1b, consistent with previous estimates from OB stars. Indicators such as the equivalent width of Ha and near-IR excesses show that while a substantial fraction of accreting disks remain at ages ~5 Myr, inner disks are essentially dissipated by 10 Myr.

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arXiv2004

Herbig Ae/Be Stars in nearby OB associations

We have carried out a study of the early type stars in nearby OB associations spanning an age range of $\sim$ 3 to 16 Myr, with the aim of determining the fraction of stars which belong to the Herbig Ae/Be class. We studied the B, A, and F stars in the nearby ($\le 500$ pc) OB associations Upper Scorpius, Perseus OB2, Lacerta OB1, and Orion OB1, with membership determined from Hipparcos data. We obtained spectra for 440 Hipparcos stars in these associations, from which we determined accurate spectral types, visual extinctions, effective temperatures, luminosities and masses, using Hipparcos photometry. Using colors corrected for reddening, we find that the Herbig Ae/Be stars and the Classical Be stars (CBe) occupy clearly different regions in the JHK diagram. Thus, we use the location on the JHK diagram, as well as the presence of emission lines and of strong 12 microns flux relative to the visual to identify the Herbig Ae/Be stars in the associations. We find that the Herbig Ae/Be stars constitute a small fraction of the early type stellar population even in the younger associations. Comparing the data from associations with different ages and assuming that the near-infrared excess in the Herbig Ae/Be stars arises from optically thick dusty inner disks, we determined the evolution of the inner disk frequency with age. We find that the inner disk frequency in the age range 3 - 10 Myr in intermediate mass stars is lower than that in the low mass stars (< 1msun). This indicates that the time-scales for disk evolution are much shorter in the intermediate mass stars, which could be a consequence of more efficient mechanisms of inner disk dispersal (viscous evolution, dust growth and settling toward the midplane).

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arXiv2003

New Low-Mass Members of the Taurus Star-Forming Region

Briceno et al. recently used optical imaging, data from the Two-Micron All-Sky Survey (2MASS), and follow-up spectroscopy to search for young low-mass stars and brown dwarfs in 8 square degrees of the Taurus star-forming region. By the end of that study, there remained candidate members of Taurus that lacked the spectroscopic observations needed to measure spectral types and determine membership. In this work, we have obtained spectroscopy of the 22 candidates that have A_V<=8, from which we find six new Taurus members with spectral types of M2.75 through M9. The new M9 source has the second latest spectral type of the known members of Taurus (~0.02 M_sun). Its spectrum contains extremely strong emission in H_alpha (W~950 A) as well as emission in He I 6678 A and the Ca II IR triplet. This is the least massive object known to exhibit emission in He I and Ca II, which together with the strong H_alpha are suggestive of intense accretion.

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arXiv2003

Massive Variability Surveys from Venezuela

At the Venezuela National Astronomical Observatory we are carrying out variability surveys spanning many hundreds of square degrees near the celestial equator, using an 8k x 8k CCD Mosaic Camera optimized for drift-scanning on a 1m Schmidt telescope. Among the initial efforts was a project to obtain the first moderately deep, homogeneous sample of young stars over an area of ~180sqr.deg. encompassing the entire Orion OB1 association, one of the nearest and most active regions of star formation. The results show that variability is a powerful technique to identify pre-main sequence populations, specially in sparse areas devoid of gas and dust. We are currently developing a massive database, equipped with web-based data mining tools, that will make our data and results available to the astronomical community.

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arXiv2003

Accretion in Young Stellar/Substellar Objects

We present a study of accretion in a sample of 45 young, low mass objects in a variety of star forming regions and young associations, about half of which are likely substellar. Based primarily on the presence of broad, asymmetric Halpha emission, we have identified 13 objects (~30% of our sample) which are strong candidates for ongoing accretion. At least 3 of these are substellar. We do not detect significant continuum veiling in most of the accretors with late spectral types (M5-M7). Accretion shock models show that lack of measurable veiling allows us to place an upper limit to the mass accretion rates of <~ 10^{-10} Msun/yr. Using magnetospheric accretion models with appropriate (sub)stellar parameters, we can successfully explain the accretor Halpha emission line profiles, and derive quantitative estimates of accretion rates in the range 10^{-12} < Mdot < 10^{-9} Msun/yr. There is a clear trend of decreasing accretion rate with stellar mass, with mean accretion rates declining by 3-4 orders of magnitude over ~ 1 - 0.05 Msun.

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arXiv2002

The Brown Dwarf Deficit in Taurus: Evidence for a Non-Universal IMF

We present the results of a deep, optical/IR wide field imaging survey of selected fields in the nearby (d ~ 140 pc) Taurus star-forming region. We report the discovery of 9 new members with spectral types M5.75-M9.5. We derive an Initial Mass Function encompassing 54% of the known members in Taurus. Comparison with dense regions like the Trapezium Cluster in Orion shows that Taurus has produced x2 less brown dwarfs. We suggest that the lower frequency of brown dwarfs in Taurus may result from the low-density star-forming environment, leading to larger minimum Jeans masses.

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arXiv2001

Disk Accretion at 10 Myr: Results from the TW Hydrae Association

We present an analysis of the accretion properties of the members of the TW Hydrae association. The Halpha emission line profile provides explicit evidence of an active accretion flow in 3 out of 23 observed members of the association; we use radiative transfer models to derive approximate accretion rates for these 3 objects. The resulting accretion rates for these 10 Myr-old stars (<10^-9 Msun/yr) are well over an order of magnitude lower than typical values in 1 Myr-old T Tauri stars. The small fraction of TW Hydrae association objects still accreting (13%, compared with ~70% in 1 Myr-old regions like Taurus), along with the very small accretion rates, points to significant disk evolution over 10 Myr, providing an important constraint on the timescales for planet formation.

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arXiv1999

The CIDA-QUEST Large Scale Variability Survey in the Orion OB Association: initial results

Using the 8k x 8k CCD Mosaic Camera on the 1m Schmidt telescope in Venezuela, we are conducting a large-scale, deep optical, multiepoch, photometric (BVRIHa) survey over 120 sq.deg. in the Orion OB association, aimed at identifying the low mass stellar populations with ages less than about 10 Myr. We present initial results for a 34 sq.deg. area spanning Orion 1b, 1a and the B Cloud. Using variability as our main selection criterion we derive much cleaner samples than with the usual single-epoch photometric selection, allowing us to attain a much higher efficiency in follow up spectroscopy and resulting in an preliminary list of 74 new low-mass (~ 0.4 Msun) pre-main sequence stars. Though preliminary, this list of new T Tauri stars already suggests that the fraction of accreting young stars in 1a is much lower than in 1b, which would be expected if 1a is indeed older than 1b. We are analyzing in detail the light curves of these new stars, and spectroscopy of further candidates is under way.

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arXiv1998

Search for X-ray emission from bona-fide and candidate brown dwarfs

Following the recent classification of the X-ray detected object V410 x-ray 3 with a young brown dwarf candidate (Briceno et al. 1998) and the identification of an X-ray source in Chamaeleon as young bona-fide brown dwarf (Neuhaeuser & Comeron 1998), we investigate all ROSAT All-Sky Survey and archived ROSAT PSPC and HRI pointed observations with bona-fide or candidate brown dwarfs in the field of view with exposure times ranging from 0.13 to 221 ks, including dedicated 64 ks and 42 ks deep ROSAT HRI pointed observations on the low-mass star BRI0021-0214 and the brown dwarf Calar 3, respectively. Out of 26 bona-fide brown dwarfs, one is newly detected in X-rays, namely rho Oph GY 202. Also, four out of 57 brown dwarf candidates studied here are detected in X-rays, namely the young Taurus brown dwarf candidates MHO-4, MHO-5, V410 Anon 13, and V410 x-ray 3. The M9.5-type star BRI0021-0214 is not detected. In the appendix, we also present catalogued, but as yet unnoticed B- and R-band data for some of the objects studied here.

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