301 research outputs found
A first checklist of the lichen-forming fungi of the Venezuelan Andes
Basing on an evaluation of the literature and some unpublished collections, 745 taxa of lichen forming-fungi are reported from the Venezuelan Andes, including 10 infraspecific taxa. Of these taxa 37 are new records for Venezuela: Actinoplaca vulgaris (Müll. Arg.) Vezda & Poelt, Asterothyrium decipiens (Rehm) R. Sant., Calopadia foliicola (Fée) Vezda, C. fusca (Müll. Arg.) Vezda, C. phyllogena (Müll. Arg.) Vezda, Chrysothrix chlorina (Ach.) J. R. Laundon, Cystocoleus ebeneus (Dillwyn) Thwaites, Diploschistes scruposus (Schreb.) Norm., Echinoplacaleucotrichoides (Vain.) R. Sant., Fellhanerabouteillei (Desm.) Vezda, F.dominicana (Vain.) Vezda, F. fuscatula (Müll. Arg.) Vezda, F. sublecanorina (Nyl.) Vezda, Gyalectidium filicinum Müll. Arg., Lecidea limosa Ach., Lepraria neglecta Auct., Ochrolechia africana Vain., Peltigera vainioi Gyelnik, Phyllobathelium nigrum R. Sant. & Tibell, Phyllophialealba R. Sant., Polymeridiumalbidum (Müll. Arg.) R. C. Harris, Porina epiphylla (Fée) Fée, Racodium rupestre Pers., Ramalina asahinae W. Culb. & C. Culb., R. calcarata Krog & Swinsc., R. puiggarii Müll Arg., R. rectangularis Nyl., Rhizoplaca melanophthalma (DC.) Leuckert & Poelt, Strigula antillarum (Fée) Müll. Arg., S. platypoda (Müll. Arg.) R. C. Harris, Tapellaria epiphylla (Müll. Arg.) R. Sant., T.nana (Fée) R. Sant., Tephromela aglaea (Sommerf.) Hertel & Rambold, Tricharia vainioi R. Sant., Trichothelium epiphyllum Müll. Arg., T. bipindense F. Schill. and Vezdaea foliicola Sérusiaux. Another 4 species are new records only for the Venezuelan Andes: Dimerellaepiphylla (Müll. Arg.) Malme, Porinaatrocoerulea Müll. Arg., Ramalina peruviana Ach. and Woessia apiahica (Müll. Arg.) Sérus. A list of synonyms used in the consulted literature is added. Four new combinations are proposed: Dictyonema zahlbruckneri (Schiffn.) V. Marcano, Heterodermia tropica (Kurok.) Sipman, Parmelinopsis cleefii (Sipman) V. Marcano & Sipman and Phyllobaeislinearis (De Vries) V. Marcano & Sipman
A submillimeter search for pre- and proto-brown dwarfs in Chamaeleon II
Context. Chamaeleon II molecular cloud is an active star forming region that
offers an excellent opportunity for studying the formation of brown dwarfs in
the southern hemisphere. Aims. Our aims are to identify a population of pre-
and proto- brown dwarfs (5 sigma mass limit threshold of ~0.015 Msun) and
provide information on the formation mechanisms of substellar objects. Methods.
We performed high sensitivity observations at 870 microns using the LABOCA
bolometer at the APEX telescope towards an active star forming region in
Chamaeleon II. The data are complemented with an extensive multiwavelength
catalogue of sources from the optical to the far-infrared to study the nature
of the LABOCA detections. Results. We detect fifteen cores at 870 microns, and
eleven of them show masses in the substellar regime. The most intense objects
in the surveyed field correspond to the submillimeter counterparts of the well
known young stellar objects DK Cha and IRAS 12500-7658. We identify a possible
proto-brown dwarf candidate (ChaII-APEX-L) with IRAC emission at 3.6 and 4.5
microns. Conclusions. Our analysis indicates that most of the spatially
resolved cores are transient, and that the point-like starless cores in the
sub-stellar regime (with masses between 0.016 Msun and 0.066 Msun) could be
pre-brown dwarfs cores gravitationally unstable if they have radii smaller than
220 AU to 907 AU (1.2" to 5" at 178 pc) respectively for different masses. ALMA
observations will be the key to reveal the energetic state of these pre-brown
dwarfs candidates.Comment: 11 pages, 6 figure
Gas in the protoplanetary disc of HD169142: Herschel’s view
In an effort to simultaneously study the gas and dust components of the disc surrounding the young Herbig Ae star HD169142, we present far-IR observations obtained with the PACS instrument onboard the Herschel Space Observatory. This work is part of the open time key program GASPS,
which is aimed at studying the evolution of protoplanetary discs. To constrain the gas properties in the outer disc, we observed the star at several key gas-lines, including [OI] 63.2 and 145.5 μm, [CII] 157.7 μm, CO 72.8 and 90.2 μm, and o-H_2O 78.7 and 179.5 μm. We only detect the
[OI] 63.2 μm line in our spectra, and derive upper limits for the other lines. We complement our data set with PACS photometry and ^(12/13)CO data obtained with the Submillimeter Array. Furthermore, we derive accurate stellar parameters from optical spectra and UV to mm photometry. We model the dust continuum with the 3D radiative transfer code MCFOST and use this model as an input to analyse the gas lines with the thermochemical code ProDiMo. Our dataset is consistent with a simple model in which the gas and dust are well-mixed in a disc with a continuous structure between 20 and 200 AU, but this is not a unique solution. Our modelling effort allows us to constrain the gas-to-dust mass ratio as well as the relative abundance of the PAHs in the disc by simultaneously fitting the lines of several species that originate in different regions. Our results
are inconsistent with a gas-poor disc with a large UV excess; a gas mass of 5.0 ± 2.0 × 10^(−3)M_⊙ is still present in this disc, in agreement with earlier CO observations
Panchromatic observations and modeling of the HV Tau C edge-on disk
We present new high spatial resolution (<~ 0.1") 1-5 micron adaptive optics
images, interferometric 1.3 mm continuum and 12CO 2-1 maps, and 350 micron, 2.8
and 3.3 mm fluxes measurements of the HV Tau system. Our adaptive optics images
reveal an unusually slow orbital motion within the tight HV Tau AB pair that
suggests a highly eccentric orbit and/or a large deprojected physical
separation. Scattered light images of the HV Tau C edge-on protoplanetary disk
suggest that the anisotropy of the dust scattering phase function is almost
independent of wavelength from 0.8 to 5 micron, whereas the dust opacity
decreases significantly over the same range. The images further reveal a marked
lateral asymmetry in the disk that does not vary over a timescale of 2 years.
We further detect a radial velocity gradient in the disk in our 12CO map that
lies along the same position angle as the elongation of the continuum emission,
which is consistent with Keplerian rotation around an 0.5-1 Msun central star,
suggesting that it could be the most massive component in the triple system. We
use a powerful radiative transfer model to compute synthetic disk observations
and use a Bayesian inference method to extract constraints on the disk
properties. Each individual image, as well as the spectral energy distribution,
of HV Tau C can be well reproduced by our models with fully mixed dust provided
grain growth has already produced larger-than-interstellar dust grains.
However, no single model can satisfactorily simultaneously account for all
observations. We suggest that future attempts to model this source include more
complex dust properties and possibly vertical stratification. (Abridged)Comment: 26 pages, 11 figures, editorially accepted for publication in Ap
YSOVAR: Mid-IR variability in the star forming region Lynds 1688
The emission from young stellar objects (YSOs) in the mid-IR is dominated by
the inner rim of their circumstellar disks. We present an IR-monitoring survey
of about 800 objects in the direction of the Lynds 1688 (L1688) star forming
region over four visibility windows spanning 1.6 years using the \emph{Spitzer}
space telescope in its warm mission phase. Among all lightcurves, 57 sources
are cluster members identified based on their spectral-energy distribution and
X-ray emission. Almost all cluster members show significant variability. The
amplitude of the variability is larger in more embedded YSOs. Ten out of 57
cluster members have periodic variations in the lightcurves with periods
typically between three and seven days, but even for those sources, significant
variability in addition to the periodic signal can be seen. No period is stable
over 1.6 years. Non-periodic lightcurves often still show a preferred timescale
of variability which is longer for more embedded sources. About half of all
sources exhibit redder colors in a fainter state. This is compatible with
time-variable absorption towards the YSO. The other half becomes bluer when
fainter. These colors can only be explained with significant changes in the
structure of the inner disk. No relation between mid-IR variability and stellar
effective temperature or X-ray spectrum is found.Comment: accepted by ApJ, 24 pages, 17 figure
A search for substellar members in the Praesepe and sigma Orionis clusters
(Abridged). We have conducted deep photometric searches for substellar
members of the Praesepe (0.5-1 Gyr) and sigma Orionis (3 Myr) star clusters
using the Sloan i' and z' filters, the 3.5-m and the 5-m Hale telescopes on the
Calar Alto and Palomar Observatories, respectively. The total area surveyed was
1177 arcmin^2 (Praesepe) and 1122 arcmin^2 (sigma Orionis) down to 5-sigma
detection limits of i'= 24.5 and z'= 24 mag, corresponding to masses of 50-55
M_Jup (Praesepe) and 6 M_Jup (sigma Orionis). Besides recovering previously
known cluster members reported in the literature, we have identified new
photometric candidates in both clusters whose masses expand the full range
covered by our study. In sigma Orionis, follow-up NIR photometry has allowed us
to confirm the likely cluster membership of three newly discovered
planetary-mass objects. The substellar mass function of sigma Orionis, which is
complete from the star-brown dwarf borderline down to 7 M_Jup, keeps rising
smoothly with a slope of alpha = 0.6^{+0.5}_{-0.1}. Very interestingly, one of
the faintest Praesepe candidates for which we have also obtained follow-up JHK
photometry nicely fits the expected optical and infrared photometric sequence
of the cluster. From its colors, we have estimated its spectral type in the
L4-L6 range. If confirmed as a true Praesepe member, it would become the first
L-type brown dwarf (50-60 M_Jup) identified in an intermediate-age star
cluster. Our derivation of the Praesepe mass function depends strongly on the
cluster age. For the youngest possible ages (500-700 Myr), our results suggest
that there is a deficit of Praesepe brown dwarfs in the central regions of the
cluster, while the similarity between the Praesepe and sigma Orionis mass
functions increases qualitatively for models older than 800 Myr.Comment: Accepted for publication in A&A. Figures 1, 7, and 9-12 are available
in jpeg forma
Young Stellar Object Variability (YSOVAR): Long Timescale Variations in the Mid-Infrared
The YSOVAR (Young Stellar Object VARiability) Spitzer Space Telescope
observing program obtained the first extensive mid-infrared (3.6 & 4.5 um)
time-series photometry of the Orion Nebula Cluster plus smaller footprints in
eleven other star-forming cores (AFGL490, NGC1333, MonR2, GGD 12-15, NGC2264,
L1688, Serpens Main, Serpens South, IRAS 20050+2720, IC1396A, and Ceph C).
There are ~29,000 unique objects with light curves in either or both IRAC
channels in the YSOVAR data set. We present the data collection and reduction
for the Spitzer and ancillary data, and define the "standard sample" on which
we calculate statistics, consisting of fast cadence data, with epochs about
twice per day for ~40d. We also define a "standard sample of members",
consisting of all the IR-selected members and X-ray selected members. We
characterize the standard sample in terms of other properties, such as spectral
energy distribution shape. We use three mechanisms to identify variables in the
fast cadence data--the Stetson index, a chi^2 fit to a flat light curve, and
significant periodicity. We also identified variables on the longest timescales
possible of ~6 years, by comparing measurements taken early in the Spitzer
mission with the mean from our YSOVAR campaign. The fraction of members in each
cluster that are variable on these longest timescales is a function of the
ratio of Class I/total members in each cluster, such that clusters with a
higher fraction of Class I objects also have a higher fraction of long-term
variables. For objects with a YSOVAR-determined period and a [3.6]-[8] color,
we find that a star with a longer period is more likely than those with shorter
periods to have an IR excess. We do not find any evidence for variability that
causes [3.6]-[4.5] excesses to appear or vanish within our data; out of members
and field objects combined, at most 0.02% may have transient IR excesses.Comment: Accepted to AJ; 38 figures, 93 page
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