168 research outputs found
Aperture Synthesis Imaging of V892 Tau and PV Cep: Disk Evolution
I present a study of two Herbig Ae stars that are in completely different
evolutionary stages: V892 Tau and PV Cep. Using sub arc-second interferometric
observations obtained with the Combined Array for Research in Millimeter-wave
Astronomy (CARMA) at the 1.3 and 2.7 mm wavelengths, I have for the first time
resolved their disks. I deduce that the 5 Myr old V892 Tau has a low dust
opacity index =1.1 and a disk mass of 0.03 \msun. These values
correspond to the growth of its dust into large up to centimeters size
structures. In contrast, the very young (a few 10yrs) PV Cep has a quite
high opacity index =1.75 and a more massive disk 0.8 \msun. PV Cep
has the youngest resolved disk around any Herbig Ae star. Unlike the youngest T
Tauri and Class 0 stars, which contain large and processed grains, the young
Herbig Ae star, PV Cep, disk contains ISM-like unprocessed dust. This suggests
that PV Cep's dust evolution is slower than T Tauri stars'. I also present high
spatial resolution interferometric observations of the PV Cep outflow. The
outflow inclination is consistent with the orientation of the known Herbig-Haro
flow in that region, HH315.Comment: 15 pages, Accepted for publication in Ap
Seasonal variation in the diet and the morphometric parameters of the genus Pseudophoxinus sp. (Cyprinidae) in Eastern Algeria
The diet and the morphometric parameters of the Pseudophoxinus sp. (Cyprinidae) in the El Mellah ravine (M’sila, Algeria) were studied by determining body measurements and analyzing the contents of digestive tracts of 320 fish from December 2018 to November 2019. The obtained results show marked diversity within the fish species diets, which can be grouped into three main fractions: (i) a plant fraction; (ii) an animal fraction composed mainly of invertebrate prey; and (iii) fraction consisting of fish eggs (digested by males during the breeding season). The plant fraction was present in all the analyzed digestive tracts, mainly being composed of phytoplankton (algae), namely, Spirogyra sp., Melosira sp. and Navicula sp. with the total relative abundances of 38.7%, 34.4% and 4.1%, respectively. On the other hand, the animal fraction was represented by the Chironomus sp., Austrosimulium sp., and Ecdyonurus sp. dipterans with a total relative abundance of 7.3%, 6.7% and 5.9%, respectively. Pseudophoxinus sp. eggs can also be considered as part of this fraction with the total relative abundance of 2.9%. The age of the fish seems to have an influence on their feeding behaviour. The juveniles of Pseudophoxinus sp. in the study area had a purely phytophagous diet, while adults showed a phytophagous behaviour with an omnivorous tendency. The present study shows that not only the age of the fish but also the seasonal changes (climatic conditions and entry into the breeding season) may have a significant influence on the diet and morphometric parameters of Pseudophoxinus sp. in El Mellah ravine
THERMAL SHOCK RESISTANCE OF A SODA LIME GLASS
We studied the thermal shock of a three millimeters thickness soda lime glass using the hot-cold thermal shock technique. The cooling was made by ambient air jet on previously warmed samples. The heat transfer coefficient was about 600 W/°C.m2 (Biot number β = 0.3). The thermal shock duration was fixed at 6 seconds. The hot temperature was taken between 100°C and 550°C while the cold temperature of the air flux was kept constant at 20°C. The acoustic emission technique was used for determining the failure time and the critical temperature difference (ΔTC). By referring to experimental results, thermal shock modelling computations are conducted. Our aim is especially focused on the fracture initiation moments during the cooling process and on the crack initiation sites. The used modeling is based on the local approach of the thermal shock during the experimental data treatment. For each test, the temperature profile and the transient stress state through the samples thickness are determined. By applying the linear superposition property of the stress intensity factors, evolution of the stress intensity factor KI in function of the pre-existing natural flaws in the glass surface is established. The size of the critical flaw is determined by the linear fracture mechanics laws. Computation results confirm the experimental values of the critical difference temperature obtained that is the source of the glass degradation
The Structure of the DoAr 25 Circumstellar Disk
We present high spatial resolution (< 0.3" = 40\Sigma
\propto r^{-p}$ with p = 0.34, significantly less steep than a steady-state
accretion disk (p = 1) or the often adopted minimum mass solar nebula (p =
1.5). Even though the total mass of material is large (M_d = 0.10 M_sun), the
densities inferred in the inner disk for such a model may be too low to
facilitate any mode of planet formation. However, alternative models with
steeper density gradients (p = 1) can explain the observations equally well if
substantial grain growth in the planet formation region (r < 40 AU) has
occurred. We discuss these data in the context of such models with dust
properties that vary with radius and highlight their implications for
understanding disk evolution and the early stages of planet formation.Comment: ApJL in pres
Resolving and probing the circumstellar disk of the Herbig Ae star MWC 480 at 1.4 mm: Evolved dust?
We present high resolution 0.45" x 0.32" observations from the BIMA array
toward the Herbig Ae system MWC 480 in the lambda = 1.4 mm dust continuum. We
resolve a circumstellar disk of radius ~170 AU and constrain the disk
parameters by comparing the observations to flat disk models. These results
show that the typical fit parameters of the disk, such as the mass, Md ~
0.04-0.18 Mo, and the surface density power law index, p=0.5 or 1, are
comparable to those of the lower mass T Tauri stars. The dust in the MWC 480
disk can be modeled as processed dust material (beta ~ 0.8), similar to the
Herbig Ae star CQ Tau disk; the fitted disk parameters are also consistent with
less-evolved dust (beta ~ 1.2). The possibility of grain growth in the MWC 480
circumstellar disk is supported by the acceptable fits with beta ~ 0.8. The
surface density power-law profiles of p=0.5 and p=1 can be easily fit to the
MWC 480 disk; however, a surface density power-law profile similiar to the
minimum mass solar nebula model p=1.5 is ruled out at an 80% confidence level.Comment: ApJ Accepted. 17 page
Simultaneous monitoring of the photometric and polarimetric activity of the young star PV Cep in the optical/near-infrared bands
We present the results of a simultaneous monitoring, lasting more than 2
years, of the optical and near-infrared photometric and polarimetric activity
of the variable protostar PV Cep. During the monitoring period, an outburst has
occurred in all the photometric bands, whose declining phase (J
3 mag) lasted about 120 days. A time lag of 30 days between
optical and infrared light curves has been measured and interpreted in the
framework of an accretion event. This latter is directly recognizable in the
significant variations of the near-infrared colors, that appear bluer in the
outburst phase, when the star dominates the emission, and redder in declining
phase, when the disk emission prevails. All the observational data have been
combined to derive a coherent picture of the complex morphology of the whole PV
Cep system, that, in addition to the star and the accretion disk, is composed
also by a variable biconical nebula. In particular, the mutual interaction
between all these components is the cause of the high value of the polarization
( 20%) and of its fluctuations. The observational data concur to
indicate that PV Cep is not a genuine EXor star, but rather a more complex
object; moreover the case of PV Cep leads to argue about the classification of
other recently discovered young sources in outburst, that have been considered,
maybe over-simplifying, as EXor.Comment: Accepted for publication on Ap
Inner disc rearrangement revealed by dramatic brightness variations in the young star PV Cep
Young Sun-like stars at the beginning of the pre-main sequence (PMS)
evolution are surrounded by accretion discs and remnant protostellar envelopes.
Photometric and spectroscopic variations of these stars are driven by
interactions of the star with the disc. Time scales and wavelength dependence
of the variability carry information on the physical mechanisms behind these
interactions. We conducted multi-epoch, multi-wavelength study of PV Cep, a
strongly variable, accreting PMS star. By combining our own observations from
2004-2010 with archival and literature data, we show that PV Cep started a
spectacular fading in 2005, reaching an I_C-band amplitude of 4 mag. Analysis
of variation of the optical and infrared fluxes, colour indices, and emission
line fluxes suggests that the photometric decline in 2005-2009 resulted from an
interplay between variable accretion and circumstellar extinction: since the
central luminosity of the system is dominated by accretion, a modest drop in
the accretion rate could induce the drastic restructuring of the inner disc.
Dust condensation in the inner disc region might have resulted in the
enhancement of the circumstellar extinction.Comment: 11 pages, 4 figures, accepted for publication by MNRAS. 3 online
tables adde
Disk Imaging Survey of Chemistry with SMA (DISCS): I. Taurus Protoplanetary Disk Data
Chemistry plays an important role in the structure and evolution of
protoplanetary disks, with implications for the composition of comets and
planets. This is the first of a series of papers based on data from DISCS, a
Submillimeter Array survey of the chemical composition of protoplanetary disks.
The six Taurus sources in the program (DM Tau, AA Tau, LkCa 15, GM Aur, CQ Tau
and MWC 480) range in stellar spectral type from M1 to A4 and offer an
opportunity to test the effects of stellar luminosity on the disk chemistry.
The disks were observed in 10 different lines at ~3" resolution and an rms of
~100 mJy beam-1 at ~0.5 km s-1. The four brightest lines are CO 2-1, HCO+ 3-2,
CN 2_3-1_2 and HCN 3-2 and these are detected toward all sources (except for
HCN toward CQ Tau). The weaker lines of CN 2_2-1_1, DCO+ 3-2, N2H+ 3-2, H2CO
3_03-2_02 and 4_14-3_13 are detected toward two to three disks each, and DCN
3-2 only toward LkCa 15. CH3OH 4_21-3_12 and c-C3H2 are not detected. There is
no obvious difference between the T Tauri and Herbig Ae sources with regard to
CN and HCN intensities. In contrast, DCO+, DCN, N2H+ and H2CO are detected only
toward the T Tauri stars, suggesting that the disks around Herbig Ae stars lack
cold regions for long enough timescales to allow for efficient deuterium
chemistry, CO freeze-out, and grain chemistry.Comment: 29 pages, 4 figures, accepted for publication in Ap
- …