arXiv subjects

L. Loinard

Publications and source records attributed to L. Loinard.

51 records · Page 3Linked to original sources

arXiv2012

A one-sided knot ejection at the core of the HH 111 outflow

We present an astrometry study of the radio source VLA 1 at the core of the HH 111 outflow using new data (2007) as well as archival observations (1992-1996). All data were taken at 3.6 cm with the Very Large Array in its most extended (A) configuration. The source VLA 1 has undergone a dramatic morphological change, showing a one-sided knot ejection in the 2007 epoch. We also report on the detection of a 3.6 cm compact continuum source (VLA 3) located at (-10.6",98.7") from VLA 1. No significant absolute proper motions were found for VLA 1 and VLA 3 and the upper limits are consistent with those found for (embedded) radio sources in the Orion Nebula. We favor the interpretation that in the continuum at 3.6 cm we are observing two nearly perpendicular jets. HH 111 presents a new case of one-sided jet ejection in a young stellar object. The Galactic (or extragalactic) nature of VLA 3 remains unclear.

astro-ph.GA↗

arXiv2011

A Surprising Dynamical Mass for V773~Tau~B

We report on new high-resolution imaging and spectroscopy on the multiple T Tauri star system V773 Tau over the 2003 -- 2009 period. With these data we derive relative astrometry, photometry between the A and B components, and radial velocity (RV) of the A-subsystem components. Combining these new data with previously published astrometry and RVs, we update the relative A-B orbit model. This updated orbit model, the known system distance, and A subsystem parameters yields a dynamical mass for the B component for the first time. Remarkably the derived B dynamical mass is in the range of 1.7 -- 3.0 M$_\sun$. This is much higher than previous estimates, and suggests that like A, B is also a multiple stellar system. Among these data, spatially-resolved spectroscopy provide new insight into the nature of the B component. Similar to A, these near-IR spectra indicate that the dominant source in B is of mid-K spectral type. If B is in fact a multiple star system as suggested by the dynamical mass estimate, the simplest assumption is that B is composed of similar $\sim$ 1.2 M$_\sun$ PMS stars in a close ($<$ 1 AU) binary system. This inference is supported by line-shape changes in near-IR spectroscopy of B, tentatively interpreted as changing RV among components in V773 Tau B. Relative photometry indicate that B is highly variable in the near-IR. The most likely explanation for this variability is circum-B material resulting in variable line-of-sight extinction. The distribution of this material must be significantly affected by both the putative B multiplicity, and the A-B orbit.

astro-ph.SR↗

arXiv2011

VLBA determination of the distance to nearby star-forming regions V. Dynamical mass, distance and radio structure of V773 Tau A

(ABRIDGED) We present multi-epoch Very Long Baseline Array (VLBA) observations of V773 Tau A, the 51-day binary subsystem in the multiple young stellar system V773 Tau. Combined with previous interferometric and radial velocity measurements, these new data enable us to improve the characterization of the physical orbit of the A subsystem. In particular, we infer updated dynamical masses for the primary and the secondary components of 1.55 pm 0.11 Msun, and 1.293 pm 0.068 Msun, respectively, and an updated orbital parallax distance to the system of 135.7 pm 3.2 pc, all consistent with previous estimates. Using the improved orbit, we can calculate the absolute coordinates of the barycenter of V773 Tau A at each epoch of our VLBA observations, and fit for its trigonometric parallax and proper motion. This provides a direct measurement of the distance to the system almost entirely independent of the orbit modeling. The best fit yields a distance of 129.9 pm 3.2 pc, in good agreement (i.e. within 1 sigma) with the distance estimate based on the orbital fit. Taking the mean value of the orbital and trigonometric parallaxes, we conclude that V773 Tau is located at d =132.8 pm 2.3 pc. The accuracy of this determination is nearly one order of magnitude better than that of previous estimates. In projection, V773 Tau and two other young stars (Hubble 4 and HDE 283572) recently observed with the VLBA are located toward the dark cloud Lynds 1495, in the central region of Taurus. These three stars appear to have similar trigonometric parallaxes, radial velocities, and proper motions, and we argue that the weighted mean and dispersion of their distances (d = 131.4 pc and sigma_d = 2.4 pc) provide a good estimate of the distance to and depth of Lynds 1495 and its associated stellar population.

astro-ph.SR↗

arXiv2011

Radio continuum emission from knots in the DG Tau jet

Context: HH 158, the jet from the young star DG Tau, is one of the few sources of its type where jet knots have been detected at optical and X-ray wavelengths. Aims: To search, using Very Large Array observations of this source, radio knots and if detected, compare them with the optical and X-ray knots. To model the emission from the radio knots. Methods: We analyzed archive data and also obtained new Very Large Array observations of this source, as well as an optical image, to measure the present position of the knots. We also modeled the radio emission from the knots in terms of shocks in a jet with intrinsically time-dependent ejection velocities. Results: We detected radio knots in the 1996.98 and 2009.62 VLA data. These radio knots are,within error, coincident with optical knots. We also modeled satisfactorily the observed radio flux densities as shock features from a jet with intrinsic variability. All the observed radio, optical, and X-ray knot positions can be intepreted as four successive knots, ejected with a period of 4.80 years and traveling away from the source with a velocity of 198 km s$^{-1}$ in the plane of the sky. Conclusions: The radio and optical knots are spatially correlated and our model can explain the observed radio flux densities. However, the X-ray knots do not appear to have optical or radio counterparts and their nature remains poorly understood.

astro-ph.GA↗

arXiv2011

VLBA determination of the distance to nearby star-forming regions VI. The distance to the young stellar object HW 9 in Cepheus A

Using the Very Long Baseline Array (VLBA), we have observed the radio continuum emission from the young stellar object HW 9 in the Cepheus A star-forming region at ten epochs between 2007 February and 2009 November. Due to its strong radio variability, the source was detected at only four of the ten epochs. From these observations, the trigonometric parallax of HW 9 was determined to be $π$ = 1.43 $\pm$ 0.07 mas, in excellent agreement with a recent independent VLBA determination of the trigonometric parallax of a methanol maser associated with the nearby young stellar source HW 2 ($π$ = 1.43 $\pm$ 0.08 mas). This concordance in results, obtained in one case from continuum and in the other from line observations, confirms the reliability of Very Long Baseline Array trigonometric parallax measurements. By combining the two results, we constrain the distance to Cepheus A to be 700$_{-28}^{+31}$ pc, an uncertainty of 3.5%.

astro-ph.GA↗

arXiv2011

The Gould's Belt distance survey

Very Long Baseline Interferometry (VLBI) observations can provide the position of compact radio sources with an accuracy of order 50 micro-arcseconds. This is sufficient to measure the trigonometric parallax and proper motions of any object within 500 pc of the Sun to better than a few percent. Because they are magnetically active, young stars are often associated with compact radio emission detectable using VLBI techniques. Here we will show how VLBI observations have already constrained the distance to the most often studied nearby regions of star-formation (Taurus, Ophiuchus, Orion, etc.) and have started to provide information on their internal structure and kinematics. We will then briefly describe a large project (called The Gould's Belt Distance Survey) designed to provide a detailed view of star-formation in the Solar neighborhood using VLBI observations.

astro-ph.GA↗

arXiv2010

A reassessment of the kinematics of PV Cephei based on accurate proper motion measurements

We present two Very Large Array observations of the pre-main-sequence star PV Cephei, taken with a separation of 10.5 years. These data show that the proper motions of this star are $μ_α\cos δ= +10.9 \pm 3.0$ mas yr$^{-1}$; $μ_δ= +0.2 \pm 1.8$ mas yr$^{-1}$, very similar to those -previously known- of HD 200775, the B2Ve star that dominates the illumination of the nearby reflection nebula NGC 7023. This result suggests that PV Cephei is not a rapidly moving run-away star as suggested by previous studies. The large velocity of PV Cephei had been inferred from the systematic eastward displacement of the bisectors of successive pairs of Herbig Haro knots along its flow. These systematic shifts might instead result from an intrinsic dissymmetry in the ejection mechanisms, or from an asymmetric distribution of the circumstellar material.

astro-ph.GA↗

arXiv2010

Confirmation of a recent bipolar ejection in the very young hierarchical multiple system IRAS 16293-2422

We present and analyze two new high-resolution (approx 0.3 arcsec), high-sensitivity (approx 50 uJy beam-1) Very Large Array 3.6 cm observations of IRAS 16293-2422 obtained in 2007 August and 2008 December. The components A2alpha and A2beta recently detected in this system are still present, and have moved roughly symmetrically away from source A2 at a projected velocity of 30-80 km s-1. This confirms that A2alpha and A2beta were formed as a consequence of a very recent bipolar ejection from A2. Powerful bipolar ejections have long been known to occur in low-mass young stars, but this is -to our knowledge-- the first time that such a dramatic one is observed from its very beginning. Under the reasonable assumption that the flux detected at radio wavelengths is optically thin free-free emission, one can estimate the mass of each ejecta to be of the order of 10^-8 Msun. If the ejecta were created as a consequence of an episode of enhanced mass loss accompanied by an increase in accretion onto the protostar, then the total luminosity of IRAS 16293-2422 ought to have increased by 10-60% over the course of at least several months. Between A2alpha and A2beta, component A2 has reappeared, and the relative position angle between A2 and A1 is found to have increased significantly since 2003-2005. This strongly suggests that A1 is a protostar rather than a shock feature, and that the A1/A2 pair is a tight binary system. Including component B, IRAS 16293-2422 therefore appears to be a very young hierarchical multiple system.

astro-ph.GA↗

arXiv2009

Motions of Galaxies in the Local Group and Beyond: an Astro2010 Science White Paper

Recent advances with the VLBA have resulted in ~10 micro-arcsec astrometry for compact sources in external galaxies, and measurement of the proper motion of Local Group galaxies has been demonstrated. With improved telescopes and equipment, we could greatly improve upon and expand these measurements, including a measurement of the proper motion of the Andromeda galaxy, which is key to understanding the history and fate of the Local Group. The combination of optical velocities and radio astrometric data would allow detailed modeling of the mass distributions of the disks, bulges, and dark matter halos of galaxies in clusters.

astro-ph.GA↗

arXiv2008

Milli-arcsecond Binaries

As part of an astrometric program, we have used the Very Long Baseline Array to measure the trigonometric parallax of several young stars in the Taurus and Ophiuchus star-forming regions with great accuracy. Additionally, we have obtained an unprecedented sample of high-resolution (~ 1 mas) images of several young stellar systems. These images revealed that about 70% of the stars in our sample are very tight binary stars (with separations of a few mas). Since it is highly unlikely that 70% of all stars are such tight binaries, we argue that selection effects are at work.

astro-ph↗

arXiv2006

Detection of HCO+ Emission toward the Planetary Nebula K 3-35

We report the detection, for the first time, of HCO+ (J=1-0) emission as well as marginal CO (J=1-0) emission toward the planetary nebula (PN) K3-35 as a result of a molecular survey carried out toward this source. We also report new observations of the previously detected CO (J=2-1) and water maser emission, as well as upper limits for the emission of the SiO, H13CO+, HNC, HCN, HC3OH, HC5N, CS, HC3N, 13CO, CN, and NH3 molecules. From the ratio of CO (J=2-1) to CO (J=1-0) emission we have estimated the kinetic temperature of the molecular gas, obtaining a value of ~20 K. Using this result, we have estimated a molecular mass for the envelope of ~ 0.017 M_Sun, and an HCO+ abundance relative to H_2 of 6 X 10^-7, similar to the abundances found in other PNe. K~3-35 is remarkable because it is one of the two PNe reported to exhibit water maser emission, which is present in the central region as well as at a distance of $\simeq$ 5000 AU away from the center. The presence of molecular emission provides some clues that could help to understand the persistence of water molecules in the envelope of K 3-35. The HCO$^{+}$ emission could be arising in dense molecular clumps, that may provide the shielding mechanism which protects water molecules in this source.

astro-ph↗

arXiv2006

New observational frontiers in the multiplicity of young stars

It has now been known for over a decade that low-mass stars located in star-forming regions are very frequently members of binary and multiple systems, even more so than main sequence stars in the solar neighborhood. This high multiplicity rate has been interpreted as the consequence of the fragmentation of small molecular cores into a few seed objects that accrete to their final mass from the remaining material and dynamically evolve into stable multiple systems, possibly producing a few ejecta in the process. Analyzing the statistical properties of young multiple systems in a variety of environments therefore represents a powerful approach to place stringent constraints on star formation theories. In this contribution, we first review a number of recent results related to the multiplicity of T Tauri stars. We then present a series of studies focusing on the multiplicity and properties of optically-undetected, heavily embedded protostars. These objects are much younger than the previously studied pre-main sequence stars, and they therefore offer a closer look at the primordial population of multiple systems. In addition to these observational avenues, we present new results of a series of numerical simulations that attempt to reproduce the fragmentation of small molecular cores into multiple systems, and compare these results to the observations.

astro-ph↗

arXiv2004

HDO abundance in the envelope of the solar-type protostar IRAS16293-2422

We present IRAM 30m and JCMT observations of HDO lines towards the solar-type protostar IRAS 16293-2422. Five HDO transitions have been detected on-source, and two were unfruitfully searched for towards a bright spot of the outflow of IRAS 16293-2422. We interpret the data by means of the Ceccarelli, Hollenbach and Tielens (1996) model, and derive the HDO abundance in the warm inner and cold outer parts of the envelope. The emission is well explained by a jump model, with an inner abundance of 1e-7 and an outer abundance lower than 1e-9 (3 sigma). This result is in favor of HDO enhancement due to ice evaporation from the grains in theinner envelope. The deuteration ratio HDO/H2O is found to be f_in=3% and f_out < 0.2% (3 sigma) in the inner and outer envelope respectively and therefore, the fractionation also undergoes a jump in the inner part of the envelope. These results are consistent with the formation of water in the gas phase during the cold prestellar core phase and storage of the molecules on the grains, but do not explain why observations of H2O ices consistently derive a H2O ice abundance of several 1e-5 to 1e-4, some two orders of magnitude larger than the gas phase abundance of water in the hot core around IRAS 16293-2422.

astro-ph↗

arXiv2002

Detection of doubly-deuterated methanol in the solar-type protostar IRAS16293-2422

We report the first detection of doubly-deuterated methanol (CHD2OH), as well as firm detections of the two singly-deuterated isotopomers of methanol (CH2DOH and CH3OD), towards the solar-type protostar IRAS16293-2422. From the present multifrequency observations, we derive the following abundance ratios: [CHD2OH]/[CH3OH] = 0.2 +/- 0.1, [CH2DOH]/[CH3OH] = 0.9 +/- 0.3, [CH3OD]/[CH3OH] = 0.04 +/- 0.02. The total abundance of the deuterated forms of methanol is greater than that of its normal hydrogenated counterpart in the circumstellar material of IRAS16293-2422, a circumstance not previously encountered. Formaldehyde, which is thought to be the chemical precursor of methanol, possesses a much lower fraction of deuterated isotopomers (~ 20%) with respect to the main isotopic form in IRAS16293-2422. The observed fractionation of methanol and formaldehyde provides a severe challenge to both gas-phase and grain-surface models of deuteration. Two examples of the latter model are roughly in agreement with our observations of CHD2OH and CH2DOH if the accreting gas has a large (0.2-0.3) atomic D/H ratio. However, no gas-phase model predicts such a high atomic D/H ratio, and hence some key ingredient seems to be missing.

astro-ph↗

arXiv1999

Mid-IR and Far-UV observations of the star-forming ring of M 31

We present mid-IR images of a 15'x15' field in the south-west part of the Andromeda galaxy M 31 obtained with the ISOCAM camera on board ISO. We also present a 20" resolution far-UV image of a larger field at 200 nm obtained with the balloon-borne telescope FOCA 1000. These images are compared with HI, CO(1-0) and Halpha maps. The mid-IR emission as seen through filters at 7 and 15 mic is extremely well correlated with the distribution of neutral gas as shown by the HI and CO(1-0) maps, while the correlation is poorer with the distribution of the ionized gas seen through its Halpha emission. There is some correlation with the UV radiation, but it appears that the contribution of UV photons to the excitation of the carriers of the mid-IR emission is not dominant in most of M 31. The spectro-imaging observations of Cesarsky et al. (1998) show that the mid-IR spectra of several regions of M 31 are dominated by a strong emission band at 11.3 mic while emission in the other classical Infrared Bands at 6.2, 7.7 and 8.6 mic is faint or absent. More precisely, we find that the mid-IR spectral variations are not clearly related to the UV radiation field. The present observations have important consequences on our understanding of excitation of the interstellar mid-IR emission. In particular, we conclude that like for M 31, excitation in the Galactic cirruses may not be dominated by UV photons but rather by another mechanism which remains to be identified (visible photons ?). The UV excitation appears to become important when the UV radiation density is of the order of twice that near the Sun.

astro-ph↗