The VISTA Orion mini-survey: star formation in the Lynds 1630 North cloud

The Orion cloud complex presents a variety of star formation mechanisms and properties and it is still one of the most intriguing targets for star formation studies. We present VISTA/VIRCAM near-infrared observations of the L1630N star forming region, including the stellar clusters NGC 2068 and NGC 2071, in the Orion molecular cloud B and discuss them in combination with Spitzer data. We select 186 young stellar object (YSO) candidates in the region on the basis of multi-colour criteria, confirm the YSO nature of the majority of them using published spectroscopy from the literature, and use this sample to investigate the overall star formation properties in L1630N. The K-band luminosity function of L1630N is remarkably similar to that of the Trapezium cluster, i.e., it presents a broad peak in the range 0.3-0.7 M$_\odot$ and a fraction of sub-stellar objects of $\sim$20%. The fraction of YSOs still surrounded by disk/envelopes is very high ($\sim$85%) compared to other star forming regions of similar age (1-2 Myr), but includes some uncertain corrections for diskless YSOs. Yet, a possibly high disk fraction together with the fact that 1/3 of the cloud mass has a gas surface density above the threshold for star formation ($\sim$129 M$_\odot$ pc$^{-2}$), points towards a still on-going star formation activity in L1630N. The star formation efficiency (SFE), star formation rate (SFR) and density of star formation of L1630N are within the ranges estimated for galactic star forming regions by the Spitzer "core to disk" and "Gould's Belt" surveys. However, the SFE and SFR are lower than the average value measured in the Orion A cloud and, in particular, lower than that in the southern regions of L1630. This might suggest different star formation mechanisms within the L1630 cloud complex.Comment: 22 pages, 9 figure

When the tale comes true: multiple populations and wide binaries in the Orion Nebula Cluster

The high-quality OmegaCAM photometry of the 3x3 deg around the Orion Nebula Cluster (ONC) in r, and i filters by Beccari et al.(2017) revealed three well-separated pre-main sequences in the color-magnitude diagram (CMD). The objects belonging to the individual sequences are concentrated towards the center of the ONC. The authors concluded that there are two competitive scenarios: a population of unresolved binaries and triples with an exotic mass ratio distribution, or three stellar populations with different ages. We use Gaia DR2 in combination with the photometric OmegaCAM catalog to test and confirm the presence of the putative three stellar populations. We also study multiple stellar systems in the ONC for the first time using Gaia DR2. We confirm that the second and third sequence members are more centrally concentrated towards the center of the ONC. In addition we find an indication that the parallax and proper motion distributions are different among the members of the stellar sequences. The age difference among stellar populations is estimated to be 1-2 Myr. We use Gaia measurements to identify and remove as many unresolved multiple system candidates as possible. Nevertheless we are still able to recover two well-separated sequences with evidence for the third one, supporting the existence of the three stellar populations. We were able to identify a substantial number of wide binary objects (separation between 1000-3000 au). This challenges previously inferred values that suggested no wide binary stars exist in the ONC. Our inferred wide-binary fraction is approx 5%. We confirm the three populations correspond to three separated episodes of star formation. Based on this result, we conclude that star formation is not happening in a single burst in this region. (abridged)Comment: Astronomy and Astrophysics (A&A) accepted. 12 pages, 9 figures + appendix. New version with language corrections and new ID values in Tab.A.

Spectroscopic signatures of magnetospheric accretion in Herbig Ae/Be stars. I. The case of HD101412

Models of magnetically-driven accretion and outflows reproduce many observational properties of T Tauri stars. This concept is not well established for the more massive Herbig Ae/Be stars. We intend to examine the magnetospheric accretion in Herbig Ae/Be stars and search for rotational modulation using spectroscopic signatures, in this first paper concentrating on the well-studied Herbig Ae star HD101412. We used near-infrared spectroscopic observations of the magnetic Herbig Ae star HD101412 to test the magnetospheric character of its accretion disk/star interaction. We reduced and analyzed 30 spectra of HD101412, acquired with the CRIRES and X-shooter spectrographs installed at the VLT (ESO, Chile). The spectroscopic analysis was based on the He I lambda 10,830 and Pa gamma lines, formed in the accretion region. We found that the temporal behavior of these diagnostic lines in the near-infrared spectra of HD101412 can be explained by rotational modulation of line profiles generated by accreting gas with a period P = 20.53+-1.68 d. The discovery of this period, about half of the magnetic rotation period P_m = 42.076 d previously determined from measurements of the mean longitudinal magnetic field, indicates that the accreted matter falls onto the star in regions close to the magnetic poles intersecting the line-of-sight two times during the rotation cycle. We intend to apply this method to a larger sample of Herbig Ae/Be stars.Comment: 8 pages, 1 table, 7 figures, accepted for publication in A&

The nearby eclipsing stellar system delta Velorum - I. Origin of the infrared excess from VISIR and NACO imaging

- Context: The triple stellar system delta Vel system presents a significant infrared excess, whose origin is still being debated. A large infrared bow shock has been discovered using Spitzer/MIPS observations. Although it appears as a significant contributor to the measured IR excess, the possibility exists that a circumstellar IR excess is present around the stars of the system. - Aims: The objective of the present VISIR and NACO observations is to identify whether one of the stars of the delta Vel system presents a circumstellar photometric excess in the thermal IR domain and to quantify it. - Methods: We observed delta Vel using the imaging modes of the ESO/VLT instruments VISIR (in BURST mode) and NACO to resolve the A-B system (0.6" separation) and obtain the photometry of each star. We also obtained one NACO photometry epoch precisely at the primary (annular) eclipse of delta Vel Aa by Ab. - Results: Our photometric measurements with NACO (2.17 mic), complemented by the existing visible photometry allowed us to reconstruct the spectral energy distribution of the three stars. We then compared the VISIR photometry (8.6-12.8 mic) to the expected photospheric emission from the three stars at the corresponding wavelengths. - Conclusions: We can exclude the presence of a circumstellar thermal infrared excess around delta Vel A or B down to a few percent level. This supports the conclusions of Gaspar et al. (2008) that the IR excess of delta Vel has an interstellar origin, although a cold circumstellar disk could still be present. In addition, we derive the spectral types of the three stars Aa, Ab, and B (respectively A2IV, A4V and F8V), and we estimate the age of the system around 400-500 Myr.Comment: 8 pages, 9 figures, A&A, in pres

VLTI monitoring of the dust formation event of the Nova V1280 Scorpii

This is the author accepted manuscript. The final version is available from EDP Sciences via the DOI in this record.Context. We present the first high spatial-resolution monitoring of the dust-forming nova V1280 Sco, performed with the Very Large Telescope Interferometer (VLTI). Aims. These observations promise to improve the distance determination of such events and constrain the mechanisms leading to very efficient dust formation under the harsh physical conditions encountered in novae ejecta. Methods. Spectra and visibilities were regularly acquired between the onset of dust formation, 23 days after discovery (or 11 days after maximum), and day 145, using the beam-combiner instruments AMBER (near-IR) and MIDI (mid-IR). These interferometric observations were complemented by near-infrared data from the 1.2 m Mt. Abu Infrared Observatory, India. The observations are initially interpreted in terms of simple uniform models; however more complex models, probably involving a second shell, are required to explain data acquired following t = 110 d after outburst. This behavior is in accordance with the light curve of V1280 Sco, which exhibits a secondary peak at about t = 106 d, followed by a new, steep decline, suggesting a new dust-forming event. Spherical dust shell models generated with the DUSTY code are used to investigate the parameters of the main dust shell. Results. Using uniform disk models, these observations allow us to determine an apparent linear expansion rate for the dust shell of 0.35 ± 0.03 mas day−1 and the approximate ejection time of the matter in which dust formed of tejec = 10.5 ± 7 d, i.e. close to the maximum brightness. This information, combined with the expansion velocity of 500 ± 100 km s−1, implies a distance estimate of 1.6 ± 0.4 kpc. The sparse uv coverage does not enable deviations from spherical symmetry to be clearly discerned. The dust envelope parameters were determined. The dust mass generated was typically 2–8 × 10−9 M day−1, with a probable peak in production at about 20 days after the detection of dust and another peak shortly after t = 110 d, when the amount of dust in the shell was estimated as 2.2 × 10−7 M. Considering that the dust-forming event lasted at least 200–250 d, the mass of the ejected material is likely to have exceeded 10−4 M. The conditions for the formation of multiple shells of dust are also discussed. K

The VMC Survey. XXXII. Pre-main-sequence populations in the Large Magellanic Cloud

Context Detailed studies of intermediate- and low-mass pre-main-sequence (PMS) stars outside the Galaxy have so far been conducted only for small targeted regions harbouring known star formation complexes. The VISTA Survey of the Magellanic Clouds (VMC) provides an opportunity to study PMS populations down to solar masses on a galaxy-wide scale. Aims Our goal is to use near-infrared data from the VMC survey to identify and characterise PMS populations down to ∼ 1 M� across the Magellanic Clouds. We present our colour−magnitude diagram method, and apply it to a ∼ 1.5 deg2 pilot field located in the Large Magellanic Cloud. Methods The pilot field is divided into equal-size grid elements. We compare the stellar population in every element with the population in nearby control fields by creating Ks/(Y−Ks) Hess diagrams; the observed density excesses over the local field population are used to classify the stellar populations. Results Our analysis recovers all known star formation complexes in this pilot field (N 44, N 51, N 148, and N 138) and for the first time reveals their true spatial extent. In total, around 2260 PMS candidates with ages . 10 Myr are found in the pilot field. PMS structures, identified as areas with a significant density excess of PMS candidates, display a power-law distribution of the number of members with a slope of −0.86 ± 0.12. We find a clustering of the young stellar populations along ridges and filaments where dust emission in the far-infrared (FIR) (70 µm – 500 µm) is bright. Regions with young populations lacking massive stars show a lower degree of clustering and are usually located in the outskirts of the star formation complexes. At short FIR wavelengths (70 µm, 100 µm) we report a strong dust emission increase in regions hosting young massive stars, which is less pronounced in regions populated only by less massive (. 4 M�) PMS stars

The circumstellar environment of the YSO TMR-1 and a revisit to the candidate very low-mass object TMR-1C

TMR-1 (IRAS~04361+2547) is a class~I proto-stellar source located in the nearby Taurus star-forming region. Its circumstellar environment is characterized by extended dust emission with complex structures and conspicuous filaments. A faint companion, called TMR-1C, located near the proto-star had been detected in previous studies, but its nature as a very young substellar object remained inconclusive. To improve the constraints on the nature of TMR-1C, and to investigate the process of very low-mass star formation in the TMR-1 system we use very sensitive infrared imaging observations as well as NIR spectroscopy. We construct the SED of TMR-1C over a much larger wavelength range as had been possible in previous work and compare it with models of extincted background stars, young sub-stellar objects, and very low-mass stars with circumstellar disk and envelope emission. We also search for additional low-luminosity objects in the immediate environment of the TMR-1, study the surrounding NIR dust morphology, and analyse the emission line spectrum of a filamentary structure in the physical context of a bow-shock model. We find that the observed SED of TMR-1C is inconsistent with an extincted background star, nor can be fitted with available models for a young extremely low-mass (<12M_Jup) object. Our near-IR spectrum indicates an effective temperature of at least ~3000K. Based on a good match of TMR-1C's SED with radiation transfer models of young stellar objects with circumstellar disks, we propose that TMR-1C is most likely a very low-mass star with M~0.1-0.2M_sun surrounded by a circumstellar disk with high inclination, i>80deg. Moreover, we detect an additional very faint source, which we call TMR-1D, and that shows a quite striking symmetry in position with TMR-1C. TMR-1C and TMR-1D may have been formed from a common triggered star-formation event, caused by... (abstract abridged)Comment: 15 pages, 11 figures, accepted for publication in A&

The VMC survey XXVIII. Improved measurements of the proper motion of the Galactic globular cluster 47 Tucanae

We use deep multi-epoch point-spread function (PSF) photometry taken with the Visible and Infrared Survey Telescope for Astronomy (VISTA) to measure and analyze the proper motions of stars within the Galactic globular cluster 47 Tucanae (47 Tuc, NGC 104). The observations are part of the ongoing near-infrared VISTA survey of the Magellanic Cloud system (VMC). The data analyzed in this study correspond to one VMC tile, which covers a total sky area of 1.77 deg(2). Absolute proper motions with respect to similar to 9070 background galaxies are calculated from a linear regression model applied to the positions of stars in 11 epochs in the K-s filter. The data extend over a total time baseline of about 17 months. We found an overall median proper motion of the stars within 47 Tuc of (mu alpha cos(delta), mu(delta)) = (+5.89 +/- 0.02 (statistical) +/- 0.13 (systematic), -2.14 +/- 0.02 (statistical) +/- 0.08 (systematic)) mas yr(-1), based on the measurements of similar to 35 000 individual sources between 5’ and 42’ from the cluster center. We compared our result to the proper motions from the newest US Naval Observatory CCD Astrograph Catalog (UCAC5), which includes data from the Gaia data release 1. Selecting cluster members (similar to 2700 stars), we found a median proper motion of (mu(alpha)cos(delta), mu(delta)) = (+5.30 +/- 0.03 (statistical) +/- 0.70 (systematic), -2.70 +/- 0.03 (statistical) +/- 0.70 (systematic)) mas yr(-1). Comparing the results with measurements in the literature, we found that the values derived from the VMC data are consistent with the UCAC5 result, and are close to measurements obtained using the Hubble Space Telescope. We combined our proper motion results with radial velocity measurements from the literature and reconstructed the orbit of 47 Tuc, finding that the cluster is on an orbit with a low ellipticity and is confined within the inner similar to 7.5 kpc of the Galaxy. We show that the use of an increased time baseline in combination with PSF-determined stellar centroids in crowded regions significantly improves the accuracy of the method. In future works, we will apply the methods described here to more VMC tiles to study in detail the kinematics of the Magellanic Clouds

Weak Magnetic Fields in Two Herbig Ae Systems: The SB2 AK Sco and the Presumed Binary HD 95881

We report the detection of weak mean longitudinal magnetic fields in the Herbig Ae double-lined spectroscopic binary AK Sco and in the presumed spectroscopic Herbig Ae binary HD95881 using observations with the High Accuracy Radial velocity Planet Searcher polarimeter (HARPSpol) attached to the European Southern Observatory's (ESO's) 3.6m telescope. Employing a multi-line singular value decomposition (SVD) method, we detect a mean longitudinal magnetic field =-83+-31G in the secondary component of AK Sco on one occasion. For HD95881, we measure =-93+-25G and =105+-29G at two different observing epochs. For all the detections the false alarm probability is smaller than 10^-5. For AK Sco system, we discover that accretion diagnostic Na I doublet lines and photospheric lines show intensity variations over the observing nights. The double-lined spectral appearance of HD95881 is presented here for the first time