arXiv subjects

C. F. Manara

Publications and source records attributed to C. F. Manara.

At least 91 records · Page 5Linked to original sources

arXiv2018

Variable Outer Disk Shadowing Around the Dipper Star RX J1604.3-2130

Low brightness dips have been recently observed in images of protoplanetary disks, and they are believed to be shadows by the inner disk. We present VLT/SPHERE polarimetric differential imaging of the transition disk around the dipper star RX J1604.3-2130. We gathered 11 epochs that cover a large temporal baseline, to search for variability over timescales of years, months, weeks, and days. Our observations unambiguously reveal two dips along an almost face-on narrow ring (with a width of $\sim$20 au), and the location of the peak of this ring is at $\sim$65 au. The ring lies inside the ring-like structure observed with ALMA, which peaks at $\sim$83 au. This segregation can result from particle trapping in pressure bumps, potentially due to planet(s). We find that the dips are variable, both in morphology and in position. The eastern dip, at a position angle (PA) of $\sim$83.7$\pm$13.7$^\circ$, has an amplitude that varies between 40% to 90%, and its angular width varies from 10$^{\circ}$ to 34$^{\circ}$. The western dip, at a PA of $\sim$265.90$\pm$13.0$^\circ$, is more variable, with amplitude and width variations of 31% to 95% and 12$^{\circ}$ to 53$^{\circ}$, respectively. The separation between the dips is 178.3$^{\circ}\pm$14.5$^{\circ}$, corresponding to a large misalignment between the inner and outer disk, supporting the classification of J1604 as an aperiodic dipper. The variability indicates that the innermost regions are highly dynamic, possibly due to a massive companion or to a complex magnetic field topology.

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arXiv2018

The ALMA Lupus protoplanetary disk survey: evidence for compact gas disks and molecular rings from CN

The cyanide radical CN is abundant in protoplanetary disks, with line fluxes often comparable to those of $^{13}$CO. It is known to be sensitive to UV irradiation of the upper disk atmosphere, with models predicting ring-shaped emission. We seek to characterize the CN emission from 94 Class II disks in the Lupus star forming region, compare it to observations in other regions, and interpret our observations with a grid of models. The CN emission morphology is discussed for two primordial disks, Sz 71 and Sz 98, and modeled in more detail. ALMA observed CN N=3-2 in Lupus disks down to sensitivities better than previous surveys. Models constructed with the physico-chemical code DALI are used to study the integrated fluxes of the disks and resolved emission of CN in disks without (dust) substructures. We find that CN N=3-2 is bright, and detected in 38% of sources, but its disk-integrated flux is not strongly correlated to either $^{13}$CO or continuum flux. Compared to pre-ALMA single-dish surveys, no significant difference in the CN flux distributions in Lupus and Taurus-Auriga is found, although $ρ$ Ophiuchus disks may be fainter on average. We find ring-shaped CN emission with peak radii of ~50 AU in two resolved disks. We conclude that a large fraction of sources is faint in CN; only exponential gas surface density cutoffs at $R_{c} \leq 15$ AU can reconcile observations with models. This is the first observational evidence of such a compact gas disk population in Lupus. Absolute intensities and the emission morphology of CN are reproduced by DALI models without the need for any continuum substructure; they are unrelated to the CO snowline location. These observations and the successful modeling of these rings provide a new probe of the structure and conditions in disks, and particularly their incident UV radiation field, if disk size is determined by the data.

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arXiv2018

Why do protoplanetary disks appear not massive enough to form the known exoplanet population?

When and how planets form in protoplanetary disks is still a topic of discussion. Exoplanet detection surveys and protoplanetary disk surveys are now providing results that allow us to have new insights. We collect the masses of confirmed exoplanets and compare their dependence with stellar mass with the same dependence for protoplanetary disk masses measured in ~1-3 Myr old star-forming regions. The latter are recalculated by us using the new estimates of their distances derived from Gaia DR2 parallaxes. We note that single and multiple exoplanetary systems form two different populations, probably pointing to a different formation mechanism for massive giant planets around very low mass stars. While expecting that the mass in exoplanetary systems is much lower than the measured disk masses, we instead find that exoplanetary systems masses are comparable or higher than the most massive disks. This same result is found also by converting the measured planet masses into heavy-element content (core masses for the giant planets and full masses for the super-Earth systems) and by comparing this value with the disk dust masses. Unless disk dust masses are heavily underestimated, this is a big conundrum. An extremely efficient recycling of dust particles in the disk cannot solve this conundrum. This implies that either the cores of planets have formed very rapidly (<0.1-1 Myr) and large amount of gas is expelled on the same timescales from the disk, or that disks are continuously replenished of fresh planet-forming material from the environment. These hypotheses can be tested by measuring disk masses in even younger targets and by better understanding if and how the disks are replenished by their surroundings.

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arXiv2018

Gaia DR2 view of the Lupus V-VI clouds: the candidate diskless young stellar objects are mainly background contaminants

Extensive surveys of star-forming regions with Spitzer have revealed populations of disk-bearing young stellar objects. These have provided crucial constraints, such as the timescale of dispersal of protoplanetary disks, obtained by carefully combining infrared data with spectroscopic or X-ray data. While observations in various regions agree with the general trend of decreasing disk fraction with age, the Lupus V and VI regions appeared to have been at odds, having an extremely low disk fraction. Here we show, using the recent Gaia data release 2 (DR2), that these extremely low disk fractions are actually due to a very high contamination by background giants. Out of the 83 candidate young stellar objects (YSOs) in these clouds observed by Gaia, only five have distances of 150 pc, similar to YSOs in the other Lupus clouds, and have similar proper motions to other members in this star-forming complex. Of these five targets, four have optically thick (Class II) disks. On the one hand, this result resolves the conundrum of the puzzling low disk fraction in these clouds, while, on the other hand, it further clarifies the need to confirm the Spitzer selected diskless population with other tracers, especially in regions at low galactic latitude like Lupus V and VI. The use of Gaia astrometry is now an independent and reliable way to further assess the membership of candidate YSOs in these, and potentially other, star-forming regions.

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arXiv2018

Digitization and astrometric calibration of Carte du Ciel photographic plates with Gaia~DR1

We want to study whether the astrometric and photometric accuracies obtained for the Carte du Ciel plates digitized with a commercial digital camera are high enough for scientific exploitation of the plates. We use a digital camera Canon EOS~5Ds, with a 100mm macrolens for digitizing. We analyze six single-exposure plates and four triple-exposure plates from the Helsinki zone of Carte du Ciel (+39 degr < delta < 47 degr). Each plate is digitized using four images, with a significant central area being covered twice for quality control purposes. The astrometric calibration of the digitized images is done with the data from the Gaia TGAS (Tycho-Gaia Astrometric Solution) of the first Gaia data release (Gaia DR1), Tycho-2, HSOY (Hot Stuff for One Year), UCAC5 (USNO CCD Astrograph Catalog), and PMA catalogs. The best astrometric accuracy is obtained with the UCAC5 reference stars. The astrometric accuracy for single-exposure plates is sigma(R.A.)=0.16" and sigma(Dec.)=0.15" expressed as a Gaussian deviation of the astrometric residuals. For triple-exposure plates the astrometric accuracy is sigma(R.A.)=0.12" and sigma(Dec.)=0.13". The 1-sigma uncertainty of photometric calibration is about 0.28 mag and 0.24 mag for single- and triple-exposure plates, respectively. We detect the photographic adjacency (Kostinsky) effect in the triple-exposure plates. We show that accuracies at least of the level of scanning machines can be achieved with a digital camera, without any corrections for possible distortions caused by our instrumental setup. This method can be used to rapidly and inexpensively digitize and calibrate old photographic plates enabling their scientific exploitation.

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arXiv2018

V1094 Sco: a rare giant multi-ringed disk around a T Tauri star

A wide variety of ring-like dust structures has been detected in protoplanetary disks, but their origin and frequency are still unclear. We characterize the structure of an extended, multi-ringed disk discovered serendipitously in the ALMA Lupus disk survey and put it in the context of the Lupus disk population. ALMA observations in Band 6 at 234 GHz and Band 7 at 328 GHz at 0.3" resolution toward the K6 star V1094 Sco in Lupus III are presented, and its disk structure is analyzed. The spectral index $α_{mm}$ is determined in the inner 150 AU of the disk. The ALMA continuum data show a very extended disk with two gap/ring pairs. The gaps are located at 100 AU and 170 AU, the bright rings at 130 AU and 220 AU. Continuum emission is detected out to a 300 AU distance, similar to IM Lup but a factor of 5 larger than typically found for Lupus disks at this sensitivity and resolution. The bright central region of the disk (within 35 AU) is possibly optically thick at 1 mm wavelengths, and has a brightness temperature of only 13 K. The spectral index increases between the inner disk and the first ring, at the location of the first gap. Due to the low temperature of the disk midplane, snow lines can be excluded as the drivers behind the ring and gap formation in this disk. Disks the size of V1094 Sco are rare, and only 2.1+-1.5% of disks in Lupus show continuum emission beyond 200 AU. Possible connections between the large primordial disk population, transition disks, and exoplanets are discussed.

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arXiv2018

First direct detection of a polarized companion outside of a resolved circumbinary disk around CS Cha

In the present study we aim to investigate the circumstellar environment of the spectroscopic binary T Tauri star CS Cha. From unresolved mid- to far-infrared photometry it is predicted that CS Cha hosts a disk with a large cavity. In addition, SED modeling suggests significant dust settling, pointing towards an evolved disk that may show signs of ongoing or completed planet formation. We observed CS Cha with the high contrast imager VLT/SPHERE in polarimetric differential imaging mode to resolve the circumbinary disk in near infrared scattered light. These observations were followed-up by VLT/NACO L-band observations and complemented by archival VLT/NACO K-band and HST/WFPC2 I-band data. We resolve the compact circumbinary disk around CS Cha for the first time in scattered light. We find a smooth, low inclination disk with an outer radius of $\sim$55 au (at 165 pc). We do not detect the inner cavity but find an upper limit for the cavity size of $\sim$15 au. Furthermore, we find a faint co-moving companion with a projected separation of 210 au from the central binary outside of the circumbinary disk. The companion is detected in polarized light and shows an extreme degree of polarization (13.7$\pm$0.4 \% in J-band). The companion's J- and H-band magnitudes are compatible with masses of a few M$_\mathrm{Jup}$. However, K-, L- and I-band data draw this conclusion into question. We explore with radiative transfer modeling whether an unresolved circum-companion disk can be responsible for the high polarization and complex photometry. We find that the set of observations is best explained by a heavily extincted low mass ($\sim 20 \mathrm{M}_\mathrm{Jup}$) brown dwarf or high mass planet with an unresolved disk and dust envelope.

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arXiv2018

Homogeneous Analysis of the Dust Morphology of Transition Disks Observed with ALMA: Investigating dust trapping and the origin of the cavities

We analyze the dust morphology of 29 transition disks (TDs) observed with ALMA at (sub-) millimeter-emission. We perform the analysis in the visibility plane to characterize the total flux, cavity size, and shape of the ring-like structure. First, we found that the $M_{\rm{dust}}-M_\star$ relation is much flatter for TDs than the observed trends from samples of class II sources in different star forming regions. This relation demonstrates that cavities open in high (dust) mass disks, independent of the stellar mass. The flatness of this relation contradicts the idea that TDs are a more evolved set of disks. Two potential reasons (not mutually exclusive) may explain this flat relation: the emission is optically thick or/and millimeter-sized particles are trapped in a pressure bump. Second, we discuss our results of the cavity size and ring width in the context of different physical processes for cavity formation. Photoevaporation is an unlikely leading mechanism for the origin of the cavity of any of the targets in the sample. Embedded giant planets or dead zones remain as potential explanations. Although both models predict correlations between the cavity size and the ring shape for different stellar and disk properties, we demonstrate that with the current resolution of the observations, it is difficult to obtain these correlations. Future observations with higher angular resolution observations of TDs with ALMA will help to discern between different potential origins of cavities in TDs.

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arXiv2018

ALMA Survey of Lupus Protoplanetary Disks II: Gas Disk Radii

We present ALMA Band 6 observations of a complete sample of protoplanetary disks in the young (1-3 Myr) Lupus star-forming region, covering the 1.33 mm continuum and the 12CO, 13CO, and C18O J=2-1 lines. The spatial resolution is 0.25 arcsec with a medium 3-sigma continuum sensitivity of 0.30 mJy, corresponding to M_dust ~ 0.2 M_earth. We apply "Keplerian masking" to enhance the signal-to-noise ratios of our 12CO zero-moment maps, enabling measurements of gas disk radii for 22 Lupus disks; we find that gas disks are universally larger than mm dust disks by a factor of two on average, likely due to a combination of the optically thick gas emission as well as the growth and inward drift of the dust. Using the gas disk radii, we calculate the dimensionless viscosity parameter, alpha_visc, finding a broad distribution and no correlations with other disk or stellar parameters, suggesting that viscous processes have not yet established quasi-steady states in Lupus disks. By combining our 1.33 mm continuum fluxes with our previous 890 micron continuum observations, we also calculate the mm spectral index, alpha_mm, for 70 Lupus disks; we find an anti-correlation between alpha_mm and mm flux for low-mass disks (M_dust < 5), followed by a flattening as disks approach alpha_mm = 2, which could indicate faster grain growth in higher-mass disks, but may also reflect their larger optically thick components. In sum, this work demonstrates the continuous stream of new insights into disk evolution and planet formation that can be gleaned from unbiased ALMA disk surveys.

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arXiv2018

A UV-to-NIR Study of Molecular Gas in the Dust Cavity Around RY Lupi

We present a study of molecular gas in the inner disk $\left(r < 20 \, \text{AU} \right)$ around RY Lupi, with spectra from HST-COS, HST-STIS, and VLT-CRIRES. We model the radial distribution of flux from hot gas in a surface layer between $r = 0.1-10$ AU, as traced by Ly$α$-pumped H$_2$. The result shows H$_2$ emission originating in a ring centered at $\sim$3 AU that declines within $r < 0.1$ AU, which is consistent with the behavior of disks with dust cavities. An analysis of the H$_2$ line shapes shows that a two-component Gaussian profile $\left(\text{FWHM}_{broad, H_2} = 105 \pm 15 \, \text{km s}^{-1}; \, \text{FWHM}_{narrow, H_2} = 43 \pm 13 \, \text{km s}^{-1} \right)$ is statistically preferred to a single-component Gaussian. We interpret this as tentative evidence for gas emitting from radially separated disk regions $\left( \left \langle r_{broad, H_2} \right \rangle \sim 0.4 \pm 0.1 \, \text{AU}; \, \left \langle r_{narrow, H_2} \right \rangle \sim 3 \pm 2 \, \text{AU} \right)$. The 4.7 $μ$m $^{12}$CO emission lines are also well fit by two-component profiles $\left( \left \langle r_{broad, CO} \right \rangle = 0.4 \pm 0.1 \, \text{AU}; \, \left \langle r_{narrow, CO} \right \rangle = 15 \pm 2 \, \text{AU} \right)$. We combine these results with 10 $μ$m observations to form a picture of gapped structure within the mm-imaged dust cavity, providing the first such overview of the inner regions of a young disk. The HST SED of RY Lupi is available online for use in modeling efforts.

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arXiv2017

Mapping young stellar populations towards Orion with Gaia DR1

We use the first data release of the Gaia mission to explore the three dimensional arrangement and the age ordering of the many stellar groups towards the Orion OB association, aiming at a new classification and characterization of the stellar population. We make use of the parallaxes and proper motions provided in the Tycho Gaia Astrometric Solution (TGAS) sub-set of the Gaia catalogue, and of the combination of Gaia and 2MASS photometry. In TGAS we find evidence for the presence of a young population, at a parallax $\varpi \sim 2.65 \, \mathrm{mas}$, loosely distributed around some known clusters: 25 Ori, $ε$ Ori and $σ$ Ori, and NGC 1980 ($ι$ Ori). The low mass counterpart of this population is visible in the color-magnitude diagrams constructed by combining Gaia and 2MASS photometry. We study the density distribution of the young sources in the sky. We find the same groups as in TGAS, and also some other density enhancements that might be related to the recently discovered Orion X group, the Orion dust ring, and to the $λ$ Ori complex. We estimate the ages of this population and we infer the presence of an age gradient going from 25 Ori (13-15 Myr) to the ONC (1-2 Myr). We confirm this age ordering by repeating the Bayesian fit using the Pan-STARRS1 data. The estimated ages towards the NGC 1980 cluster span a broad range of values. This can either be due to the presence of two populations coming from two different episodes of star formation or to a large spread along the line of sight of the same population. Our results form the first step towards using the Gaia data to unravel the complex star formation history of the Orion region in terms of the different star formation episodes, their duration, and their effects on the surrounding interstellar medium.

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arXiv2017

Connection between jets, winds and accretion in T Tauri stars: the X-shooter view

We have analysed the [OI]6300 A line in a sample of 131 young stars with discs in the Lupus, Chamaeleon and signa Orionis star forming regions, observed with the X-shooter spectrograph at VLT. The stars have mass accretion rates spanning from 10^{-12} to 10^{-7} Mo/yr. The line profile was deconvolved into a low velocity component (LVC, < 40 km/s) and a high velocity component (HVC, > 40 km/s ), originating from slow winds and high velocity jets, respectively. The LVC is by far the most frequent component, with a detection rate of 77%, while only 30% of sources have a HVC. The [OI]6300 luminosity of both the LVC and HVC, when detected, correlates with stellar and accretion parameters of the central sources (i.e. Lstar , Mstar , Lacc , Macc), with similar slopes for the two components. The line luminosity correlates better with the accretion luminosity than with the stellar luminosity or stellar mass. We suggest that accretion is the main drivers for the line excitation and that MHD disc-winds are at the origin of both components. In the sub-sample of Lupus sources observed with ALMA a relationship is found between the HVC peak velocity and the outer disc inclination angle, as expected if the HVC traces jets ejected perpendicularly to the disc plane. Mass loss rates measured from the HVC span from ~ 10^{-13} to ~10^{-7} Mo/yr. The corresponding Mloss/Macc ratio ranges from ~0.01 to ~0.5, with an average value of 0.07. However, considering the upper limits on the HVC, we infer a ratio < 0.03 in more than 40% of sources. We argue that most of these sources might lack the physical conditions needed for an efficient magneto-centrifugal acceleration in the star-disc interaction region. Systematic observations of populations of younger stars, that is, class 0/I, are needed to explore how the frequency and role of jets evolve during the pre-main sequence phase.

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arXiv2017

Gaia view of low-mass star formation

Understanding how young stars and their circumstellar disks form and evolve is key to explain how planets form. The evolution of the star and the disk is regulated by different processes, both internal to the system or related to their environment. The former include accretion of material onto the central star, wind emission, and photoevaporation of the disk due to high-energy radiation from the central star. These are best studied spectroscopically, and the distance to the star is a key parameter in all these studies. Here we present new estimates of the distance to a complex of nearby star-forming clouds obtained combining TGAS distances with measurement of extinction on the line of sight. Furthermore, we show how we plan to study the effects of the environment on the evolution of disks with Gaia, using a kinematic modelling code we have developed to model young star-forming regions.

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arXiv2017

Physical properties of dusty protoplanetary disks in Lupus: evidence for viscous evolution?

The formation of planets strongly depends on the total amount as well as on the spatial distribution of solids in protoplanetary disks. Thanks to the improvements in resolution and sensitivity provided by ALMA, measurements of the surface density of mm-sized grains are now possible on large samples of disks. Such measurements provide statistical constraints that can be used to inform our understanding of the initial conditions of planet formation. We analyze spatially resolved observations of 36 protoplanetary disks in the Lupus star forming complex from our ALMA survey at 890 micron, aiming to determine physical properties such as the dust surface density, the disk mass and size and to provide a constraint on the temperature profile. We fit the observations directly in the uv-plane using a two-layer disk model that computes the 890 micron emission by solving the energy balance at each disk radius. For 22 out of 36 protoplanetary disks we derive robust estimates of their physical properties. The sample covers stellar masses between ~0.1 and ~2 Solar masses, and we find no trend between the average disk temperatures and the stellar parameters. We find, instead, a correlation between the integrated sub-mm flux (a proxy for the disk mass) and the exponential cut-off radii (a proxy of the disk size) of the Lupus disks. Comparing these results with observations at similar angular resolution of Taurus-Auriga/Ophiuchus disks found in literature and scaling them to the same distance, we observe that the Lupus disks are generally fainter and larger at a high level of statistical significance. Considering the 1-2 Myr age difference between these regions, it is possible to tentatively explain the offset in the disk mass/disk size relation with viscous spreading, however with the current measurements other mechanisms cannot be ruled out.

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arXiv2017

X-shooter spectroscopy of young stellar objects in Lupus: Lithium, iron, and barium elemental abundances

With the purpose of performing a homogeneous determination of elemental abundances for members of the Lupus T association, we analyzed three chemical elements: lithium, iron, and barium. The aims were: to derive the Li abundance for ~90% of known class II stars in the Lupus I, II, III, IV clouds; to perform chemical tagging of a region where few Fe abundance measurements have been obtained in the past, and no determination of the Ba content has been done up to now. We also investigated possible Ba enhancement, as this element has become increasingly interesting in the last years following the evidence of Ba over-abundance in young clusters, the origin of which is still unknown. Using X-shooter@VLT, we analyzed the spectra of 89 cluster members, both class II and III stars. We measured the strength of the Li line and derived the abundance of this element through equivalent width measurements and curves of growth. For six class II stars we also measured the Fe and Ba abundances using the spectral synthesis and the code MOOG. The veiling contribution was taken into account for all three elements. We find a dispersion in the strength of the Li line at low Teff and identify three targets with severe Li depletion. The nuclear age inferred for these highly Li-depleted stars is around 15 Myr, which exceeds the isochronal one. As in other star-forming regions, no metal-rich members are found in Lupus, giving support to a recent hypothesis that the Fe abundance distribution of most of the nearby young regions could be the result of a common and widespread star formation episode involving the Galactic thin disk. We find that Ba is over-abundant by ~0.7 dex with respect to the Sun. Since current theoretical models cannot reproduce this Ba abundance pattern, we investigated whether this unusually large Ba content might be related to effects due to stellar parameters, stellar activity, and accretion.

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arXiv2017

X-Shooter study of accretion in Chamaeleon I: II. A steeper increase of accretion with stellar mass for very low mass stars?

The dependence of the mass accretion rate on the stellar properties is a key constraint for star formation and disk evolution studies. Here we present a study of a sample of stars in the Chamaeleon I star forming region carried out using the VLT/X-Shooter spectrograph. The sample is nearly complete down to M~0.1Msun for the young stars still harboring a disk in this region. We derive the stellar and accretion parameters using a self-consistent method to fit the broad-band flux-calibrated medium resolution spectrum. The correlation between the accretion luminosity to the stellar luminosity, and of the mass accretion rate to the stellar mass in the logarithmic plane yields slopes of 1.9 and 2.3, respectively. These slopes and the accretion rates are consistent with previous results in various star forming regions and with different theoretical frameworks. However, we find that a broken power-law fit, with a steeper slope for stellar luminosity smaller than ~0.45 Lsun and for stellar masses smaller than ~ 0.3 Msun, is slightly preferred according to different statistical tests, but the single power-law model is not excluded. The steeper relation for lower mass stars can be interpreted as a faster evolution in the past for accretion in disks around these objects, or as different accretion regimes in different stellar mass ranges. Finally, we find two regions on the mass accretion versus stellar mass plane empty of objects. One at high mass accretion rates and low stellar masses, which is related to the steeper dependence of the two parameters we derived. The second one is just above the observational limits imposed by chromospheric emission. This empty region is located at M~0.3-0.4Msun, typical masses where photoevaporation is known to be effective, and at mass accretion rates ~10^-10 Msun/yr, a value compatible with the one expected for photoevaporation to rapidly dissipate the inner disk.

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arXiv2017

An extensive VLT/X-Shooter library of photospheric templates of pre-main sequence stars

Studies of the formation and evolution of young stars and their disks rely on the knowledge of the stellar parameters of the young stars. The derivation of these parameters is commonly based on comparison with photospheric template spectra. Furthermore, chromospheric emission in young active stars impacts the measurement of mass accretion rates, a key quantity to study disk evolution. Here we derive stellar properties of low-mass pre-main sequence stars without disks, which represent ideal photospheric templates for studies of young stars. We also use these spectra to constrain the impact of chromospheric emission on the measurements of mass accretion rates. The spectra in reduced, flux-calibrated, and corrected for telluric absorption form are made available to the community. We derive the spectral type for our targets by analyzing the photospheric molecular features present in their VLT/X-Shooter spectra by means of spectral indices and comparison of the relative strength of photospheric absorption features. We also measure effective temperature, gravity, projected rotational velocity, and radial velocity from our spectra by fitting them with synthetic spectra with the ROTFIT tool. The targets have negligible extinction and spectral type from G5 to M8. We perform synthetic photometry on the spectra to derive the typical colors of young stars in different filters. We measure the luminosity of the emission lines present in the spectra and estimate the noise due to chromospheric emission in the measurements of accretion luminosity in accreting stars. We provide a calibration of the photospheric colors of young PMS stars as a function of their spectral type in a set of standard broad-band optical and near-infrared filters. For stars with masses of ~ 1.5Msun and ages of ~1-5 Myr, the chromospheric noise converts to a limit of measurable mass accretion rates of ~ 3x10^-10 Msun/yr.

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arXiv2017

A Tale of Three Cities: OmegaCAM discovers multiple sequences in the color-magnitude diagram of the Orion Nebula Cluster

As part of the Accretion Discs in H$α$ with OmegaCAM (ADHOC) survey, we imaged in r, i and H-alpha a region of 12x8 square degrees around the Orion Nebula Cluster. Thanks to the high-quality photometry obtained, we discovered three well-separated pre-main sequences in the color-magnitude diagram. The populations are all concentrated towards the cluster's center. Although several explanations can be invoked to explain these sequences we are left with two competitive, but intriguing, scenarios: a population of unresolved binaries with an exotic mass ratio distribution or three populations with different ages. Independent high-resolution spectroscopy supports the presence of discrete episodes of star formation, each separated by about a million years. The stars from the two putative youngest populations rotate faster than the older ones, in agreement with the evolution of stellar rotation observed in pre-main sequence stars younger than 4 Myr in several star forming regions. Whatever the final explanation, our results prompt for a revised look at the formation mode and early evolution of stars in clusters.

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