No abstract

We present an analysis of spectrosopic and astrometric data from APOGEE-2 and Gaia DR2 to identify structures towards the Orion Complex. By applying a hierarchical clustering algorithm to the 6-dimensional stellar data, we identify spatially and/or kinematically distinct groups of young stellar objects with ages ranging from 1 to 12 Myr. We also investigate the star forming history within the Orion Complex, and identify peculiar sub-clusters. With this method we reconstruct the older populations in the regions that are presently largely devoid of molecular gas, such as Orion C (which includes the σ Ori cluster), and Orion D (the population that traces Ori OB1a, OB1b, and Orion X). We report on the distances, kinematics, and ages of the groups within the Complex. The Orion D groups is in the process of expanding. On the other hand, Orion B is still in the process of contraction. In λ Ori the proper motions are consistent with a radial expansion due to an explosion from a supernova; the traceback age from the expansion exceeds the age of the youngest stars formed near the outer edges of the region, and their formation would have been triggered when they were half-way from the cluster center to their current positions. We also present a comparison between the parallax and proper motion solutions obtained by Gaia DR2, and those obtained towards star-forming regions by Very Long Baseline Array.

The present observational understanding of the evolution of the mass accretion rates (Ṁ acc ) in premain sequence stars is limited by the lack of accurate measurements ofṀ acc over homogeneous and large statistical samples of young stars. Such observational effort is needed to properly constrain the theory of star formation and disk evolution. Based on HST/WFPC2 observations, we present a study ofṀ acc for a sample of ∼ 700 sources in the Orion Nebula Cluster, ranging from the Hydrogen-burning limit to M * ∼ 2M . We deriveṀ acc from both the U -band excess and the Hα luminosity (L Hα ), after determining empirically both the shape of the typical accretion spectrum across the Balmer jump and the relation between the accretion luminosity (L acc ) and L Hα , that is L acc /L = (1.31±0.03)·L Hα /L + (2.63± 0.13). Given our large statistical sample, we are able to accurately investigate relations betweenṀ acc and the parameters of the central star such as mass and age. We clearly findṀ acc to increase with stellar mass, and decrease over evolutionary time, but we also find strong evidence that the decay ofṀ acc with stellar age occurs over longer timescales for more massive PMS stars. Our best fit relation between these parameters is given by: log(Ṁ acc /M ·yr)=(-5.12 ± 0.86) -(0.46 ± 0.13) · log(t/yr) -(5.75 ± 1.47)· log(M * /M ) + (1.17 ± 0.23)· log(t/yr) · log(M * /M ). These results also suggest that the similarity solution model could be revised for sources with M * 0.5M . Finally, we do not find a clear trend indicating environmental effects on the accretion properties of the sources.

We present a new analysis of the stellar population of the Orion Nebula Cluster (ONC) based on multi-band optical photometry and spectroscopy. We study the color-color diagrams in BV I, plus a narrow-band filter centered at 6200Å, finding evidences that intrinsic color scales valid for main-sequence dwarfs are incompatible with the ONC in the M spectral type range, while a better agreement is found employing intrinsic colors derived from synthetic photometry, constraining the surface gravity value as predicted by a pre-main sequence isochrone. We refine these model colors even further, empirically, by comparison with a selected sample of ONC stars with no accretion and no extinction. We consider the stars with known spectral types from the literature, and extend this sample with the addition of 65 newly classified stars from slit spectroscopy and 182 M-type from narrow-band photometry; in this way we isolate a sample of about 1000 stars with known spectral type. We introduce a new method to self-consistently derive the stellar reddening and the optical excess due to accretion from the location of each star in the BV I color-color diagram. This enables us to accurately determine the extinction of the ONC members, together with an estimate of their accretion luminosities. We adopt a lower distance for the Orion Nebula than previously assumed, based on recent parallax measurements. With a careful choice also of the spectral type-temperature transformation, we produce the new Hertzsprung-Russell diagram of the ONC population, more populated than previous works. With respect to previous works, we find higher luminosity for late-type stars and a slightly lower luminosity for early types. We determine the age distribution of the population, peaking from ∼ 2 to ∼ 3 Myr depending on the model, a higher age than previously estimated. We study the distribution of the members in the mass-age plane, and find that taking into account selection effects due to incompleteness removes an apparent correlation between mass and age. We derive the IMF for low-and intermediate-mass members of the ONC, which turns out to be model-dependent, and shows a turn-over at M 0.2 M ⊙ .

We present a new census of the Orion Nebula Cluster over a large field of view ( 30 × 30 ), significantly increasing the known population of stellar and substellar cluster members with precisely determined properties. We develop and exploit a technique to determine stellar effective temperatures from optical colors, nearly doubling the previously available number of objects with effective temperature determinations in this benchmark cluster. Our technique utilizes colors from deep photometry in the I band and in two medium-band filters at λ ∼ 753 and 770 nm, which accurately measure the depth of a molecular feature present in the spectra of cool stars. From these colors we can derive effective temperatures with a precision corresponding to better than one-half spectral subtype, and importantly this precision is independent of the extinction to the individual stars. Also, because this technique utilizes only photometry redward of 750 nm, the results are only mildly sensitive to optical veiling produced by accretion. Completing our census with previously available data, we place some 1750 sources in the Hertzsprung-Russell diagram and assign masses and ages down to 0.02 solar masses. At faint luminosities, we detect a large population of background sources which is easily separated in our photometry from the bona fide cluster members. The resulting initial mass function of the cluster has good completeness well into the substellar mass range, and we find that it declines steeply with decreasing mass. This suggests a deficiency of newly formed brown dwarfs in the cluster compared to the Galactic disk population.