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Register a new userProtoplanetary and Transitional Disks in the Open Stellar Cluster IC 2395
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We present new deep UBVRI images and high-resolution multi-object optical spectroscopy of the young (~ 6 - 10 Myr old), relatively nearby (800 pc) open cluster IC 2395. We identify nearly 300 cluster members and use the photometry to estimate their spectral types, which extend from early B to middle M. We also present an infrared imaging survey of the central region using the IRAC and MIPS instruments on board the Spitzer Space Telescope, covering the wavelength range from 3.6 to 24 microns. Our infrared observations allow us to detect dust in circumstellar disks originating over a typical range of radii ~ 0.1 to ~ 10AU from the central star. We identify 18 Class II, 8 transitional disk, and 23 debris disk candidates, respectively 6.5%, 2.9%, and 8.3% of the cluster members with appropriate data. We apply the same criteria for transitional disk identification to 19 other stellar clusters and associations spanning ages from ~ 1 to ~ 18 Myr. We find that the number of disks in the transitional phase as a fraction of the total with strong 24 micron excesses ([8] - [24] > 1.5) increases from 8.4 +- 1.3% at ~ 3 Myr to 46 +- 5% at ~ 10 Myr. Alternative definitions of transitional disks will yield different percentages but should show the same trend.
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Young open clusters are ideal laboratories to understand star formation process. We present deep UBV I and Halpha photometry for the young open cluster IC 1590 in the center of the H II region NGC 281. Early-type members are selected from UBV photometric diagrams, and low-mass pre-main sequence (PMS) members are identified by using Halpha photometry. In addition, the published X-ray source list and Gaia astrometric data are also used to isolate probable members. A total of 408 stars are selected as members. The mean reddening obtained from early-type members is <E(B-V) = 0.40 +/- 0.06 (s.d.). We confirm the abnormal extinction law for the intracluster medium. The distance modulus to the cluster determined from the zero-age main-sequence fitting method is 12.3 +/- 0.2 mag (d = 2.88 +/- 0.28 kpc), which is consistent with the distance d = 2.70 ^+0.24 _-0.20 kpc from the recent Gaia parallaxes. We also estimate the ages and masses of individual members by means of stellar evolutionary models. The mode of the age of PMS stars is about 0.8 Myr. The initial mass function of IC 1590 is derived. It appears a steeper shape (Gamma = -1.49 +/- 0.14) than that of the Salpeter/Kroupa initial mass function for the high mass regime (m > 1 M_sun). The signature of mass segregation is detected from the difference in the slopes of the initial mass functions for the inner (r < 2.5) and outer region of this cluster. We finally discuss the star formation history in NGC 281.
The dominant reservoirs of elemental nitrogen in protoplanetary disks have not yet been observationally identified. Likely candidates are HCN, NH$_3$ and N$_2$. The relative abundances of these carriers determine the composition of planetesimals as a function of disk radius due to strong differences in their volatility. A significant sequestration of nitrogen in carriers less volatile than N$_2$ is likely required to deliver even small amounts of nitrogen to the Earth and potentially habitable exo-planets. While HCN has been detected in small amounts in inner disks ($<10$ au), so far only relatively insensitive upper limits on inner disk NH$_3$ have been obtained. We present new Gemini-TEXES high resolution spectroscopy of the 10.75 $mu$m band of warm NH$_3$, and use 2-dimensional radiative transfer modeling to improve previous upper limits by an order of magnitude to $rm [NH_3/H_{nuc}]<10^{-7}$ at 1 au. These NH$_3$ abundances are significantly lower than those typical for ices in circumstellar envelopes ($[{rm NH_3/H_{nuc}}]sim 3times 10^{-6}$). We also consistently retrieve the inner disk HCN gas abundances using archival Spitzer spectra, and derive upper limits on the HCN ice abundance in protostellar envelopes using archival ground-based 4.7 $mu$m spectroscopy ([HCN$_{rm ice}$]/[H$_2$O$_{rm ice}$]$<1.5-9$%). We identify the NH$_3$/HCN ratio as an indicator of chemical evolution in the disk, and use this ratio to suggest that inner disk nitrogen is efficiently converted from NH$_3$ to N$_2$, significantly increasing the volatility of nitrogen in planet-forming regions.
Aims: The two stable isotopes of nitrogen, 14N and 15N, exhibit a range of abundance ratios both inside and outside the solar system. The elemental ratio in the solar neighborhood is 440. Recent ALMA observations showed HCN/HC15N ratios from 83 to 156 in six T Tauri and Herbig disks and a CN/C15 N ratio of 323 +/- 30 in one T Tauri star. We aim to determine the dominant mechanism responsible for these enhancements of 15N: low-temperature exchange reactions or isotope-selective photodissociation of N2. Methods: Using the thermochemical code DALI, we model the nitrogen isotope chemistry in circumstellar disks with a 2D axisymmetric geometry. Our chemical network is the first to include both fractionation mechanisms for nitrogen. The model produces abundance profiles and isotope ratios for several key N-bearing species. We study how these isotope ratios depend on various disk parameters. Results: The formation of CN and HCN is closely coupled to the vibrational excitation of H2 in the UV-irradiated surface layers of the disk. Isotope fractionation is completely dominated by isotope-selective photodissociation of N2. The column density ratio of HCN over HC15N in the disks inner 100 au does not depend strongly on the disk mass, the flaring angle or the stellar spectrum, but it is sensitive to the grain size distribution. For larger grains, self-shielding of N2 becomes more important relative to dust extinction, leading to stronger isotope fractionation. Between disk radii of ~50 and 200 au, the models predict HCN/HC15N and CN/C15N abundance ratios consistent with observations of disks and comets. The HCN/HC15N and CN/C15N column density ratios in the models are a factor of 2-3 higher than those inferred from the ALMA observations.
Stars spend most of their lifetimes on the `main sequence (MS) in the Hertzsprung--Russell diagram. The obvious double MSs seen in the equivalent color--magnitude diagrams characteristic of Milky Way open clusters pose a fundamental challenge to our traditional understanding of star clusters as `single stellar populations. The clear MS bifurcation of early-type stars with masses greater than $sim1.6 M_odot$ is thought to result from a range in the stellar rotation rates. However, direct evidence connecting double MSs to stellar rotation properties has yet to emerge. Here, we show through analysis of the projected stellar rotational velocities ($vsin i$, where $i$ represents the stars inclination angle) that the well-separated double MS in the young, $sim200Myr$-old Milky Way open cluster NGC 2287 is tightly correlated with a dichotomous distribution of stellar rotation rates. We discuss whether our observations may reflect the effects of tidal locking affecting a fraction of the clusters member stars in stellar binary systems. We show that the slow rotators could potentially be initially rapidly rotating stars that have been slowed down by tidal locking by a low mass-ratio companion in a cluster containing a large fraction of short-period, low-mass-ratio binaries. This demonstrates that stellar rotation drives the split MSs in young, $lessapprox 300$Myr-old star clusters. However, special conditions, e.g., as regards the mass-ratio distribution, might be required for this scenario to hold.
The $sigma$ Orionis cluster is important for studying protoplanetary disk evolution, as its intermediate age ($sim$3-5 Myr) is comparable to the median disk lifetime. We use ALMA to conduct a high-sensitivity survey of dust and gas in 92 protoplanetary disks around $sigma$ Orionis members with $M_{ast}gtrsim0.1 M_{odot}$. Our observations cover the 1.33 mm continuum and several CO $J=2-1$ lines: out of 92 sources, we detect 37 in the mm continuum and six in $^{12}$CO, three in $^{13}$CO, and none in C$^{18}$O. Using the continuum emission to estimate dust mass, we find only 11 disks with $M_{rm dust}gtrsim10 M_{oplus}$, indicating that after only a few Myr of evolution most disks lack sufficient dust to form giant planet cores. Stacking the individually undetected continuum sources limits their average dust mass to 5$times$ lower than that of the faintest detected disk, supporting theoretical models that indicate rapid dissipation once disk clearing begins. Comparing the protoplanetary disk population in $sigma$ Orionis to those of other star-forming regions supports the steady decline in average dust mass and the steepening of the $M_{rm dust}$-$M_{ast}$ relation with age; studying these evolutionary trends can inform the relative importance of different disk processes during key eras of planet formation. External photoevaporation from the central O9 star is influencing disk evolution throughout the region: dust masses clearly decline with decreasing separation from the photoionizing source, and the handful of CO detections exist at projected separations $>1.5$ pc. Collectively, our findings indicate that giant planet formation is inherently rare and/or well underway by a few Myr of age.
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