The extreme outer Galaxy (EOG), the region with a Galactic radius of more than 18 kpc, is known to have very low metallicity, about one-tenth that of the solar neighborhood. We obtained deep near-infrared (NIR) images of two very young ($\sim$0.5 Myr) star-forming clusters that are one of the most distant embedded clusters in the EOG. We find that in both clusters the fraction of stars with NIR excess, which originates from the circumstellar dust disk at radii of $\leq$0.1 AU, is significantly lower than those in the solar neighborhood. Our results suggest that most stars forming in the low-metallicity environment experience disk dispersal at an earlier stage ($<$1 Myr) than those forming in the solar metallicity environment (as much as $\sim$5--6 Myr). Such rapid disk dispersal may make the formation of planets difficult, and the shorter disk lifetime with lower metallicity could contribute to the strong metallicity dependence of the well-known "planet-metallicity correlation", which states the probability of a star hosting a planet increases steeply with stellar metallicity. The reason for the rapid disk dispersal could be increase of the mass accretion rate and/or the effective far-ultraviolet photoevaporation due to the low extinction; however, another unknown mechanism for the EOG environment could be contributing significantly.
Protoplanetary disk lifetimes vs stellar mass and possible implications for giant planet populations
我们利用附近年轻恒星形成区的大量年轻恒星样本，研究了原行星盘演化对恒星质量的依赖性。我们更新了我们最近的工作（本系列的论文I）中提出的原行星盘分数，这些分数是从1到1亿年的22个附近（< 500 pc）的星协导出的。我们使用一个子样本的1428光谱确认的成员，研究恒星质量对原行星盘演化的影响。我们把这个样本分为两个恒星质量箱（2 M $_{\odot}$边界）和两个年龄箱（3百万年边界），并使用红外线过度的光球发射分类对象分为三组：原行星盘，进化盘，无盘。均匀分析和偏倚校正允许对所得结果进行统计学显著性相互比较。我们发现强大的统计证据磁盘演化与恒星质量的依赖。我们的研究结果，结合以前的研究磁盘的演变，确认原行星盘演变更快和/或更早的高质量（> 2 M $_{\odot}$）的恒星。我们还发现了一个大致恒定的水平，在整个年龄和恒星质量谱的演变盘。我们得出结论，原行星盘的演化依赖于恒星质量。这种依赖性可能对气体巨行星形成和迁移有重要的影响，并有助于解释大质量恒星周围热彗星的明显缺乏。
A protoplanetary disk typically forms a dead zone near its midplane at the distance of a few au from the central protostar. Accretion through such a magnetically layered disk can be intrinsically unstable and has been associated with episodic outbursts in young stellar objects. We present the first investigation into the effects of low metallicity environment on the structure of the dead zone as well as the resulting outbursting behavior of the protoplanetary disk. We conducted global numerical hydrodynamic simulations of protoplanetary disk formation and evolution in the thin-disk limit. The …
We study the influence of different metallicities on the physical, thermal, and chemical properties of protoplanetary disks, and in particular on the formation and destruction of carbon-based molecules. With the thermo-chemical code ProoDIMO we investigate the impact of lower metallicities on the radiation field, disk temperature, and the abundance of different molecules (H$_2$O, CH$_4$, CO, CO$_2$, HCN, CN, HCO$^+$ and N$_2$H$^+$). We use a fiducial disk model as a reference model and produce two models with lower metallicity. The resulting influence on different chemical species is studied b…
HD\,139614 is known to be a $\sim$14-Myr-old, possibly pre-main-sequence star in the Sco-Cen OB association in the Upper Centaurus-Lupus subgroup, with a slightly warped circumstellar disc containing ring structures hinting at one or more planets. The star's chemical abundance pattern is metal-deficient except for volatile elements, which places it in the $λ$ Boo class and suggests it has recently accreted gas-rich but dust-poor material. We identify seven dipole and four radial pulsation modes among its $δ$ Sct pulsations using the TESS light curve and an échelle diagram. Precision modelling …
This study presents sensitive MIR photometry obtained with the Spitzer/IRAC for a young cluster in Sh 2-208 (S208) located in one of the lowest-metallicity HII regions in the Galaxy, ${\rm [O/H]} = -0.8$ dex. Previous studies suggested that the cluster is $\sim$0.5-Myr old and has a distance of $D = 4$ kpc, which is consistent with the astrometric distance from Gaia EDR3. In $\sim$$3.5 \times 4$-arcmin field, 96 sources were detected in at least one MIR band at $\ge$10$σ$, covering intermediate-mass stars with $\sim$1.0-$M_\odot$ mass detection limit. Total 41 probable cluster members were ide…
The time-scale over which and modality by which young stellar objects (YSOs) disperse their circumstellar discs dramatically influences the eventual formation and evolution of planetary systems. By means of extensive radiative transfer (RT) modelling, we have developed a new set of diagnostic diagrams in the infrared colour-colour plane (K-[24] vs. K-[8]), to aid with the classiffication of the evolutionary stage of YSOs from photometric observations. Our diagrams allow the differentiation of sources with un-evolved (primordial) discs from those evolving according to different clearing scenari…
The effects of an accretion disk are crucial to understanding the evolution of young stars. During the combined evolution, stellar and disk parameters influence each other, motivating a combined stellar and disk model. This makes a combined numerical model, evolving the disk alongside the star, the next logical step in the progress of studying early stellar evolution. We aim to understand the effects of metallicity on the accretion disk and the stellar spin evolution during the T~Tauri phase. We combine the numerical treatment of a hydrodynamic disk with stellar evolution, including a stellar …
The dispersal of protoplanetary disks sets the timescale available for planets to assemble, and thus it is one of the fundamental parameters in theories of planetary formation. Disk dispersal is determined by several properties of the central star, the disk itself, and the surrounding environment. In particular, the metallicity of disks may impact their evolution, even if to date controversial results exist: in low-metallicity clusters disks seem to rapidly disperse, while in the Magellanic Clouds some evidence supports the existence of accreting disks few tens of Myrs old. In this paper we st…
Recent multi-wavelength observations suggest that inner parts of protoplanetary disks (PPDs) have shorter lifetimes for heavier host stars. Since PPDs around high-mass stars are irradiated by strong ultra-violet radiation, photoevaporation may provide an explanation for the observed trend. We perform radiation hydrodynamics simulations of photoevaporation of PPDs for a wide range of host star mass of $M_* =0.5$-$7.0 M_{\odot}$. We derive disk mass-loss rate $\dot{M}$, which has strong stellar dependence as $\dot{M} \approx 7.30\times10^{-9}(M_{*}/M_{\odot})^{2}M_{\odot}\rm{yr}^{-1}$. The absol…
The gas dissipation from a protoplanetary disk is one of the key processes affecting planet formation, and it is widely accepted that it happens on timescales of a few million years for disks around single stars. Over the last years, several protoplanetary disks have been discovered in multiple star systems, and despite the complex environment in which they find themselves, some of them seem to be quite old, a situation that may favor planet formation. A clear example of this is the disk around HD 98800 B, a binary in a hierarchical quadruple stellar system, which at a $\sim$10 Myr age seems t…
Protoplanetary Disk Evolution in a Low-Metallicity Environment: JWST's First Mid-Infrared Census of Low-Mass Stars
National Astronomical Observatory of Japan（日本国立天文台）；SOKENDAI (The Graduate University for Advanced Studies)（综合研究大学院大学）；Graduate School of Science, Nagoya University（名古屋大学理学研究科）；Institute of Astronomy, School of Science, University of Tokyo（东京大学理学部天体物理系）；Kiso Observatory, Institute of Astronomy, School of Science, University of Tokyo（东京大学理学部天文科学系木曾观测站）；Jet Propulsion Laboratory, California Institute of Technology（加州理工学院喷气推进实验室）
We investigate the roles of magnetically driven disk wind (MDW) and thermally driven photoevaporative wind (PEW) in the long-time evolution of protoplanetary disks. We start simulations from the early phase in which the disk mass is $0.118\,{\mathrm{M}_{\odot}}$ around a $1\,{\mathrm{M}_{\odot}}$ star and track the evolution until the disk is completely dispersed. We incorporate the mass loss by PEW and the mass loss and magnetic braking (wind torque) by MDW, in addition to the viscous accretion, viscous heating, and stellar irradiation. We find that MDW and PEW respectively have different rol…
We study the evolution and final dispersal of protoplanetary discs that evolve under the action of internal and external photoevaporation, and different degrees of viscous transport. We identify five distinct dispersal pathways, which are i) very long lived discs ($>20\,$Myr), ii) inside-out dispersal where internal photoevaporation dominates and opens inner holes, iii) outside-in dispersal where external photoevaporation dominates through disc truncation and two intermediate regimes characterised by lingering material in the inner disc with the outer disc dispersed predominantly by either int…
Debris disks are classically considered to be gas-less systems, but recent (sub)millimeter observations have detected tens of those with rich gas content. The origin of the gas component remains unclear; namely, it can be protoplanetary remnants and/or secondary products deriving from large bodies. In order to be protoplanetary in origin, the gas component of the parental protoplanetary disk is required to survive for $\gtrsim10{\,\rm Myr}$. However, previous models predict $\lesssim 10{\,\rm Myr}$ lifetimes because of efficient photoevaporation at the late stage of disk evolution. In the pres…
The evolution and lifetime of protoplanetary disks (PPDs) play a central role in the formation and architecture of planetary systems. Astronomical observations suggest that PPDs evolve in two timescales, accreting onto the star for up to several million years (Myr) followed by gas dissipation within <1 Myr. Because solar nebula magnetic fields are sustained by the gas of the protoplanetary disk, we can use paleomagnetic measurements to infer the lifetime of the solar nebula. Here, we use paleomagnetic measurements of meteorites to constrain this lifetime and investigate whether the solar nebul…
Insight into the Formation of $β$ Pic b through the Composition of Its Parent Protoplanetary Disk as Revealed by the $β$ Pic Moving Group Member HD 181327
通过推断移动星群成员 HD 181327 的丰度作为代理，本研究确认 $β$ Pic b 具有超恒星碳氧丰度和低于恒星的 C/O 比，支持其通过核吸积形成的理论，并提出 FGK 矮星丰度可代理同群早型星原行星盘成分。
The prevalence of short-period super-Earths that are independent of host metallicity challenges the theoretical construction of their origin. We propose that dust trapping in the global pressure bump induced by magnetospheric truncation in evolved protoplanetary discs (PPDs) around T Tauri stars offers a promising formation mechanism for super-Earths, where the host metallicity is already established. To better understand this planet forming scenario, we construct a toy inner disc model and focus on the evolution of dust trapped in the bump, taking into account the supply from drifting pebbles…
The volatile composition of a planet is determined by the inventory of gas and ice in the parent disk. The volatile chemistry in the disk is expected to evolve over time, though this evolution is poorly constrained observationally. We present ALMA observations of C18O, C2H, and the isotopologues H13CN, HC15N, and DCN towards five Class 0/I disk candidates. Combined with a sample of fourteen Class II disks presented in Bergner et al. (2019b), this data set offers a view of volatile chemical evolution over the disk lifetime. Our estimates of C18O abundances are consistent with a rapid depletion …
Evolution of Thermally Pulsing Asymptotic Giant Branch Stars V: Constraining the Mass Loss and Lifetimes of Intermediate Mass, Low Metallicity AGB Stars
Estimating The Mean Linewidth And Lifetime of Solar-like Oscillations of Stars
\textbf{Scaling formulas were}推导出描述恒星基本参数与恒星类日振荡平均值\textbf{linewidth and lifetime}之间关系的公式。恒星类太阳振荡\textbf{are dependent on the large frequency separation, the effective temperature, and the acoustic impedance ($ρc$) in the photosphere}的平均\textbf{linewidth and}寿命。平均寿命\textbf{can be}与含$ν\simν_{max}$的模式的寿命相近。将标度关系的结果与\textit{Kepler}和\textit{CoRoT}观测到的类太阳振荡的平均寿命进行了比较，结果表明标度关系能很好地再现观测到的类太阳振荡的平均寿命.平均寿命对\textbf{the}大频率间隔、有效温度和声阻抗的依赖性\textbf{indicates}表明类太阳振荡的寿命依赖于恒星的质量和演化阶段。此外，我们的计算表明，恒星的$p$模式的平均寿命可以受到金属丰度的影响。
Globular clusters (GCs) are known to harbor multiple stellar populations. To explain these observations Bastian et al. suggested a scenario in which a second population is formed by the accretion of enriched material onto the low-mass stars in the initial GC population. The idea is that the low-mass, pre-main sequence stars sweep up gas expelled by the massive stars of the same generation into their protoplanetary disc as they move through the GC core. We perform simulations with 2 different smoothed particle hydrodynamics codes to investigate if a low-mass star surrounded by a protoplanetary …
(Abridged) The lifetime of protoplanetary disks around young stars limits the timescale when planets form. A disk dissipation timescale < 10 Myr was inferred from surveys providing the fraction of stars with disks in young stellar clusters with different ages. However, most previous surveys focused on the compact region within ~ 2 pc from the clusters' centers, for which the disk fraction information considering the outer part is practically absent. We aim to test if disk fraction estimates change when inferred from an extended region around the clusters' centers. Gaia EDR3 data and a best-sui…
Rapid destruction of protoplanetary discs due to externalphotoevaporation in star-forming regions
我们分析了恒星形成区的N体模拟，以研究大质量恒星外部远紫外光蒸发对原行星盘的影响。通过改变模拟恒星形成区域的初始条件，如空间分布，净体积运动（维里比）和密度，我们调查哪些参数最影响光盘分散由于外部光蒸发的速率。我们发现，光盘分散由于外部光蒸发是更快的高度亚结构的恒星形成区域比在光滑和集中的地区。亚维里恒星形成区域发生坍缩也显示出比处于维里平衡或正在膨胀的区域更高的星盘分散率。在中等密度（$\sim$ 100 M $_{\odot}$ pc $^{-3}$）区域，半径$\geq$ 100 AU的所有原行星盘中有一半在100万年内被光蒸发，比目前观测研究提出的速度快三倍。在低密度恒星形成区（$\sim$ 10 M $_{\odot}$ pc $^{-3}$）的星盘存活时间更长，但一半的星盘仍然分散在较短的时间尺度上（$\sim$ 2 Myr）。这表明恒星形成区的初始条件将极大地影响原行星盘的演化和寿命。这些结果也意味着，要么气体巨星行星形成是非常迅速的，发生在盘的气体成分被蒸发之前，要么气体巨星只形成在低密度的恒星形成区域，那里没有大质量恒星存在，以从原行星盘中光蒸发气体。
