WISE J080822.18$-$644357.3 was recently identified as a new M dwarf debris disc system and a candidate member of the 45 Myr-old Carina association. Given that the strength of its infrared excess ($L_{\rm IR}/L_{\rm \star}\simeq 0.1$) appears to be more consistent with a young protoplanetary disc, we present the first optical spectra of the star and reassess its evolutionary and membership status. We find WISE J0808-6443 to be a Li-rich M5 star with strong H$α$ emission ($-125 < \textrm{EW} < -65$ Å over 4 epochs) whose strength and broad width are consistent with accretion at a low level ($\sim$10$^{-10}$ $M_{\odot}$ yr$^{-1}$) from its disc. The spectral energy distribution of the star is consistent with a primordial disc and is well-fit using a two-temperature blackbody model with $T_{\rm inner}\simeq$ 1100 K and $T_{\rm outer}\simeq$ 240 K. AllWISE multi-epoch photometry shows the system exhibits significant variability in the 3.4 $μ$m and 4.6 $μ$m bands. We calculate an improved proper motion based on archival astrometry, and combined with a new radial velocity, the kinematics of the star are consistent with membership in Carina at a kinematic distance of $90\pm9$ pc. The spectroscopic and photometric data are consistent with WISE J0808-6443 being a $\sim$0.1 $M_{\odot}$ Classical T-Tauri star and one of the oldest known accreting M-type stars. These results provide further evidence that the upper limit on the lifetimes of gas-rich discs - and hence the timescales to form and evolve protoplanetary systems - around the lowest mass stars may be longer than previously recognised, or some mechanism may be responsible for regenerating short-lived discs at later stages of pre-main sequence evolution.
Many binary companions to the central stars of planetary nebulae (PNe) are found to be inflated, perhaps indicating that accretion onto the central star might occur during the PN phase. The discovery of a handful of nova eruptions and supersoft X-ray sources inside PNe supports this hypothesis. In this paper, we investigate the impact that hosting a steadily-accreting WD would have on the properties and evolution of a PN. By pairing the published accreting nuclear-burning WD models with radiation transfer simulations, we extract the time evolution of the emission line spectra and ionization pr…
New members of the TW Hydrae Association and two accreting M-dwarfs in Scorpius-Centaurus
我们报告了在寻找30-40万年前的Octans协会新成员时发现的几颗年轻的中M星的偶然发现。只有一颗恒星可能被认为是Octans（近）成员。然而，这两颗恒星的自行、运动学距离、径向速度、测光和Li I 6708 AA的测量结果与800万至1000万年前的长蛇座TW星协成员一致。另一个可能是长蛇座TW的外围成员，但其速度与Octans成员的预测相似。我们还确定了邻近的天蝎座-半人马座OB协会（Sco-Cen）的两个新的富锂成员。两者都表现出大的12和22微米的过剩和强大的，可变的H-α发射，我们归因于吸积从拱盘。这样的恒星被认为在Sco-Cen的1600万年中位年龄是非常罕见的，它们只加入了上天蝎座之外的一个其他确认的M型和三个更高质量的吸积物。偶然发现的两颗吸积恒星承载着大量的星周物质，可能表明斯科-岑的年龄分布相当大，或者进一步证明在低质量恒星周围，圆盘分散和行星形成的时间尺度更长。为了帮助未来的研究Sco-Cen，我们还提供了一个新编制的目录305早期型依巴谷成员的光谱径向速度来源于文献。
We investigate the nature of an unusually faint member of the $ε$ Cha Association ($D\sim100$ pc, age $\sim5$ Myr), the nearest region of star formation of age $<$8 Myr. This object, 2MASS J11550336-7919147 (2M1155$-$79B), is a wide ($\sim$580 AU) separation, comoving companion to low-mass (M3) $ε$ Cha Association member 2MASS J11550485-7919108 (2M1155$-$79A). We present near-infrared spectra of both components, along with analysis of photometry from Gaia EDR3, 2MASS, VHS, and WISE. The near-IR spectrum of 2M1155$-$79B displays strong He I 1.083 emission, a sign of active accretion and/or accr…
As host to two accreting planets, PDS 70 provides a unique opportunity to probe the chemical complexity of atmosphere-forming material. We present ALMA Band 6 observations of the PDS~70 disk and report the first chemical inventory of the system. With a spatial resolution of 0.4''-0.5'' ($\sim$50 au), 12 species are detected, including CO isotopologues and formaldehyde, small hydrocarbons, HCN and HCO+ isotopologues, and S-bearing molecules. SO and CH3OH are not detected. All lines show a large cavity at the center of the disk, indicative of the deep gap carved by the massive planets. The radia…
The low water content of the terrestrial planets in the solar system suggests that the protoplanets formed within the water snow line. Accurate prediction of the snow line location moving with time provides a clue to constrain the formation process of the planets. In this paper, we investigate the migration of the snow line in protoplanetary disks whose accretion is controlled by laminar magnetic fields, which have been proposed by various nonideal magnetohydrodynamic (MHD) simulations. We propose an empirical model of the disk temperature based on our nonideal MHD simulations, which show that…
Planets around young stars trace the early evolution of planetary systems. We report the discovery and validation of two planetary systems with ages $\lesssim 300$ Myr from observations by the Transiting Exoplanet Survey Satellite. TOI-251 is a 40-320 Myr old G star hosting a 2.74 +0.18/-0.18 REarth mini-Neptune with a 4.94 day period. TOI-942 is a 20-160 Myr old K star hosting a system of inflated Neptune-sized planets, with TOI-942b orbiting with a period of 4.32 days, with a radius of 4.81 +0.20/-0.20 REarth, and TOI-942c orbiting in a period of 10.16 days with a radius of 5.79 +0.19/-0.18 …
We have followed up on our observations of the ~ 40-Myr, and still accreting, PMC Delorme 1 (AB)b. We used high-resolution spectroscopy to characterise the accretion process further by accessing the wealth of emission lines in the near-UV. With VLT/UVES, we obtained R ~ 50000 spectroscopy at 330--452 nm. After separating the emission of the companion from that of the M5 low-mass binary, we performed a detailed emission-line analysis, which included planetary accretion shock modelling. We reaffirm ongoing accretion in Delorme 1 (AB)b and report the first detections in a (super-Jovian) protoplan…
NUV Excess in Slowly Accreting T Tauri Stars: Limits Imposed by Chromospheric Emission
被质量吸积率非常低的圆盘包围的年轻恒星可能处于内部圆盘演化的最后阶段，因此特别值得研究。利用哈勃太空望远镜（HST）上的宇宙起源光谱仪（COS）和空间望远镜成像光谱仪（STIS）对500 - 900万年前的两颗恒星RECX-1和RECX-11进行了紫外（UV）观测，并对它们进行了光学和近红外光谱观测。这两颗恒星的近紫外辐射水平相似，尽管光谱证据表明RECX-11正在吸积，而RECX-1没有。在RECX-11中，H α和He I 10830的谱线轮廓显示出随时间变化的宽和窄的红移吸收分量，揭示了吸积流的复杂性。我们发现，吸积指标通常用于测量质量吸积率，例如U带过剩光度或钙II三线光度是不可靠的低吸积物，至少在中间K光谱范围。以RECX-1作为光球层和色球层发射本征水平的模板，我们确定RECX-11的上限为3 x 10^-10太阳质量/年。在如此低的吸积率下，最近的光蒸发模型预测，在圆盘中应该已经形成了一个内孔。然而，RECX-11的光谱能量分布显示，在近红外波段的通量与金牛座的中值相当，这表明仍然存在大量的尘埃。在RECX-11中观测到了在最内侧盘中形成的荧光H_2发射线，表明在内侧盘中存在气体和尘埃。
The Trifid Nebula is a young, nearby star-forming region where star formation is proposed to have been triggered by cloud-cloud collision (CCC), based on observations of molecular clouds. It offers a unique opportunity to test whether the CCC hypothesis is supported by the spatial distribution and star formation chronology of young stars. We present the first study of the optically visible pre-main sequence (PMS) population of the region using riH$α$ imaging and Gaia astrometry. Combined with an analysis of young stellar objects (YSOs) using infrared imaging, we capture the spatial distributio…
We summarize Hubble Space Telescope (HST) UV observations of the weak-lined T Tauri star (wTTS) PDS 70 obtained with the Space Telescope Imaging Spectrograph (STIS). These observations provide the first far-UV (FUV) and near-UV (NUV) spectra of PDS 70. Ground-based observations have so far revealed two formative giant planets orbiting in a wide gap in its circumstellar disk. Both the star and young planets are thought to still be accreting. The HST spectra provide new insight into physical conditions in the star's outer atmosphere and circumstellar environment. The spectra are dominated by chr…
A radial velocity study by Donati et al. (2016) reported the detection of a close-in giant planet in a 4.93 d orbit around the ~2 Myr old weak-lined T Tauri star V830 Tau. Because of the stringent timescale constraints that a very young host star like V830 Tau would place on hot Jupiter formation models and inward migration mechanisms, independent confirmation of the planet's existence is needed but so far has not been obtained. We present new Chandra X-ray observations of V830 Tau. The Chandra observations in combination with previous XMM-Newton observations reveal strong variable X-ray emiss…
The Transiting Exoplanet Survey Satellite (TESS) mission searches for new exoplanets. The observing strategy of TESS results in high-precision photometry of millions of stars across the sky, allowing for detailed asteroseismic studies of individual systems. In this work, we present a detailed asteroseismic analysis of the giant star HD 76920 hosting a highly eccentric giant planet ($e = 0.878$) with an orbital period of 415 days, using 5 sectors of TESS light curve that cover around 140 days of data. Solar-like oscillations in HD 76920 are detected around $52 \, μ$Hz by TESS for the first time…
