Mira Variable Stars From LAMOST DR4 Data: Emission Features, Temperature Types, and Candidate Selection

Based on an extensive spectral study of a photometrically confirmed sample of Mira variables, we find a relationship between relative Balmer emission-line strength and spectral temperature of O-rich Mira stars. The $F_{\rm H\delta}/F_{\rm H\gamma}$ flux ratio increases from less than unity to five as stars cool down from M0 to M10, which is likely driven by increasing TiO absorption above the deepest shock-emitting regions. We also discuss the relationship between the equivalent widths of the Balmer emission lines and the photometric luminosity phase of our Mira sample stars. Using our 291 Mira spectra as templates for reference, 191 Mira candidates are newly identified from the LAMOST DR4 catalog. We summarize the criteria adopted to select Mira candidates based on emission-line indices and molecular absorption bands. This enlarged spectral sample of Mira variables has the potential to contribute significantly to our knowledge of the optical properties of Mira stars and will facilitate further studies of these late-type, long-period variables.Comment: 21 pages; ApJS, in pres

High Energy Transients Powered by Black Holes

The accretion of matter onto black holes heats the surrounding materials to extremely high temperatures and drives outflows, producing a sudden and intense emission of light. Some of the most well-known examples include gamma-ray bursts, X-ray binaries (XRBs), and tidal disruption events (TDEs). In recent years, modern wide-field time domain sky surveys, such as the optical Zwicky Transient Facility (ZTF) and the Spektrum-Roentgen-Gamma (SRG) X-ray satellite, have opened up the discovery space of fast-evolving transients and enabled population analysis. In this thesis, I conducted a series of observational studies to understand the inner workings, environments, and demographics of high-energy transients powered by black holes. The first part of my thesis presents detailed studies on AT2019wey and AT2020mrf --- two transients discovered by SRG. First, I established that AT2019wey is a Galactic XRB with a low-mass companion star, and provided evidence that its central compact object is a black hole. Next, I demonstrated that AT2020mrf is a massive star explosion likely powered by fall-back accretion onto a newly formed black hole (with a rapidly spinning magnetar as an alternative power source). My work supports the idea that luminous fast blue optical transients form a rare class of engine-driven stellar explosions. The second part of my thesis concerns TDEs. I contributed to the discovery of two X-ray bright TDEs (AT2021ehb and AT2022cmc) and led comprehensive follow-up campaigns to track their long-term evolution. In both objects, using the NuSTAR and NICER X-ray telescopes, I identified novel TDE spectral features, which probe massive black hole accretion and jet launching. Additionally, using ZTF, I constructed the largest flux-limited sample of 33 TDEs, which enabled robust estimates of the optical TDE luminosity functions, host galaxy preference, and the black hole mass function. The emerging functional forms resulting from the large sample size represent significant advancements. My work lays a foundation for both TDE population studies with future sky surveys and theoretical inquiries.</p

Multiple Linear Regression Model of Blood Oxygen Saturation

We used standardized regression analysis to analyze the influence of age and recent smoking status on blood oxygen saturation. We found that blood oxygen saturation was negatively correlated with age and recent smoking status, but whether smoking had a greater influence on blood oxygen saturation than age. In order to further explore whether age and recent smoking status could jointly affect oxygen saturation of blood, we used k-means clustering algorithm and took age as the control variable to conduct clustering analysis. Is obtained by polynomial fitting, and then the blood oxygen saturation under all ages about age, smoking status of recent expression of partial derivatives of recent smoking status, get the following conclusion: smoking is oxygen desaturation, and compared with the young, the elderly smoking more effect on the blood oxygen saturation degree, are more likely to suffer from cardiovascular disease.When the regression results were tested for significance, the significance of all variables was investigated, and it was found that only age and current smoking status were significant for oxygen saturation, while BMI and gender showed no difference from zero in T test, indicating that oxygen saturation was not affected by BMI and gender. Then, taking oxygen saturation as the dependent variable and age and current smoking status as independent variables, the spline interpolation method with the best fitting effect was found, and its expression was given in the text

IN-SYNC. VIII. Primordial Disk Frequencies in NGC 1333, IC 348, and the Orion A Molecular Cloud

In this paper, we address two issues related to primordial disk evolution in three clusters (NGC 1333, IC 348, and Orion A) observed by the INfrared Spectra of Young Nebulous Clusters (IN-SYNC) project. First, in each cluster, averaged over the spread of age, we investigate how disk lifetime is dependent on stellar mass. The general relation in IC 348 and Orion A is that primordial disks around intermediate mass stars (2--5$M_{\odot}$) evolve faster than those around loss mass stars (0.1--1$M_{\odot}$), which is consistent with previous results. However, considering only low mass stars, we do not find a significant dependence of disk frequency on stellar mass. These results can help to better constrain theories on gas giant planet formation timescales. Secondly, in the Orion A molecular cloud, in the mass range of 0.35--0.7$M_{\odot}$, we provide the most robust evidence to date for disk evolution within a single cluster exhibiting modest age spread. By using surface gravity as an age indicator and employing 4.5 $\mu m$ excess as a primordial disk diagnostic, we observe a trend of decreasing disk frequency for older stars. The detection of intra-cluster disk evolution in NGC 1333 and IC 348 is tentative, since the slight decrease of disk frequency for older stars is a less than 1-$\sigma$ effect.Comment: 25 pages, 26 figures; submitted for publication (ApJ