A Catalog of 45,206 H{\alpha} Emission-line Stars from LAMOST MRS DR7
Stars that exhibit prominent emission lines in their spectra are referred to as emission-line stars, encompassing a wide range of stellar types and indicative of intriguing physical properties. The Large Sky Area Multi-Object fiber Spectroscopic Telescope (LAMOST) has released millions of spectra from its Medium-Resolution Survey (MRS). A small fraction of these spectra exhibit emission lines, yet they remain undiscovered and unanalyzed due to being buried in the vast dataset. We have developed a method based on Derivative Spectroscopy (DS), which provides a novel approach for detecting and identifying emission-line stars by extracting signals from complex backgrounds and estimating spectral line profiles. Applying this method to the $\mathrm{H\alpha}$ spectral line profiles from the LAMOST-MRS Data Release 7 (DR7), we compiled a catalog of emission-line stars using the second- and third-derivative spectra for automated peak detection. This approach also facilitates the classification of $\mathrm{H\alpha}$ emission-line morphologies through a simplified scheme. The catalog comprises 56\,649 spectra with relatively prominent $\mathrm{H\alpha}$ emission lines from 45\,206 unique stars, with each emission-line component accompanied by approximate estimates of its wavelength, amplitude, and width. All $\mathrm{H\alpha}$ spectral lines were classified into three morphological classes under a unified classification scheme: single emission peak (83.0 \%), double emission peaks (5.6 \%), and P Cygni-type profiles (11.5 \%), which encompass both P Cygni and inverse P Cygni features. Through cross-referencing with SIMBAD, 39\,497 stars represent new emission-line sources discovered in our research.