Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userDetermination of Sodium Abundance Ratio from Low-Resolution Stellar Spectra and Its Applications
68
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We present a method to determine sodium-abundance ratios ([Na/Fe]) using the Na I D doublet lines in low-resolution ($R sim$ 2000) stellar spectra. As stellar Na I D lines are blended with those produced by the interstellar medium (ISM), we developed a technique for removing the interstellar Na I D lines using the relationship between extinction, which is proportional to $E(B-V)$, and the equivalent width (EW) of the interstellar Na I D absorption lines. When measuring [Na/Fe], we also considered corrections for non-local thermodynamic equilibrium (NLTE) effects. Comparisons with data from high-resolution spectroscopic surveys suggest that the expected precision of [Na/Fe] from low-resolution spectra is better than 0.3 dex for stars with [Fe/H] $>$ $-$3.0. We also present a simple application employing the estimated [Na/Fe] values for a large number of stellar spectra from the Sloan Digital Sky Survey (SDSS). After classifying the SDSS stars into Na-normal, Na-high, and Na-extreme, we explore their relation to stars in Galactic globular clusters (GCs). We find that, while the Na-high SDSS stars exhibit a similar metallicity distribution function (MDF) to that of the GCs, indicating that the majority of such stars may have originated from GC debris, the MDF of the Na-normal SDSS stars follows that of typical disk and halo stars. As there is a high fraction of carbon-enhanced metal-poor stars among the Na-extreme stars, they may have a non-GC origin, perhaps due to mass-transfer events from evolved binary companions.
rate research
Read More
We present a method for the determination of [alpha/Fe] ratios from low-resolution (R = 2000) SDSS/SEGUE stellar spectra. By means of a star-by-star comparison with degraded spectra from the ELODIE spectral library and with a set of moderately high-resolution (R = 15,000) and medium-resolution (R = 6000) spectra of SDSS/SEGUE stars, we demonstrate that we are able to measure [alpha/Fe] from SDSS/SEGUE spectra (with S/N > 20/1) to a precision of better than 0.1 dex, for stars with atmospheric parameters in the range Teff = [4500, 7000] K, log g = [1.5, 5.0], and [Fe/H] = [-1.4, +0.3], over the range [alpha/Fe] = [-0.1, +0.6]. For stars with [Fe/H] < -1.4, our method requires spectra with slightly higher signal-to-noise to achieve this precision (S/N > 25/1). Over the full temperature range considered, the lowest metallicity star for which a confident estimate of [alpha/Fe] can be obtained from our approach is [Fe/H] ~ -2.5; preliminary tests indicate that a metallicity limit as low as [Fe/H] ~ -3.0 may apply to cooler stars. As a further validation of this approach, weighted averages of [alpha/Fe] obtained for SEGUE spectra of likely member stars of Galactic globular clusters (M15, M13, and M71) and open clusters (NGC 2420, M67, and NGC 6791) exhibit good agreement with the values of [alpha/Fe] from previous studies. The results of the comparison with NGC 6791 imply that the metallicity range for the method may extend to ~ +0.5.
The Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) is an unbiased, massively multiplexed spectroscopic survey, designed to measure the expansion history of the universe through low-resolution ($Rsim750$) spectra of Lyman-Alpha Emitters. In its search for these galaxies, HETDEX will also observe a few 10$^{5}$ stars. In this paper, we present the first stellar value-added catalog within the internal second data release of the HETDEX Survey (HDR2). The new catalog contains 120,571 low-resolution spectra for 98,736 unique stars between $10 < G < 22$ spread across the HETDEX footprint at relatively high ($bsim60^circ$) Galactic latitudes. With these spectra, we measure radial velocities (RVs) for $sim$42,000 unique FGK-type stars in the catalog and show that the HETDEX spectra are sufficient to constrain these RVs with a 1$sigma$ precision of 28.0 km/s and bias of 3.5 km/s with respect to the LAMOST surveys and 1$sigma$ precision of 27.5 km/s and bias of 14.0 km/s compared to the SEGUE survey. Since these RVs are for faint ($Ggeq16$) stars, they will be complementary to Gaia. Using t-Distributed Stochastic Neighbor Embedding (t-SNE), we also demonstrate that the HETDEX spectra can be used to determine a stars T${rm{eff}}$, and log g and its [Fe/H]. With the t-SNE projection of the FGK-type stars with HETDEX spectra we also identify 416 new candidate metal-poor ([Fe/H] $< -1$~dex) stars for future study. These encouraging results illustrate the utility of future low-resolution stellar spectroscopic surveys.
To study the effect of supermassive black holes (SMBHs) on their host galaxies it is important to study the hosts when the SMBH is near its peak activity. A method to investigate the host galaxies of high luminosity quasars is to obtain optical spectra at positions offset from the nucleus where the relative contribution of the quasar and host are comparable. However, at these extended radii the galaxy surface brightness is often low (20-22 mag per arcsec$^{2}$) and the resulting spectrum might have such low S/N that it hinders analysis with standard stellar population modeling techniques. To address this problem we have developed a method that can recover galaxy star formation histories (SFHs) from rest frame optical spectra with S/N $sim$ 5~AA$^{-1}$. This method uses the statistical technique diffusion k-means to tailor the stellar population modeling basis set. Our diffusion k-means minimal basis set, composed of 4 broad age bins, is successful in recovering a range of galaxy SFHs. Additionally, using an analytic prescription for seeing conditions, we are able to simultaneously model scattered quasar light and the SFH of quasar host galaxies (QHGs). We use synthetic data to compare results of our novel method with previous techniques. We also present the modeling results on a previously published QHG and show that galaxy properties recovered from a diffusion k-means basis set are less sensitive to noise added to this quasar host galaxy spectrum. Our new method has a clear advantage in recovering information from QHGs and could also be applied to the analysis of other low S/N galaxy spectra such as those typically obtained for high redshift objects or integral field spectroscopic surveys.
Based on MUSE integral-field data we present evidence for a radial variation at the low-mass end of the stellar initial-mass function (IMF) in the central regions of the giant early-type galaxy NGC4486 (M87). We used state-of-the-art stellar population models and the observed strength of various IMF-sensitive absorption-line features to solve for the best low-mass tapered bimodal form of the IMF, while accounting also for radial variations in stellar metallicity, the overall $alpha$-elements abundance, and the abundance of individual elements such as Ti, O, Na and Ca. Our analysis reveals a strong IMF gradient in M87, corresponding to an exceeding fraction of low-mass stars compared to the case of the Milky Way toward the center of M87 that drops to nearly Milky-way levels by 0.4 $R_e$. This IMF gradient is found to correlate well with both the radial profile for stellar metallicity and for $alpha$-elements abundance but not with stellar velocity dispersion. Such IMF variations correspond to over a factor two increase in stellar mass-to-light M/L ratio compared to the case of a Milky-way like IMF, consistent with other investigations into IMF gradients in early-type galaxies, including recent dynamical constraints on M/L radial variations in M87 by Oldham & Auger. In addition to constraining the IMF in M87 we also looked into the abundance of Sodium, which turned up to be super-Solar over the entire radial range of our MUSE observations and to exhibit a considerable negative gradient. These findings suggest an additional role of metallicity in boosting the Na-yields in the central, metal-rich regions of M87 during its early and brief star-formation history. Our work adds the case of M87 to the few objects that as of today have radial constraints on their IMF or [Na/Fe] abundance, while also illustrating the accuracy that MUSE could bring to this kind of investigations.
We present the determination of stellar parameters and individual elemental abundances for 6 million stars from $sim$8 million low-resolution ($Rsim1800$) spectra from LAMOST DR5. This is based on a modeling approach that we dub $The$ $Data$--$Driven$ $Payne$ ($DD$--$Payne$), which inherits essential ingredients from both {it The Payne} citep{Ting2019} and $The$ $Cannon$ citep{Ness2015}. It is a data-driven model that incorporates constraints from theoretical spectral models to ensure the derived abundance estimates are physically sensible. Stars in LAMOST DR5 that are in common with either GALAH DR2 or APOGEE DR14 are used to train a model that delivers stellar parameters ($T_{rm eff}$, $log g$, $V_{rm mic}$) and abundances for 16 elements (C, N, O, Na, Mg, Al, Si, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, and Ba) when applied to LAMOST spectra. Cross-validation and repeat observations suggest that, for ${rm S/N}_{rm pix}ge 50$, the typical internal abundance precision is 0.03--0.1,dex for the majority of these elements, with 0.2--0.3,dex for Cu and Ba, and the internal precision of $T_{rm eff}$ and $log g$ is better than 30,K and 0.07,dex, respectively. Abundance systematics at the $sim$0.1,dex level are present in these estimates, but are inherited from the high-resolution surveys training labels. For some elements, GALAH provides more robust training labels, for others, APOGEE. We provide flags to guide the quality of the label determination and to identify binary/multiple stars in LAMOST DR5. The abundance catalogs are publicly accessible via href{url}{http://dr5.lamost.org/doc/vac}.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
