Model atmosphere analysis of two new early–type O4 dwarfs stars

Early-type O stars play an excessively significant role in field of astrophysics. they have high effective temperatures (T_eff > 30, 000K), strong ionizing radiation fields and display many times intense mass-loss over their stellar winds, making it very difficult to describe stellar evolution and the atmosphere models. It is known that these stars are the ancestors of the wonderful objects such as Wolf Wright Stars (W-Rs), the Red Giant(RSGs), luminous blue variables (LBVS), and As well as for some of the most active phenomena in the universe of the second type Supernovae and some gamma-ray bursts [11], [15]. As well they greatly affect their galaxies by transferring the momentum, energy and chemical elements enriched into the interstellar medium [1]; [3].

Many different studies have been dealing with O-type stars for many years, however their origin, properties and evolution are not completely clear and display many theoretical and observational challenges. their mass-loss rates (Ṁ) depend on the metallicity (Z), For example, as well as effective temperatures, surface gravity and wind structure (e.g., clumping) have been constantly discussed in the literature over the last few years (see, e.g. [2], [9], [13], [14]).

For illustrate some of the questions mentioned earlier, and for further highlight into the problem, we have analyzed the spectra of two new early-type galaxy O4V stars(ALS 19618 and BD+50886) with optical medium-resolution. we used model atmosphere codes TLUSTY204 and SYNSPEC49 to achieve their stellar physical parameters [4].

The rest of the paper is arranged as follows. In Sect.2 we describe the optical observation of those two stars from GOSSS servay. In Sect.3 we give a specification for the atmosphere codes and the selected hypotheses. In sect.4. We provide analysis of (ALS19618) and (Bd +50886). In Sect.5 The estimated physical parameters are given with comparisons to theoretical predictions/appreciation and epitomizes the main results found in this study.

Observations used in this work are retrieved from The Galactic O-Star Spectroscopic Survey (GOSSS) the third installment (see [8]) (GOSSS) is a large survey of Galactic O stars in spectroscopic, established on modern homogeneous, signal-to-noise ratio are very high, typically S/N ∼ 300, moderate resolution (R ∼ 2500), in a wavelength range ∼ 4000–5000 of a considerable amount(1000+) of O stars in the our Galaxy and estimating precise and self-compatible spectral types for all of them . the selected digital observations are from the Galactic O-Star Catalog (GOSC) [7]. This study includes two new galactic O4 dwarfs stars from GOSSS, their Observational data are shown in Table. 1.

we used model atmosphere code TLUSTY204 [4] to determine the main stellar parameters of those two stars, TLUSTY code assumes the atmosphere is a plane-parallel layers, contains line-blanketing, NLTE, hydrostatic equilibrium, radiative equilibrium and present line formed only in photosphere layer.

We use to synthesis the spectra the code SYNSPEC 49 [5] which is a general spectrum synthesis program. The input model atmospheres to SYNSPEC could be taken from TLUSTY or from the literature, for instance from the Kurucz grid of models. Then the program solves the radiative transfer equation wavelength by wavelength in a specified wavelength range and with a specified wavelength resolution. In the present calculation To start our analysis, we calculate a small grid of TLUSTY model based on the OSTAR2002 grid [6], with an effective temperature range between 40,000 K and 45,000 K, and surface gravity range between 3.80 and 4.20, Our new model atmosphere grid uses small steps in effective temperature (1000 K) and surface gravity (0.1 dex). We use the helium lines He I λ4471 and He II λ4542 ratio to derive the effective temperatures, Typical errors for T_eff is 1 kK. Our deduction of(log g) was established on the fitting to the Hγ wings, but fits other transitions such as Hβ wings and Hδ wings were also checked for consistency. For this parameter, the uncertainty is 0.1 dex. After determining the effective temperatures and surface gravities for the target stars, we turn to determine the rotational velocities(vsini). For this purpose, we use the routine rotin3 which is a part of the SYNSPEC package. To do that, a small grid of rotated spectra for a given effective temperature and surface gravity is calculated and The rotational velocities (vsini) were estimated from the fitting process.

The results of our analysis and stellar modelling of ALS 19618 and BD +50886 are presented here. For each system a brief background is given, also the results of the analysis of their spectra and stellar modelling is discussed. The values of the effective temperature(Teff), surface gravity (log g) and the rotational velocity (vsin i) is determined from the fitting process with the synthetic one. the variation between the best fit model achieved from the fitting process is small, so visual checking that make the final judgment of the reasonable model.

4.1

ALS 19618

ALS 19618 is located in the young open cluster NGC 6618 which embedded in the omega nebula (M 17), and it’s the reason for the gases of the nebula to radiate because of radiation from these young, hot stars; It is considered one of the brightest and most massive star-forming regions of the galaxy.

No line blanketed stellar model atmosphere have been applied to study this star, in Fig.1 we show our best fit model to the optical spectrum, several spectral regions were displayed which include the diagnostic lines applied to estimate the stellar parameters. The effective temperature would be very well represented. The He I λ4471/He II λ4542 ratio are well reproduced with Teff = (42.9 ± 1) kK, from H γ we estimate surface gravity log g=4.00 ± 1.0, and the best fit for rotation velocity vsini=210 km s⁻¹.

Fig. 1

the most strong lines for hydrogen and helium in the wavelength range(4000 - 5000) show in Fig.1 are: He I λ4388, He I λ4471, He I λ4713, He II λ4200, He II λ4542, He II λ4686, Hδ, Hγ, and Hβ. These same transitions are easily identified in the objects of our sample.

Fig. 2

In this study we provided a model atmosphere analysis of two new galactic early type O4 main-sequence stars. We established effective temperature (Teff), surface gravities (log g), and rotational velocities (vsin i) for each star using NLTE model. The effective temperatures are 42,900 and 44,000 K. for ALS 19618 and BD+50886 respectively. BD+50886 has The lowest surface gravity 3.95 ± 0.1 and ALS 19618 has the higher surface gravity 4.0 ± 0.1.the fit with the model performed higher rotational velocity for ALS 19618 (vsin i=210) and (vsin i=150) for BD+50886.The physical parameters of this study are displayed in Table 2. comparison of The derived stellar parameters with the recently calibration for Galactic O-type star properties of the O4 main-sequence shows a fair agreement (see [10]).