Context.
The study of high-mass stars found to be isolated in the field of the Milky Way may help to probe the feasibility of the core-accretion mechanism in the case of massive star formation. The ...existence of truly isolated stars may efficiently probe the possibility that individual massive stars can be born in isolation.
Aims.
We observed WR67a (hereafter Sapaki), an O3If* star that appears to be isolated close to the center of a well-developed giant cavity that is aptly traced by 8.0 μm hot dust emission.
Methods.
We acquired medium-resolution (
R
= 4100) and moderate signal-to-noise (S/N = 95 at 4500 Å) spectra for Sapaki in the range of 3800 − 10 500 Å with the Magellan Echellette (MagE) at Las Campanas Observatory. We computed the line-of-sight total extinctions. Additionally, we restricted its heliocentric distance by using a range of different estimators. Moreover, we measured its radial velocity from several lines in its spectrum. Finally, we analyzed its proper motions from
Gaia
to examine its possible runaway status.
Results.
The star has been classified as having the spectral type O3If* given its resemblance to standard examples of the class. In addition, we found that Sapaki is highly obscured, reaching a line-of-sight extinction value of
A
V
= 7.87. We estimated the heliocentric distance to be in the range of
d
= 4 − 7 kpc. We also estimated its radial velocity to be
V
r
= −34.2 ± 15.6 km s
−1
. We may also discard its runaway status solely based on its 2D kinematics. Furthermore, by analyzing proper motions and parallaxes provided by
Gaia
, we found only one other star with compatible measurements.
Conclusions.
Given its apparent non-runaway status and the absence of clustering, Sapaki appears to be a solid candidate for isolated high-mass star formation in the Milky Way.
ABSTRACT In this work, we present the results of a spectroscopic study of very massive stars (VMSs) found outside the center of the massive stellar cluster NGC 3603. From the analysis of the ...associated Southern Astrophysical Research (SOAR) Telescope spectroscopic data and related optical-near-IR (NIR) photometry, we confirm the existence of several VMSs in the periphery of NGC 3603. The first group of objects (MTT58, WR42e, and RF7) is composed of three new Galactic exemplars of the OIf*/WN type, all of them with probable initial masses well above 100 and estimated ages of about 1 Myr. Based on our Goodman blue-optical spectrum of another source in our sample (MTT68), we can confirm the previous finding in the NIR of the only other Galactic exemplar (besides HD 93129A) of the O2If* type known to date. Based on its position relative to a set of theoretical isochrones in a Hertzprung-Russel (H-R) diagram, we concluded that the new O2If* star could be one of the most massive (150 ) and luminous (MV = −7.3) O-stars in the Galaxy. Also, another remarkable result is the discovery of a new O2v star (MTT31), which is the first exemplar of that class so far identified in the Milk Way. From its position in the H-R diagram it is found that this new star probably had an initial mass of 80 , as well as an absolute magnitude of MV = −6.0, corresponding to a luminosity similar to other known O2v stars in the Large Magellanic Cloud. Finally, we also communicate the discovery of a new Galactic O3.5If* star (RFS8) that is quite an intriguing case. Indeed, it is located far to the south of the NGC 3603 center, in apparent isolation at a large radial projected linear distance of ∼62 pc. Its derived luminosity is similar to that of the other O3.5If* (Sh18) found in NGC 3603's innermost region, and the fact that a such high mass star is observed so isolated in the field led us to speculate that perhaps it could have been expelled from the innermost parts of the complex by a close fly-by dynamical encounter with a very massive hard binary system.
ABSTRACT
The structure of the Small Magellanic Cloud (SMC) is very complex, in particular in the periphery that suffers more from the interactions with the Large Magellanic Cloud (LMC). A wealth of ...observational evidence has been accumulated revealing tidal tails and bridges made up of gas, stars, and star clusters. Nevertheless, a full picture of the SMC outskirts is only recently starting to emerge with a 6D phase-space map plus age and metallicity using star clusters as tracers. In this work, we continue our analysis of another outer region of the SMC, the so-called West Halo, and combined it with the previously analysed Northern Bridge. We use both structures to define the Bridge and Counter-bridge trailing and leading tidal tails. These two structures are moving away from each other, roughly in the SMC–LMC direction. The West Halo form a ring around the SMC inner regions that goes up to the background of the Northern Bridge shaping an extended layer of the Counter-bridge. Four old Bridge clusters were identified at distances larger than 8 kpc from the SMC centre moving towards the LMC, which is consistent with the SMC–LMC closest distance of 7.5 kpc when the Magellanic Bridge was formed about 150Myr ago; this shows that the Magellanic Bridge was not formed only by pulled gas, but it also removed older stars from the SMC during its formation. We also found age and metallicity radial gradients using projected distances on sky, which are vanished when we use the real 3D distances.
ABSTRACT
We perform ground-based photometric observations of 22 DA white dwarf stars, 10 already known ZZ Cetis and 12 candidates with atmospheric parameters inside the classical instability strip. ...We report on the discovery of four new variable DA white dwarf stars. Two objects are near the middle of the instability strip, SDSS J082804.63+094956.6 and SDSS J094929.09+101918.8, and two red edge pulsators, GD 195 and L495−82. In addition, we classified four objects as possible variables, since evidence of variability was detected in the light curve, but the signal-to-noise ratio was not sufficient to establish a definite detection. Follow-up observations were performed for 10 known ZZ Ceti stars to verify period stability and search for new periodicities. For each confirmed variable, we perform a detailed asteroseismological fit and compare the structural parameters obtained from the best-fitting models with those obtained from spectroscopy and photometry from Gaia. Finally we present a study of the asteroseismological properties of a sample of 91 ZZ Ceti stars.
ABSTRACT
The formation scenario of the Magellanic Bridge during an encounter between the Large and Small Magellanic Clouds ∼200 Myr ago, as proposed by N-body models, would be imprinted in the ...chemical enrichment and kinematics of its stars and sites of ongoing star formation along its extension. We present an analysis of 33 Bridge star clusters using photometry obtained with the SOAR 4-m telescope equipped with adaptive optics for the VISCACHA survey. We performed a membership selection and derived self-consistent ages, metallicities, distances, and reddening values via statistical isochrone fitting, as well as tidal radii and integrated masses from structure analysis. Two groups are clearly detected: 13 well-studied clusters older than the Bridge, with 0.5–6.8 Gyr and $\hbox{Fe/H} < -0.6$ dex; and 15 clusters with <200 Myr and $\rm {Fe/H} > -0.5$ dex, probably formed in situ. The old clusters follow the overall age and metallicity gradients of the SMC, whereas the younger ones are uniformly distributed along the Bridge. The main results are as follows: (i) we derive ages and metallicities for the first time for 9 and 18 clusters, respectively; (ii) we detect two metallicity dips in the age-metallicity relation of the Bridge at ∼200 Myr and 1.5 Gyr ago for the first time, possibly chemical signatures of the formation of the Bridge and Magellanic Stream; (iii) we estimate a minimum stellar mass for the Bridge of 3–5 × 105 M⊙; (iv) we confirm that all the young Bridge clusters at ${\rm RA} < 3^h$ are metal-rich with $\rm {Fe/H} \sim -0.4$ dex.
Context.The Galactic H II region NGC 2579 has stayed undeservedly unexplored due to identification problems which persisted until recently. Both NGC 2579 and its companion ESO 370-9 have been ...misclassified as planetary or reflection nebula, confused with each other and with other objects. Due to its high surface brightness, high excitation, angular size of few arcminutes and relatively low interstellar extinction, NGC 2579 is an ideal object for investigations in the optical range. Located in the outer Galaxy, NGC 2579 is an excellent object for studying the Galactic chemical abundance gradients. Aims.To present the first comprehensive observational study on the nebular and stellar properties of NGC 2579 and ESO 370-9, including the determination of electron temperature, density structure, chemical composition, kinematics, distance, and the identification and spectral classification of the ionizing stars, and to discuss the nature of ESO 370-9. Methods.Long slit spectrophotometric data in the optical range were used to derive the nebular electron temperature, density and chemical abundances and for the spectral classification of the ionizing star candidates. Hα and $\it UBV$ CCD photometry was carried out to derive stellar distances from spectroscopic parallax and to measure the ionizing photon flux. Results.The chemical abundances of He, N, O, Ne, S, Cl, and Ar were obtained. Maps of electron density and radial velocity with a spatial resolution of 5´´ $\times$ 5´´ were composed from long slit spectra taken at different declinations. Three O stars classified as O5 V, O6.5 V, and O8 V were found responsible for the ionization of NGC 2579, while ESO 370-9 is ionized by a single O8.5 V star. The estimated mass of ionized gas of ≈25 $M_\odot$ indicates that ESO 370-9 is not a planetary nebula, but a small H II region. A photometric distance of 7.6 ± 0.9 kpc and a kinematic distance of 7.4 ± 1.4 kpc were obtained for both objects. At the galactocentric distance of 12.8 ± 0.7 kpc, NGC 2579 is one of the most distant Galactic H II regions for which direct abundance determinations have been accomplished.
The VISCACHA survey Saroon, S; Dias, B; Tsujimoto, T ...
Astronomy & astrophysics,
09/2023, Letnik:
677
Journal Article
Recenzirano
Odprti dostop
The chemical evolution history of the Small Magellanic Cloud (SMC) has been a matter of debate for decades. The challenges in understanding the SMC chemical evolution are related to a very slow star ...formation rate (SFR) combined with bursts triggered by the multiple interactions between the SMC and the Large Magellanic Cloud, a significant (∼0.5 dex) metallicity dispersion for the SMC cluster population younger than about 7.5 Gyr, and multiple chemical evolution models tracing very different paths through the observed age–metallicity relation of the SMC. There is no doubt that these processes were complex. Therefore, a step-by-step strategy is required in order to better understand the SMC chemical evolution. We adopted an existing framework to split the SMC into regions on the sky, and we focus on the west halo in this work, which contains the oldest and most metal-poor stellar populations and is moving away from the SMC, that is, in an opposite motion with respect to the Magellanic Bridge. We present a sample containing ∼60% of all west halo clusters to represent the region well, and we identify a clear age–metallicity relation with a tight dispersion that exhibits a 0.5 dex metallicity dip about 6 Gyr ago. We ran chemical evolution models and discuss possible scenarios to explain this metallicity dip, the most likely being a major merger accelerating the SFR after the event. This merger should be combined with inefficient internal gas mixing within the SMC and different SFRs in different SMC regions because the same metallicity dip is not seen in the AMR of the SMC combining clusters from all regions. We try to explain the scenario to better understand the SMC chemo-dynamical history.
The VISCACHA survey Dias, B.; Angelo, M. S.; Oliveira, R. A. P. ...
Astronomy and astrophysics (Berlin),
03/2021, Letnik:
647
Journal Article
Recenzirano
Context.
The interactions between the Small and Large Magellanic Clouds (SMC and LMC) created the Magellanic Bridge; a stream of gas and stars pulled out of the SMC towards the LMC about 150 Myr ago. ...The tidal counterpart of this structure, which should include a trailing arm, has been predicted by models but no compelling observational evidence has confirmed the Counter-Bridge so far.
Aims.
The main goal of this work is to find the stellar counterpart of the Magellanic Bridge and Counter-Bridge. We use star clusters in the SMC outskirts as they provide a 6D phase-space vector, age, and metallicity which help characterise the outskirts of the SMC.
Methods.
Distances, ages, and photometric metallicities were derived from fitting isochrones to the colour-magnitude diagrams from the VISCACHA survey. Radial velocities and spectroscopic metallicities were derived from the spectroscopic follow-up using GMOS in the CaII triplet region.
Results.
Among the seven clusters analysed in this work, five belong to the Magellanic Bridge, one belongs to the Counter-Bridge, and the other belongs to the transition region.
Conclusions.
The existence of the tidal counterpart of the Magellanic Bridge is evidenced by star clusters. The stellar component of the Magellanic Bridge and Counter-Bridge are confirmed in the SMC outskirts. These results are an important constraint for models that seek to reconstruct the history of the orbit and interactions between the LMC and SMC as well as constrain their future interaction including with the Milky Way.
The VISCACHA survey Saroon, S.; Dias, B.; Tsujimoto, T. ...
Astronomy and astrophysics (Berlin),
09/2023, Letnik:
677
Journal Article
Recenzirano
Odprti dostop
The chemical evolution history of the Small Magellanic Cloud (SMC) has been a matter of debate for decades. The challenges in understanding the SMC chemical evolution are related to a very slow star ...formation rate (SFR) combined with bursts triggered by the multiple interactions between the SMC and the Large Magellanic Cloud, a significant (∼0.5 dex) metallicity dispersion for the SMC cluster population younger than about 7.5 Gyr, and multiple chemical evolution models tracing very different paths through the observed age–metallicity relation of the SMC. There is no doubt that these processes were complex. Therefore, a step-by-step strategy is required in order to better understand the SMC chemical evolution. We adopted an existing framework to split the SMC into regions on the sky, and we focus on the west halo in this work, which contains the oldest and most metal-poor stellar populations and is moving away from the SMC, that is, in an opposite motion with respect to the Magellanic Bridge. We present a sample containing ∼60% of all west halo clusters to represent the region well, and we identify a clear age–metallicity relation with a tight dispersion that exhibits a 0.5 dex metallicity dip about 6 Gyr ago. We ran chemical evolution models and discuss possible scenarios to explain this metallicity dip, the most likely being a major merger accelerating the SFR after the event. This merger should be combined with inefficient internal gas mixing within the SMC and different SFRs in different SMC regions because the same metallicity dip is not seen in the AMR of the SMC combining clusters from all regions. We try to explain the scenario to better understand the SMC chemo-dynamical history.
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