We have observed the blue dwarf galaxy I Zw 36 with the f/96 relay of the HST Faint Object Camera and have for the first time resolved massive stars, using the broad band filters F175W, F342W, F430W and F480LP. We have measured the fluxes of 143 of these objects and studied their characteristics in color-magnitude diagrams. A few stars may be red supergiants but their contribution to the integrated light is less than 5% in the F430W filter. The F175W-F430W color of the integrated stellar population is redder than expected from the current burst of star formation, suggesting therefore the presence of an older and unresolved underlying population. The ultraviolet measurements combined with synthetic photometry calculations allow us to place the massive stars in a bolometric magnitude vs. temperature diagram. In this diagram, the stars are compared to evolutionary tracks for different stellar masses. The current burst probably has an age less than 12Myr. We infer an Initial Mass Function, with a power-law slope in the range -1.7 to -2.6 for masses M>=20M_{sun}_. This is consistent with most of the values reported for sites of star formation in the Galaxy and the Magellanic Clouds and does not support the view of an IMF flattening at low metallicity.
An increasing number of non-terminal giant eruptions are being observed by modern supernova and transient surveys. But very little is known about the origin of these giant eruptions and their progenitors, many of which are presumably very massive, evolved stars. Motivated by the small number of progenitors positively associated with these giant eruptions, we have begun a survey of the evolved massive star populations in nearby galaxies. The nearby, nearly face-on, giant spiral M101 is an excellent laboratory for studying a large population of very massive stars. In this paper, we present BVI photometry obtained from archival HST/ACS Wide Field Camera images of M101. We have produced a catalog of luminous stars with photometric errors <10% for V<24.5 and 50% completeness down to V~26.5 even in regions of high stellar crowding. Using color and luminosity criteria, we have identified candidate luminous OB-type stars and blue supergiants, yellow supergiants, and red supergiants for future observation. We examine their spatial distributions across the face of M101 and find that the ratio of blue to red supergiants decreases by two orders of magnitude over the radial extent of M101 corresponding to 0.5 dex in metallicity. We discuss the resolved stellar content in the giant star-forming complexes NGC 5458, 5453, 5461, 5451, 5462, and 5449 and discuss their color-magnitude diagrams in conjunction with the spatial distribution of the stars to determine their spatio-temporal formation histories.
In this work, we have applied a semi-empirical spectral classification method for OB-stars using the APOGEE spectrograph to a sample of candidates in the W3-W4-W5 (W345) complexes. These massive star-forming regions span over 200pc across the Perseus arm and have a notorious population of massive stars, from which a large fraction are members of various embedded and young open clusters. From 288 APOGEE spectra showing H-band spectral features typical of O- and B-type sources, 46 probably correspond to previously unknown O-type stars. Therefore, we confirm that Br11-Br13 together with HeII {lambda}16923 (7-12) and HeII {lambda}15723 (7-13) lines contained in the APOGEE spectral bands are useful in providing spectral classification down to one spectral sub-class for massive stars in regions as distant as d~2kpc. The large number of newly found O-type stars as well as the numerous intermediate-mass population confirm that W345 is a very efficient massive star factory, with an integral stellar population probably amounting several thousand solar masses.
In this work, we make use of DR14 APOGEE spectroscopic data to study a sample of 92 known OB stars. We developed a near-infrared semi-empirical spectral classification method that was successfully used in case of four new exemplars, previously classified as later B-type stars. Our results agree well with those determined independently from ECHELLE optical spectra, being in line with the spectral types derived from the "canonical" MK blue optical system. This confirms that the APOGEE spectrograph can also be used as a powerful tool in surveys aiming to unveil and study a large number of moderately and highly obscured OB stars still hidden in the Galaxy.
Young massive stars and stellar clusters continuously form in the Galactic disk, generating new HII regions within their natal giant molecular clouds and subsequently enriching the interstellar medium via their winds and supernovae. Massive stars are among the brightest infrared stars in such regions; their identification permits the characterisation of the star formation history of the associated cloud as well as constraining the location of stellar aggregates and hence their occurrence as a function of global environment. We present a stellar spectroscopic survey in the direction of the giant molecular cloud G23.3-0.3. This complex is located at a distance of ~4-5kpc, and consists of several HII regions and supernova remnants. We discovered 11 Of_K_^+^ stars, one candidate luminous blue variable, several OB stars, and candidate red supergiants. Stars with K-band extinction from ~1.3-1.9mag appear to be associated with the GMC G23.3-0.3; O and B-types satisfying this criterion have spectrophotometric distances consistent with that of the giant molecular cloud. Combining near-IR spectroscopic and photometric data allowed us to characterize the multiple sites of star formation within it. The O-type stars have masses from ~25-45M_{sun}_, and ages of 5-8Myr. Two new red supergiants were detected with interstellar extinction typical of the cloud; along with the two RSGs within the cluster GLIMPSE9, they trace an older burst with an age of 20-30Myr. Massive stars were also detected in the core of three supernova remnants - W41, G22.7-0.2, and G22.7583-0.4917. A large population of massive stars appears associated with the GMC G23.3-0.3, with the properties inferred for them indicative of an extended history of stars formation.
Using data from the (intermediate) Palomar Transient Factory (iPTF), we characterize the time variability of ~500 massive stars in M31. Our sample is those stars that are spectrally typed by Massey and collaborators, including Luminous Blue Variables, Wolf-Rayets, and warm and cool supergiants. We use the high-cadence, long-baseline (~5yr) data from the iPTF survey, coupled with data-processing tools that model complex features in the light curves. We find widespread photometric (R-band) variability in the upper Hertzsprung Russell diagram (or CMD) with an increasing prevalence of variability with later spectral types. Red stars (V-I>1.5) exhibit larger amplitude fluctuations than their bluer counterparts. We extract a characteristic variability timescale, t_ch_, via wavelet transformations that are sensitive to both continuous and localized fluctuations. Cool supergiants are characterized by longer timescales (>100 days) than the hotter stars. The latter have typical timescales of tens of days but cover a wider range, from our resolution limit of a few days to longer than 100 days. Using a 60 night block of data straddling two nights with a cadence of around 2 minutes, we extracted t_ch_ in the range 0.1-10 days with amplitudes of a few percent for 13 stars. Though there is broad agreement between the observed variability characteristics in the different parts of the upper CMD with theoretical predictions, detailed comparison requires models with a more comprehensive treatment of the various physical processes operating in these stars, such as pulsation, subsurface convection, and the effect of binary companions.
We analyse the environmental properties of 370 local early-type galaxies (ETGs) in the MASSIVE and ATLAS^3D^ surveys, two complementary volume-limited integral-field spectroscopic (IFS) galaxy surveys spanning absolute K-band magnitude - 21.5>=M_K_>=-26.6, or stellar mass 8*10^9^<~M*<~2*10^12^M{sun}. We find these galaxies to reside in a diverse range of environments measured by four methods: group membership (whether a galaxy is a brightest group/cluster galaxy, satellite or isolated), halo mass, large-scale mass density (measured over a few Mpc) and local mass density (measured within the Nth neighbour). The spatially resolved IFS stellar kinematics provide robust measurements of the spin parameter {lambda}_e_ and enable us to examine the relationship among {lambda}_e_, M* and galaxy environment. We find a strong correlation between {lambda}_e_ and M*, where the average {lambda}_e_ decreases from ~0.4 to below 0.1 with increasing mass, and the fraction of slow rotators f_slow_ increase from ~10 to 90 per cent. We show for the first time that at fixed M*, there are almost no trends between galaxy spin and environment; the apparent kinematic morphology-density relation for ETGs is therefore primarily driven by M* and is accounted for by the joint correlations between M* and spin, and between M* and environment. A possible exception is that the increased f_slow_ at high local density is slightly more than expected based only on these joint correlations. Our results suggest that the physical processes responsible for building up the present-day stellar masses of massive galaxies are also very efficient at reducing their spin, in any environment.
G79.3+0.3 is an infrared dark cloud in the Cygnus-X complex that is home to massive deeply embedded young stellar objects (YSOs). We have produced a Submillimeter Array (SMA) 1.3mm continuum image and ^12^CO line maps of the eastern section of G79.3+0.3 in which we detect five separate YSOs. We have estimated physical parameters for these five YSOs and others in the vicinity of G79.3+0.3 by fitting existing photometry from Spitzer, Herschel, and ground-based telescopes to spectral energy distribution models. Through these model fits we find that the most massive YSOs seen in the SMA 1.3mm continuum emission have masses in the 5-6M_{sun}_ range. One of the SMA sources was observed to power a massive collimated ^12^CO outflow extending at least 0.94pc in both directions from the protostar, with a total mass of 0.83M_{sun}_ and a dynamical timescale of 23kyr.
We present a catalog of variable stars in the near-infrared wavelength detected with overlapping regions of the Two Micron All Sky Survey public images, and discuss their properties. The investigated region is in the direction of the Galactic center (-30{deg}<~l<~20, |b|<~20), which covers the entire bulge. We have detected 136 variable stars, of which six are already known and 118 are distributed in the |b|<5{deg} region. Additionally, 84 variable stars have optical counterparts in Digitized Sky Survey images. The three diagrams (color-magnitude, light variance, and color-color diagrams) indicate that most of the detected variable stars should be large-amplitude and long-period variables such as Mira variables or OH/IR stars. The number density distribution of the detected variable stars implies that they trace the bar structure of the Galactic bulge.
We follow-up on a previous finding that AGB Mira variables containing the third dredge-up indicator technetium (Tc) in their atmosphere form a different sequence of K-[22] colour as a function of pulsation period than Miras without Tc. A near- to mid-infrared colour such as K-[22] is a good probe for the dust mass-loss rate of the stars. Contrary to what might be expected, Tc-poor Miras show redder K-[22] colours (i.e. higher dust mass-loss rates) than Tc-rich Miras at a given period. Here, the previous sample is extended and the analysis is expanded towards other colours and dust spectra. The most important aim is to investigate if the same two sequences can be revealed in the gas mass-loss rate. We analysed new optical spectra and expanded the sample by including more stars from the literature. Near- and mid-IR photometry and ISO dust spectra of our stars were investigated where available. Literature data of gas mass-loss rates of Miras and semi-regular variables were collected and analysed. Our results show that Tc-poor Miras are redder than Tc-rich Miras in a broad range of the mid-IR, suggesting that the previous finding based on the K-[22] colour is not due to a specific dust feature in the 22-micron band. We establish a linear relation between K-[22] and the gas mass-loss rate. We also find that the 13 micron feature disappears above K-[22]~2.17mag, corresponding to dM/dt}_g_~2.6x10^-7^M_{sun}/yr. No similar sequences of Tc-poor and Tc-rich Miras in the gas mass-loss rate vs. period diagram are found, most probably owing to limitations in the available data. Different hypotheses to explain the observation of two sequences in the P vs. K-[22] diagram are discussed and tested, but so far, none of them convincingly explains the observations. Nevertheless, we might have found an hitherto unknown but potentially important process influencing mass loss on the TP-AGB.
