arXiv subjects

Y. M. Pihlström

Publications and source records attributed to Y. M. Pihlström.

13 recordsLinked to original sources

arXiv2026

29SiO and 30SiO J=1--0 maser signatures in Galactic AGB stars: the impact of third dredge up and turbulence velocity

J=1--0 SiO masers at 43 GHz have a well-established distinctive signature in asymptotic giant branch (AGB) stars. 28SiO transitions typically dominate these spectra with the v=1 and v=2 emission being especially prominent and ubiquitous. Several predictions about enhanced 29SiO abundances in exotic stars prompt us to catalog the cases where 29SiO maser emission is enhanced compared to 28SiO. Our purpose is to catalog the known cases of 43 GHz spectra dominated by emission from isotopologue transitions (iso-dom spectra), to explore the commonalities in these sources, and to explain the cause of such maser signatures. Our catalog is drawn from SiO maser line ratios in the infrared-color-selected BAaDE survey and supplemented with a literature detection. The BAaDE catalog has cemented the typical signature of 43 GHz SiO masers, showing it is dominated by the v=1 and v=2 lines. Thirty-five iso-dom spectra are identified, meaning that this signature is seen in about 0.2% of our SiO maser-bearing stars. Their infrared colors are blue compared to other sources of the same period, similar to all sources displaying isotopologue masers. It is clear that the iso-dom nature of sources is variable, but unclear whether this is tied to stellar phase. A large abundance abnormality among the iso-dom sources is disfavored as the iso-dom sources do not appear significantly different from other stars which host isotopologue masers. Maser pumping, affecting the population inversions of specific transitions, can instead explain the enhanced signatures. We posit that iso-dom spectra appear in AGB stars which have undergone third-dredge up (enhancing the 29SiO and 30SiO abundance slightly) and which, in addition, are experiencing very low turbulence velocity (<1 km/s), creating a line overlap which pumps the maser transitions very efficiently.

astro-ph.SR↗

arXiv2023

Long-period maser-bearing Miras in the Galactic center: period-luminosity relations and extinction estimates

We establish a sample of 370 Mira variables that are likely near the Galactic center (GC). The sources have been selected from the OGLE and BAaDE surveys based on their sky coordinates, OGLE classifications, and BAaDE maser-derived line-of-sight velocities. As the distance to the GC is known to a high accuracy, this sample is a test bed for reddening and extinction studies toward the GC and in Mira envelopes. We calculated separate interstellar- and circumstellar-extinction values for individual sources, showing that there is a wide range of circumstellar extinction values (up to four magnitudes in the K$_s$ band) in the sample, and that circumstellar reddening is statistically different from interstellar reddening laws. Further, the reddening laws in the circumstellar environments of our sample and the circumstellar environments of Large Magellanic Cloud (LMC) Miras are strikingly similar despite the different metallicities of the samples. Period-magnitude relations for the mid-infrared (MIR) WISE and MSX bands are also explored, and in the WISE bands we compare these to period-magnitude relationships derived from Miras in the LMC as it is important to compare these LMC relations to those in a higher metallicity environment. Emission from the envelope itself may contaminate MIR magnitudes altering the relations, especially for sources with thick envelopes.

astro-ph.SR↗

arXiv2017

Maser, infrared and optical emission for late-type stars in the Galactic plane

Radio astrometric campaigns using VLBI have provided distances and proper motions for masers associated with young massive stars (BeSSeL survey). The ongoing BAaDE project plans to obtain astrometric information of SiO maser stars located in the inner Galaxy. These stars are associated with evolved, mass-losing stars. By overlapping optical (Gaia), infrared (2MASS, MSX and WISE) and radio (BAaDE) sources, we expect to obtain important clues on the intrinsic properties and population distribution of late-type stars. Moreover, a comparison of the Galactic parameters obtained with Gaia and VLBI can be done using radio observations on different targets: young massive stars (BeSSeL) and evolved stars (BAaDE).

astro-ph.SR↗

arXiv2017

Astrometric Galactic maser measurements cross-matched with Gaia

Using the VLBA, the BeSSeL survey has provided distances and proper motions of young massive stars, allowing an accurate measure of the Galactic spiral structure. By the same technique, we are planning to map the inner Galaxy using positions and velocities of evolved stars (provided by the BAaDE survey). These radio astrometric measurements (BeSSeL and BAaDE) will be complementary to Gaia results and the overlap will provide important clues on the intrinsic properties and population distribution of the stars in the bulge.

astro-ph.GA↗

arXiv2017

Finding evolved stars in the inner Galactic disk with Gaia

The Bulge Asymmetries and Dynamical Evolution (BAaDE) survey will provide positions and line-of-sight velocities of ~20,000 evolved, maser bearing stars in the Galactic plane. Although this Galactic region is affected by optical extinction, BAaDE targets may have Gaia cross-matches, eventually providing additional stellar information. In an initial attempt to cross-match BAaDE targets with Gaia, we have found more than 5,000 candidates. Of these, we may expect half to show SiO emission, which will allow us to obtain velocity information. The cross-match is being refined to avoid false positives using different criteria based on distance analysis, flux variability, and color assessment in the mid- and near-IR. Once the cross-matches can be confirmed, we will have a unique sample to characterize the stellar population of evolved stars in the Galactic bulge, which can be considered fossils of the Milky Way formation.

astro-ph.SR↗

arXiv2015

The First Gamma-Ray Bursts in the Universe

Gamma-ray bursts (GRBs) are the ultimate cosmic lighthouses, capable of illuminating the universe at its earliest epochs. Could such events probe the properties of the first stars at z $\sim$ 20, the end of the cosmic Dark Ages? Previous studies of Population III GRBs only considered explosions in the diffuse relic H II regions of their progenitors, or bursts that are far more more energetic than those observed to date. But the processes that produce GRBs at the highest redshifts likely reset their local environments, creating much more complicated structures than those in which relativistic jets have been modeled so far. These structures can greatly affect the luminosity of the afterglow, and hence the redshift at which it can be detected. We have now simulated Population III GRB afterglows in H II regions, winds, and dense shells ejected by the star during the processes that produce the burst. Our model, which has been used in previous work, has been extended to include contributions from reverse shocks, inverse Compton cooling and the effects of sphericity and beaming in the blast wave, and is valid in a variety of circumjet density profiles. We find that GRBs with E$_{\mathrm{iso},γ} =$ 10$^{ 51}$ - 10$^{53}$ erg will be visible at z $\gtrsim$ 20 to the next generation of near infrared and radio observatories. In many cases, the environment of the burst, and hence progenitor type, can be inferred from the afterglow light curve. Although some Population III GRBs are visible to Swift and the Very Large Array now, the optimal strategy for their detection will be future missions like EXIST and JANUS, which have large survey areas and onboard X-ray and infrared telescopes that can track their near infrared flux from the moment of the burst, thereby identifying its redshift.

astro-ph.HE↗

arXiv2013

Detection of Class I Methanol (CH3OH) Maser Candidates in Supernova Remnants

We have used the Karl G. Jansky Very Large Array (VLA) to search for 36 GHz and 44 GHz methanol (CH3OH) lines in a sample of 21 Galactic supernova remnants (SNRs). Mainly the regions of the SNRs with 1720 MHz OH masers were observed. Despite the limited spatial extent covered in our search, methanol masers were detected in both G1.4-0.1 and W28. Additional masers were found in SgrAEast. More than 40 masers were found in G1.4-0.1 which we deduce are due to interactions between the SNR and at least two separate molecular clouds. The six masers in W28 are associated with the molecular cloud that is also associated with the OH maser excitation. We discuss the possibility that the methanol maser may be more numerous in SNRs than the OH maser, but harder to detect due to observational constraints.

astro-ph.GA↗

arXiv2011

Very Large Array monitoring of 1720 MHz OH masers toward the Galactic Center

We present the first variability study of the 1720 MHz OH masers located in the Galactic Center. Most of these masers are associated with the interaction between the supernova remnant SgrAEast and the interstellar medium, but a few masers are associated with the Circumnuclear Disk. The monitoring program covered five epochs and a timescale of 20-195 days, during which no masers disappeared and no new masers appeared. All masers have previously been detected in a single epoch observation about one year prior to the start of the monitoring experiment, implying relatively stable conditions for the 1720 MHz OH masers. No extreme variability was detected. The masers associated with the northeastern interaction region between the supernova remnant and the +50km/s molecular cloud show the highest level of variability. This can be explained with the +50km/s molecular cloud being located behind the supernova remnant and with a region of high OH absorbing column density along the line of sight. Possibly the supernova remnant provides additional turbulence to the gas in this region, through which the maser emission must travel. The masers in the southern interaction region are located on the outermost edge of SgrAEast which line of sight is not covered by either absorbing OH gas or a supernova remnant, in agreement with the much lower variability level observed. Similarly, the masers associated with the CND show little variability, consistent with them arising through collisions between relatively large clumps of gas in the CND and no significant amount of turbulent gas along the line of sight.

astro-ph.GA↗

arXiv2011

Expanded Very Large Array Detection of 44.1 GHz Class I Methanol Masers in Sagittarius A

We report on the detection of 44 GHz Class I methanol (CH3OH) maser emission in the SgrA complex with the Expanded Very Large Array (EVLA). These EVLA observations show that the SgrA complex harbors at least four different tracers of shocked regions in the radio regime. The 44 GHz masers correlate with the positions and velocities of previously detected 36 GHz CH3OH masers, but less with 1720 MHz OH masers. Our detections agree with theoretical predictions that the densities and temperatures conducive for 1720 MHz OH masers may also produce 36 and 44 GHz CH3OH maser emission. However, many 44 GHz masers do not overlap with 36 GHz methanol masers, suggesting that 44 GHz masers also arise in regions too hot and too dense for 36 GHz masers to form. This agrees with the non-detection of 1720 MHz OH masers in the same area, which are thought to be excited under even cooler and less dense conditions. We speculate that the geometry of the 36 GHz masers outlines the current location of a shock front.

astro-ph.GA↗

arXiv2010

VLA observations of water masers towards 6.7 GHz methanol maser sources

22 GHz water and 6.7 GHz methanol masers are usually thought as signposts of early stages of high-mass star formation but little is known about their associations and the physical environments they occur in. The aim was to obtain accurate positions and morphologies of the water maser emission and relate them to the methanol maser emission recently mapped with Very Long Baseline Interferometry. A sample of 31 methanol maser sources was searched for 22 GHz water masers using the VLA and observed in the 6.7 GHz methanol maser line with the 32 m Torun dish simultaneously. Water maser clusters were detected towards 27 sites finding 15 new sources. The detection rate of water maser emission associated with methanol sources was as high as 71%. In a large number of objects (18/21) the structure of water maser is well aligned with that of the extended emission at 4.5 $μ$m confirming the origin of water emission from outflows. The sources with methanol emission with ring-like morphologies, which likely trace a circumstellar disk/torus, either do not show associated water masers or the distribution of water maser spots is orthogonal to the major axis of the ring. The two maser species are generally powered by the same high-mass young stellar object but probe different parts of its environment. The morphology of water and methanol maser emission in a minority of sources is consistent with a scenario that 6.7 GHz methanol masers trace a disc/torus around a protostar while the associated 22 GHz water masers arise in outflows. The majority of sources in which methanol maser emission is associated with the water maser appears to trace outflows. The two types of associations might be related to different evolutionary phases.

astro-ph.GA↗

arXiv2007

Stirring the Embers: High Sensitivity VLBI Observations of GRB030329

We present high sensitivity Very Long Baseline Interferometry (VLBI) observations 806 days after the gamma-ray burst of 2003 March 29 (GRB030329). The angular diameter of the radio afterglow is measured to be 0.347 +- 0.09 mas, corresponding to 0.99 +- 0.26 pc at the redshift of GRB030329 (z = 0.1685). The evolution of the image size favors a uniform external density over an R^-2 wind-like density profile (at distances of R >~10^18 cm from the source), although the latter cannot be ruled out yet. The current apparent expansion velocity of the image size is only mildly relativistic, suggesting a non-relativistic transition time of t_NR ~ 1 yr. A rebrightening, or at least a significant flattening in the flux decay, is expected within the next several years as the counter-jet becomes visible (this has not yet been observed). An upper limit of <1.9c is set on the proper motion of the flux centroid.

astro-ph↗

arXiv2005

A compact starburst ring traced by clumpy OH megamaser emission

We model the OH megamaser emission from the luminous infrared galaxy IIIZw35 as arising from a narrow rotating starburst ring of radius 22 pc enclosing a mass of 7 x 10^6 Msun. We show how both the compact and apparently diffuse maser emission from this ring can arise from a single phase of unsaturated maser clouds amplifying background radio continuum. The masing clouds are estimated to have a diameter of <0.7 pc and internal velocity dispersion of about 20 km/s. We find that the clouds are neither self-gravitating nor pressure confined but are freely expanding. Their dispersal lifetimes may set the vertical thickness of the ring. For an estimated internal density of 3 x 10^3 cm^-3, cloud masses are of order 24 Msun. The observed spectral features and velocity gradients indicate that the clouds must be outflowing and escaping the nucleus. The cloud mass outflow rate is estimated to be 0.8 Msun/yr, while the star formation rate is about 19 Msun/yr. Associated ionised gas, possibly generated from dissipated clouds, provides free-free absorption along the source axis, explaining the observed East-West asymmetries. We show that the clumpiness of a maser medium can have a dramatic effect on what is observed even in a relatively low gain OH megamaser. Specifically, in IIIZw35 our clumpy maser model naturally explains the large line to continuum ratios, the large 1667MHz:1665MHz line ratios and the wide velocity dispersions seen in the compact maser spots. Other astrophysical masers showing both compact and apparently diffuse emission might be explained by similar clumpy structures.

astro-ph↗

arXiv2004

Atomic and Molecular Absorption at High Redshift

Strong constraints on possible variations in fundamental constants can be derived from HI 21-cm and molecular rotational absorption lines observed towards quasars. With the aim of forming a statistical sample of constraints we have begun a program of systematic searches for such absorption systems. Here we describe molecular rotational searches in 25 damped Lyman-alpha systems where, in many cases, we set optical depth limits an order of magnitude better than that required to detect the 4 known redshifted millimeter-wave absorbers. We also discuss the contributory factors in the detectability of HI 21-cm absorption, focusing on possible biases (e.g.low covering factors) in the currently known sample of absorbers and non-detections.

astro-ph↗