Lee K.J.,Max Planck Institute for Radio Astronomy
Classical and Quantum Gravity | Year: 2013
In this paper, we focus on testing gravity theories in the radiative regime using pulsar timing array observations. After reviewing current techniques to measure the dispersion and alternative polarization of gravitational waves, we extend the framework to the most general situations, where the combinations of a massive graviton and alternative polarization modes are considered. The atlas of the Hellings-Downs functions is completed by the new calculations for these dispersive alternative polarization modes. We find that each mode and corresponding graviton mass introduce characteristic features in the Hellings-Downs function. Thus, in principal, we can not only detect each polarization mode, measure the corresponding graviton mass, but also discriminate the different scenarios. In this way, we can test gravity theories in the radiative regime in a generalized fashion, and such method is a direct experiment, where one can address the gauge symmetry of the gravity theories in their linearized limits. Although current pulsar timing still lacks enough stable pulsars and sensitivity for such practices, we expect that future telescopes with larger collecting areas could make such experiments feasible. © 2013 IOP Publishing Ltd.
Hirano N.,National Taiwan University |
Liu F.-C.,Max Planck Institute for Radio Astronomy
Astrophysical Journal | Year: 2014
Two submillimeter/millimeter sources in the Barnard 1b (B1-b) core, B1-bN and B1-bS, have been studied in dust continuum, H13CO+ J = 1-0, CO J = 2-1, 13CO J = 2-1, and C18O J = 2-1. The spectral energy distributions of these sources from the mid-IR to 7 mm are characterized by very cold temperatures of T dust < 20 K and low bolometric luminosities of 0.15-0.31 L ⊙. The internal luminosities of B1-bN and B1-bS are estimated to be <0.01-0.03 L ⊙ and 0.1-0.2 L ⊙, respectively. Millimeter interferometric observations have shown that these sources have already formed central compact objects of 100 AU sizes. Both B1-bN and B1-bS are driving the CO outflows with low characteristic velocities of 2-4 km s-1. The fractional abundance of H13CO+ at the positions of B1-bN and B1-bS is lower than the canonical value by a factor of four to eight. This implies that a significant fraction of CO is depleted onto dust grains in the dense gas surrounding these sources. The observed physical and chemical properties suggest that B1-bN and B1-bS are in an earlier evolutionary stage than most of the known class 0 protostars. In particular, the properties of B1-bN agree with those of the first hydrostatic core predicted by the MHD simulations. The CO outflow was also detected in the mid-IR source located at 15″ from B1-bS. Since the dust continuum emission was not detected in this source, the circumstellar material surrounding this source is less than 0.01 M ⊙. It is likely that the envelope of this source was dissipated by the outflow from the protostar that is located to the southwest of B1-b. © 2014. The American Astronomical Society. All rights reserved.
Schnitzeler D.H.F.M.,CSIRO |
Schnitzeler D.H.F.M.,Max Planck Institute for Radio Astronomy
Monthly Notices of the Royal Astronomical Society | Year: 2012
An accurate picture of how free electrons are distributed throughout the Milky Way leads to more reliable distances for pulsars and more accurate maps of the magnetic field distribution in the Milky Way. In this paper we test eight models of the free electron distribution in the Milky Way that have been published previously, and we introduce four additional models that explore the parameter space of possible models further. These new models consist of a simple exponential thick-disc model, and updated versions of the models by Taylor & Cordes and Cordes & Lazio with more extended thick discs. The final model we introduce uses the observed Hα intensity as a proxy for the total electron column density, also known as the dispersion measure (DM). Since accurate maps of Hα intensity are now available, this final model can in theory outperform the other models. We use the latest available data sets of pulsars with accurate distances (through parallax measurements or association with globular clusters) to optimize the parameters in these models. In the process of fitting a new scale height for the thick disc in the model by Cordes & Lazio, we discuss why this thick disc cannot be replaced by the thick disc that Gaensler et al. advocated in a recent paper. In the second part of our paper we test how well the different models can predict the DMs of these pulsars at known distances. We base our test on the ratios between the modelled and observed DMs, rather than on absolute deviations, and we identify systematic deviations between the modelled and observed DMs for the different models. For almost all models the ratio between the predicted and the observed DM cannot be described very well by a Gaussian distribution. We therefore calculate the deviations N between the modelled and observed DMs instead, and compare the cumulative distributions of N for the different models. Almost all models perform well, in that they predict DMs within a factor of 1.5-2 of the observed DMs for about 75 per cent of the lines of sight. This is somewhat surprising since the models we tested range from very simple models that only contain a single exponential thick disc to very complex models like the model by Cordes & Lazio. We show that the model by Taylor & Cordes that we updated with a more extended thick disc consistently performs better than the other models we tested. Finally, we analyse which sightlines have DMs that prove difficult to predict by most models, which indicates the presence of local features in the interstellar medium between us and the pulsar. © 2012 CSIRO. Monthly Notices of the Royal Astronomical Society © 2012 RAS.
Ohnaka K.,Max Planck Institute for Radio Astronomy
Astronomy and Astrophysics | Year: 2014
Context. The mass-loss mechanism in red giants and red supergiants is not yet understood well. The SiO fundamental lines near 8 μm are potentially useful for probing the outer atmosphere, which is essential for clarifying the mass-loss mechanism. However, these lines have been little explored until now. Aims. We present high spectral resolution spectroscopic observations of the SiO fundamental lines near 8.1 μm in 16 bright red giants and red supergiants. Our sample consists of seven normal (i.e., non-Mira) K-M giants (from K1.5 to M6.5), three Mira stars, three optically bright red supergiants, two dusty red supergiants, and the enigmatic object GCIRS3 near the Galactic center. Methods. Our program stars were observed between 8.088 μm and 8.112 μm with a spectral resolution of 30 000 using VLT/VISIR. Results. We detected SiO fundamental lines in all of our program stars except for GCIRS3. The SiO lines in normal K and M giants as well as optically bright (i.e., not dusty) red supergiants do not show P-Cyg profiles or blueshifts, which means the absence of systematic outflows in the SiO line forming region. We detected P-Cyg profiles in the SiO lines in the dusty red supergiants VY CMa and VX Sgr, with the latter object being a new detection. These SiO lines originate in the outflowing gas with the thermal dust continuum emission seen as the background. The outflow velocities of the SiO line forming region in VY CMa and VX Sgr are estimated to be 27 km s-1 and 17 km s-1, respectively. We derived basic stellar parameters (effective temperature, surface gravity, luminosity, and mass) for the normal K-M giants and optically bright red supergiants in our sample and compared the observed VISIR spectra with synthetic spectra predicted from MARCS photospheric models. Most of the SiO lines observed in the program stars warmer than ~3400 K are reasonably reproduced by the MARCS models, which allowed us to estimate the silicon abundance as well as the 28Si/ 29Si and 28Si/30Si ratios. However, we detected possible absorption excess in some SiO lines. Moreover, the SiO lines in the cooler red giants and red supergiant cannot be explained by the MARCS models at all, even if the dust emission is taken into account. This disagreement may be a signature of the dense, extended molecular outer atmosphere. © 2013 ESO.
Pfalzner S.,Max Planck Institute for Radio Astronomy
Astronomy and Astrophysics | Year: 2011
Context. Massive Galactic clusters (>1000 M ̇) exhibit a clear correlation between cluster density, size, and age and can be sorted into two categories, i.e. starburst and leaky clusters. The reason for having two types of massive clusters is still an open question; however, the answer is probably connected to the different formation histories of the two types. Aims. In this study we concentrate on leaky clusters and investigate possible formation scenarios and the gas expulsion phase. Methods. We use the existing observational data and numerical results of embedded cluster properties. Results. Assuming that a clear correlation between cluster density, size, and age exists, it is shown that the density-radius development over time for embedded clusters can be approximated by ρ ≈ 100 * r -1.3 M ⊙ pc -3. The consequences for the star formation process in leaky clusters are discussed and found to favour an inside-out star formation scenario with an initially low, but later accelerated, star formation rate. It is shown how the leaky clusters form in a unique, sequential manner and how rapid gas expulsion is responsible for the 80-90% mass loss over the next 20 Myr. © 2011 ESO.