Nancy Remage Evans

AN INTRODUCTION TO THE CHANDRA CARINA COMPLEX PROJECT

The Great Nebula in Carina provides an exceptional view into the violent massive star formation and feedback that typifies giant H II regions and starburst galaxies. We have mapped the Carina star-forming complex in X-rays, using archival Chandra data and a mosaic of 20 new 60 ks pointings using the Chandra X-ray Observatorys Advanced CCD Imaging Spectrometer, as a testbed for understanding recent and ongoing star formation and to probe Carinas regions of bright diffuse X-ray emission. This study has yielded a catalog of properties of > 14,000 X-ray point sources;> 9800 of them have multiwavelength counterparts. Using Chandras unsurpassed X-ray spatial resolution, we have separated these point sources from the extensive, spatially-complex diffuse emission that pervades the region; X-ray properties of this diffuse emission suggest that it traces feedback from Carinas massive stars. In this introductory paper, we motivate the survey design, describe the Chandra observations, and present some simple results, providing a foundation for the 15 papers that follow in this special issue and that present detailed catalogs, methods, and science results.

Chandra multiwavelength project. II. First results of X-ray source properties

The Chandra Multiwavelength Project (ChaMP) is a wide-area (~14 deg2) survey of serendipitous Chandra X-ray sources, aiming to establish fair statistical samples covering a wide range of characteristics (such as absorbed active galactic nuclei [AGNs] and high-z clusters of galaxies) at flux levels (fX ~ 10-15 to 10-14 ergs s-1 cm-2) intermediate between the Chandra Deep Field surveys and previous missions. We present the first results of X-ray source properties obtained from the initial sample of 62 observations. The data have been uniformly reduced and analyzed with techniques specifically developed for the ChaMP and then validated by visual examination. Utilizing only near-on-axis X-ray-bright sources (to avoid problems caused by incompleteness and the Eddington bias), we derive the log N- log S relation in soft (0.5-2 keV) and hard (2-8 keV) energy bands. The ChaMP data are consistent with previous results of ROSAT, ASCA, and Chandra Deep Field surveys. In particular, our data nicely fill in the flux gap in the hard band between the Chandra Deep Field data and the previous ASCA data. We check whether there is any systematic difference in the source density between cluster and noncluster fields and also search for field-to-field variation, both of which have been previously reported. We found no significant field-to-field cosmic variation in either test within the statistics (~1 σ) across the flux levels included in our sample. In the X-ray color-color plot, most sources fall in the location characterized by photon index = 1.5-2 and NH = a few × 1020 cm2, suggesting that they are typical broadline AGNs. There also exist a considerable number of sources with peculiar X-ray colors (e.g., highly absorbed, very hard, very soft). We confirm a trend that on average the X-ray color hardens as the count rate decreases. Since the hardening is confined to the softest energy band (0.3-0.9 keV), we conclude that it is most likely due to absorption. We cross-correlate the X-ray sources with other catalogs and describe their properties in terms of optical color, X-ray-to-optical luminosity ratio, and X-ray colors.

Period and light‐curve fluctuations of the Kepler Cepheid V1154 Cygni

We present a detailed period analysis of the bright Cepheid-type variable star V1154 Cygni (V1154 Cyg; V = 9.1 mag, P ≈ 4.9 d) based on almost 600 d of continuous observations by the Kepler space telescope. The data reveal significant cycle-to-cycle fluctuations in the pulsation period, indicating that classical Cepheids may not be as accurate astrophysical clocks as commonly believed: regardless of the specific points used to determine the O − C values, the cycle lengths show a scatter of 0.015–0.02 d over 120 cycles covered by the observations. A very slight correlation between the individual Fourier parameters and the O − C values was found, suggesting that the O − C variations might be due to the instability of the light-curve shape. Random-fluctuation tests revealed a linear trend up to a cycle difference 15, but for long term, the period remains around the mean value. We compare the measurements with simulated light curves that were constructed to mimic V1154 Cyg as a perfect pulsator modulated only by the light travel time effect caused by low-mass companions. We show that the observed period jitter in V1154 Cyg represents a serious limitation in the search for binary companions. While the Kepler data are accurate enough to allow the detection of planetary bodies in close orbits around a Cepheid, the astrophysical noise can easily hide the signal of the light-time effect.

Polaris: Amplitude, Period Change, and Companions

Polaris has presented us with the rare phenomenon of a Cepheid with a pulsation amplitude that has decreased over the last 50 yr. In this study we have used this property to see whether the amplitude decrease during the last 15 yr has had any effect on upper atmosphere heating. We obtained IUE high- and low-resolution spectra but found no change in either the Mg II chromospheric emission or the flux at 1800 A between 1978 and 1993 when the pulsation amplitude dropped by 50% (from 2.8 to 1.6 km s-1). The energy distribution from 1700 A through V, B, R(KC), and I(KC) is like that of a nonvariable supergiant of the same color rather than a full amplitude Cepheid in that it has more flux at 1800 A than the full amplitude Cepheid δ Cep. Polaris also has a rapidly changing period (3.2 s yr-1), in common with other overtone pulsators. We argue that this is a natural consequence of the different envelope locations that dominate pulsation growth rates in fundamental and overtone pulsation. In fundamental mode pulsators, the deeper envelope is more important in determining growth rates than for overtone pulsators. For fundamental mode pulsators, evolutionary changes in the radius produce approximately linear changes in period. In overtone pulsators, pulsation reacts to small evolutionary changes in a more unstable way because the modes are more sensitive to high envelope features such as opacity bumps, and the growth rates for the many closely spaced overtone modes change easily. Finally, the upper limit to the X-ray flux from an Einstein observation implies that the companion in the astrometric orbit is earlier than F4 V. The combination of upper and lower limits on the companion from IUE and Einstein respectively catch the companion mass between 1.7 and 1.4 M☉. The X-ray limit is consistent with the more distant companion α UMi B being a physical companion in a hierarchal triple system. However the X-ray limits require that the even more distant companions α UMi C and D are too old to be physically associated with Polaris.

A magnitude-limited survey of Cepheid companions in the ultraviolet

Results of a magnitude-limited survey of classic Cepheids brighter than 8th mag carried out to search for hot main-sequence companions are presented. Spectra of 76 stars obtained with the IUE satellite in the 2000-3200-A region were compared with the spectra of nonvariable supergiants and also the single Cepheid Delta Cep to search for excess flux at 2500 A from possible companions. Photometric companions were found for 21 percent of the sample. When the Cepheids known to be binary from either orbital motion or spectra in the 1200-2000-A region are included, the percentage of companions rises to 29 percent. If a statistical correction from stars with orbital motion is included, 34 percent have companions. This percentage is compared with that found by Abt et al. (1990) for B2-B5 main-sequence stars. If only systems with periods longer than a year and separations not more than 30 arcsec are considered, only 18 percent of the B stars will become Cepheids with companions.

Chandra Observations of Associates of η Carinae. I. Luminosities

The region around the η Carinae Nebula has three OB associations, which contain a Wolf-Rayet star and several massive O3 stars. An early Chandra ACIS-I image was centered on η Car and includes Trumpler 16 and part of Trumpler 14. The Chandra image confirms the well-known result that O and very early B stars are X-ray sources with LX 10-7Lbol over an X-ray luminosity range of about 100. Two new, anomalously strong X-ray sources have been found among the hot star population: Tr 16-244, a heavily reddened O3 I star, and Tr 16-22, a heavily reddened O8.5 V star. Two stars have an unusually large LX/Lbol: HD 93162, a Wolf-Rayet star (and possible binary), and Tr 16-22, a possible colliding-wind binary. In addition, a population of sources associated with cool stars is detected. In the color-magnitude diagram, these X-ray sources sit above the sequence of field stars in the Carina arm. The OB stars are on average more X-ray-luminous than the cool star X-ray sources. X-ray sources among A stars have X-ray luminosities similar to those of cooler stars and may be due to cooler companions. Upper limits are presented for B stars that are not detected in X-rays. These upper limits are also the upper limits for any cool companions that the hot stars may have. Hardness ratios are presented for the most luminous sources in bands 0.5-0.9, 0.9-1.5, and 1.5-2.04 keV. The available information on the binary nature of the hot stars is discussed, but binarity does not correlate with X-ray strength in a simple way.

Observations of Cepheids with the MOST satellite: contrast between pulsation modes

The quantity and quality of satellite photometric data strings is revealing details in Cepheid variation at very low levels. Specifically, we observed a Cepheid pulsating in the fundamental mode and one pulsating in the first overtone with the Canadian MOST satellite. The 3.7-d period fundamental mode pulsator (RT Aur) has a light curve that repeats precisely, and can be modeled by a Fourier series very accurately. The overtone pulsator (SZ Tau, 3.1 d period) on the other hand shows light curve variation from cycle to cycle which we characterize by the variations in the Fourier parameters. We present arguments that we are seeing instability in the pulsation cycle of the overtone pulsator, and that this is also a characteristic of the O −C curves of overtone pulsators. On the other hand, deviations from cycle to cycle as a function of pulsation phase follow a similar pattern in both stars, increasing after minimum radius. In summary, pulsation in the overtone pulsator is less stable than that of the fundamental mode pulsator at both long and short timescales.

Classical Cepheid luminosities from binary companions

Luminosities for the classical Cepheids Eta Aql, W Sgr, and SU Cas are determined from IUE spectra of their binary companions. Spectral types of the companions are determined from the spectra by comparison with the spectra of standard stars. The absolute magnitude inferred from these spectral types is used to determine the absolute magnitude of the Cepheid, either directly or from the magnitude difference between the two stars. For the temperature range of the companions (A0 V), distinctions of a quarter of a spectral subclass can be made in the comparison between the companions and standard stars. The absolute magnitudes for Eta Aql and W Sgr agree well with the period-luminosity-color relation of Feast and Walker (1987). Random errors are estimated to be 0.3 mag. SU Cas, however, is overluminous for pulsation in the fundamental mode, implying that it is pulsating in an overtone. 58 refs.

Polaris the Cepheid Returns: 4.5 Years of Monitoring from Ground and Space

We present the analysis of 4.5 years of nearly continuous observations of the classical Cepheid Polaris, which comprise the most precise data available for this star. We have made spectroscopic measurements from ground and photometric measurements from the WIRE star tracker and the SMEI instrument on the Coriolis satellite. Measurements of the amplitude of the dominant oscillation (P = 4 days), which go back more than a century, show a decrease from AV = 120 to 30 mmag around the turn of the millennium. It has been speculated that the reason for the decrease in amplitude is the evolution of Polaris toward the edge of the instability strip. However, our new data reveal an increase in the amplitude by ~30% from 2003 to 2006. It now appears that the amplitude change is cyclic rather than monotonic and most likely the result of a pulsation phenomenon. In addition, previous radial velocity campaigns have claimed the detection of long-period variation in Polaris (P > 40 days). Our radial velocity data are more precise than previous data sets, and we find no evidence for additional variation for periods in the range 3-50 days with an upper limit of 100 m s−1. However, in the WIRE data we find evidence of variation on timescales of 2-6 days, which we interpret as being due to granulation.

Chandra Observations of Associates of η Carinae. II. Spectra

The low-resolution X-ray spectra around η Car covering Trumpler 16 and part of Trumpler 14 have been extracted from a Chandra CCD ACIS image. Various analysis techniques have been applied to the spectra based on their count rates. The spectra with the greatest number of counts (HD 93162 = WR 25, HD 93129 AB, and HD 93250) have been fitted with a wind model, which uses several components with different temperatures and depths in the wind. Weaker spectra have been fitted with Raymond-Smith models. The weakest spectra are simply intercompared with strong spectra. In general, fits produce reasonable parameters based on knowledge of the extinction from optical studies and on the range of temperatures for high- and low-mass stars. Direct comparisons of spectra confirm the consistency of the fitting results and also hardness ratios for cases of unusually large extinction in the clusters. The spectra of the low-mass stars are harder than the more massive stars. Stars in the sequence evolving from the main sequence (HD 93250) through the system containing the O supergiant (HD 93129 AB) and then through the Wolf-Rayet stage (HD 93162), presumably ending in the extreme example of η Car, share the property of being unusually luminous and hard in X-rays. For these X-ray-luminous stars, their high mass and evolutionary status (from the very last stages of the main sequence and beyond) is the common feature. Their binary status is mixed, and their magnetic status is still uncertain.